Epitaxial Graphene Growth on 3C-SiC/Si(111): Towards Semiconducting Graphene
Submitted in fulfilment of the requirements for a degree of
Doctor of Philosophy
Mojtaba Amjadipour
B.Eng., M.Sc.
School of Chemistry, Physics and Mechanical Engineering
Science and Engineering Faculty
Queensland University of Technology
2018
ii
Dedicated to
My dear wife and mother for their kind support throughout this project.
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
iii
Keywords
Graphene; Epitaxial graphene on SiC; Focused ion beam (FIB); Buffer layer; Hydrogen intercalation;
Free-standing graphene; Graphene nanostructures; Effective attenuation length (EAL); Inelastic
mean free path (IMFP); Photoelectron spectroscopy.
iv
Abstract
Graphene is a promising material for a variety of advanced applications such as nanoelectronics
and sensing. Thermal decomposition of SiC has proven to be an excellent method to grow transfer-
free wafer-scale graphene. Epitaxial graphene growth on SiC thin films on Si is a cheaper alternative
to the growth on bulk SiC. Employing patterning techniques for engineering graphene dimensions is
emerging as a vital step towards its applications. The fact that graphene is a semimetal with zero
bandgap is one of the challenges preventing graphene from being used in nanoelectronics. Reducing
the width of graphene to nanoscale dimensions is one way to open a bandgap. A considerable
literature has grown up in order to explore different pathways for nanoscale graphene fabrication.
This thesis attempts to manipulate the SiC substrate using focused ion beam milling to grow graphene
over SiC nanostructures.
Epitaxial graphene growth on 3C-SiC/Si leads to creation of a graphene-like layer at the
interface between graphene and SiC substrate, which is commonly called the buffer layer. This
interface layer is partially bonded to the SiC substrate adversely affecting graphene properties.
Hydrogen intercalation is a possible pathway to eliminate the buffer layer. The effectiveness of
hydrogen intercalation in removing the buffer layer for graphene grown on 3C-SiC/Si(111) substrate
is also explored in this work.
Finally, graphene has been the subject of many electron-collection-based measurements such
as photoelectron spectroscopy and Auger electron spectroscopy. Having a precise understanding
about electron inelastic mean free path in graphene is crucial for data interpretation using such
measurements. Inelastic mean free path in materials can be experimentally estimated by measuring
electron effective attenuation length, assuming the elastic scattering effects are negligible. A direct
measurement of electron effective attenuation length in epitaxial graphene fabricated on 3C-
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
v
SiC/Si(111) using synchrotron photoelectron spectroscopy over an electron kinetic energy range of
50-1150 eV is presented, indicating unexpected variations with respect to theoretical calculations and
experimental data for graphite. The results indicate that the existing models for estimating IMFP in
bulk materials (graphite) may not adequately show the electron interactions in 2D materials
(graphene).
vi
Statement of Original Authorship
The work contained in this thesis has not been previously submitted to meet requirements for an award at this or any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.
Signature:
Mojtaba Amjadipour
Date: 9/11/2018
QUT Verified Signature
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
vii
Acknowledgment
First of all, my deepest gratitude goes to my principal supervisor Prof. Nunzio Motta for giving
me the opportunity to join his group and his kind support throughout my PhD. He has always been
available to talk to and get guidance to take the next step. I would like to thank Dr. Jennifer Macleod
for her kind support and guidance throughout this work. I kindly acknowledge the help and support I
received from Prof Francesca Iacopi and Prof Jose Alarco.
Part of the data reported in this thesis were obtained at the Central Analytical Research Facility
(CARF) operated by the Institute for Future Environments (IFE) at Queensland University of
Technology (QUT). Access to CARF was supported by generous funding from the Science and
Engineering Faculty, QUT. All CARF staff are appreciated for providing access to laboratory
equipment and training in XPS, SEM, TEM, FIB, Raman spectroscopy and sample preparation. In
particular, I would like to thank Dr. Josh Lipton-Duffin, Dr. Annalena Wolf, Dr. Peter Hines, Dr.
Jamie Riches, and Dr. Llew Rintoul.
Part of data reported in this thesis were undertaken on the Soft X-ray Beamline at the Australian
Synchrotron, part of ANSTO. I kindly acknowledge the support of the Australian Synchrotron and
ANSTO. I thank Dr Anton Tadich at the Australian Synchrotron for his help and support.
I like to thank my team mates Mr Jonathan Bradford, Mr Nima Khoshsirat, and Dr. Bharati
Gupta.
Last but not least, I wish to express my deepest appreciation to my wife and family for their
support during my PhD studies.
viii
List of Publications
The chapters included in the following thesis are based on articles that have been published in
peer-reviewed scientific journals. Each chapter is presented in a conventional form of a scientific
publication and contains an introduction, experimental details, results and discussions making them
self-contained.
[1] Chapter 4:
M. Amjadipour, J. Macleod, J. Lipton-Duffin, F. Iacopi, and N. Motta. Epitaxial graphene growth on FIB patterned 3C-SiC nanostructures on Si (111): reducing milling damage. Nanotechnology 2017, 28 (34), 345602. DOI: https://doi.org/10.1088/1361-6528/aa752e
[2] Chapter 5:
M. Amjadipour, A. Tadich, J. J. Boeckl, J. Lipton-Duffin, J. Macleod, F. Iacopi, and N. Motta. Quasi free-standing epitaxial graphene fabrication on 3C-SiC/Si(111). Nanotechnology 2018, 29 (14), 145601. DOI: https://doi.org/10.1088/1361-6528/aaab1a
[3] Chapter 6:
M. Amjadipour, J. Macleod, J. Lipton-Duffin, A. Tadich, J. J Boeckl, F. Iacopi, and N. Motta. Electron effective attenuation length in epitaxial graphene on SiC. Nanotechnology 2019, 30 (2), 025704. DOI: https://dx.doi.org/10.1088/1361-6528/aae7ec
Publications not presented in this thesis:
[4] N. Khoshsirat, F. Ali, V. Tiing Tiong, M. Amjadipour, M. Shafiei, H. Wang, N. Motta. Enhancing adhesion and conductivity of Mo based back contact for solar cell applications through incorporation of an ultra-thin Cr Layer. Beilstein Journal of Nanotechnology 2018. 9(1): p. 2700-2707.
[5] Y. Zhao, J. Liu, B. Wang, J. Sha, Y. Li, M. Amjadipour, and N. Motta. Supercapacitor electrodes with remarkable specific capacitance converted from hybrid graphene Oxide/NaCl/Urea Films, ACS Applied Materials & Interfaces 2017, 9(27) p. 22588-22596.
[6] Wang, Bin, J. Liu, Y. Zhao, Y. Li, W. Xian, M. Amjadipour, J. MacLeod, and N. Motta. Role of graphene oxide liquid crystals in hydrothermal reduction and supercapacitor performance. ACS Applied Materials & Interfaces 2016, 8(34) p. 22316-22323.
[7] M. Amjadipour, D. V. Dao, and N. Motta, Combination effect of waviness and vacancy defects on the natural frequency of single walled carbon nanotubes, Journal of Computational and Theoretical Nanoscience 2016, 13(8), p.5031-5036.
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
ix
Table of Contents
Keywords ............................................................................................................................................ iii
Abstract ............................................................................................................................................... iv
Statement of Original Authorship ....................................................................................................... vi
Acknowledgment ............................................................................................................................... vii
List of Publications ........................................................................................................................... viii
Chapter 1: Introduction .................................................................................................................... 1
1.1 Background ........................................................................................................................... 1
1.2 Objectives .............................................................................................................................. 2
1.3 Significance and Context....................................................................................................... 3
1.4 Thesis Outline........................................................................................................................ 4
1.5 References ............................................................................................................................. 5
Chapter 2: Literature Review ........................................................................................................... 7
2.1 Graphene ............................................................................................................................... 7
2.2 Synthesis Methods ................................................................................................................. 9
2.3 Nanoscale Graphene Fabrication ......................................................................................... 17
2.4 Free-Standing Epitaxial Graphene on SiC .......................................................................... 22
2.5 Inelastic Mean Free Path in Epitaxial Graphene ................................................................. 25
2.6 Literature Review Summary ............................................................................................... 27
2.7 References ........................................................................................................................... 28
Chapter 3: Methodology ................................................................................................................ 39
3.1 Experimental Data ............................................................................................................... 39
3.2 Instrumentation .................................................................................................................... 41
3.3 Refrences ............................................................................................................................. 49
Chapter 4: Graphene Growth on FIB Patterned 3C-SiC Nanostructures on Si (111) ................... 52
4.1 Abstract ............................................................................................................................... 52
4.2 Introduction ......................................................................................................................... 54
4.3 Experimental Details ........................................................................................................... 56
4.4 Results and Discussions ...................................................................................................... 57
4.5 Conclusions ......................................................................................................................... 65
4.6 Acknowledgments ............................................................................................................... 65
4.7 References ........................................................................................................................... 66
x
4.8 Supporting Information ....................................................................................................... 71
Chapter 5: Free-Standing Graphene on 3C-SiC/Si(111) ............................................................... 76
5.1 Abstract ............................................................................................................................... 76
5.2 Introduction ......................................................................................................................... 79
5.3 Experimental Details ........................................................................................................... 80
5.4 Results and Discussions ...................................................................................................... 82
5.5 Conclusions ......................................................................................................................... 92
5.6 Acknowledgments ............................................................................................................... 93
5.7 References ........................................................................................................................... 93
5.8 Supporting Information ....................................................................................................... 98
Chapter 6: Electron Effective Attenuation Length in Epitaxial Graphene on SiC ...................... 102
6.1 Abstract ............................................................................................................................. 102
6.2 Introduction ....................................................................................................................... 105
6.3 Experimental Details ......................................................................................................... 107
6.4 Results and Discussions .................................................................................................... 108
6.5 Conclusions ....................................................................................................................... 114
6.6 Acknowledgments ............................................................................................................. 115
6.7 References ......................................................................................................................... 115
6.8 Supporting Information ..................................................................................................... 120
Chapter 7: Conclusions and Future Research .............................................................................. 126
7.1 Conclusions ....................................................................................................................... 126
7.2 Future Research ................................................................................................................. 127
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
xi
List of Figures
Figure 1-1. Minimum feature size trend in semiconductor industries [2]. .......................................... 1
Figure 2-1. Graphene’s honeycomb structure (a) the ideal lattice structure of graphene with two sublattices A and B, (b) reciprocal lattice of single layer graphene [1]. .............................................. 8
Figure 2-2. (a) Graphene band structure calculated by ab initio, (b) schematic of the Fermi surface of graphene, (c) the two dimensional tight binding energy surface of graphene [13]. ........................ 9
Figure 2-3. Cross-sectional demonstration of 3C and 6H SiC polytypes. 3C polytype is shown parallel to (121) plane and 6H is parallel to (1120) plane. The cubic (3C) polytype has a linear stacking, and 6H polytype has six bilayers in which three of them are in opposite direction. Depending on the depth of the direction change, three different stacking sequences can be formed: S1 (CACBABC), S2 (BCACBAB) and S3 (ABCACBA) [38]. ........................................................ 12
Figure 2-4. AFM images of (a) initial surface of 6H-SiC, (b) graphene on 6H-SiC grown by annealing in UHV at 1280 ˚C, (c) graphene on 6H-SiC grown in a furnace with Ar atmosphere by annealing at 1650 ˚C [39]. .................................................................................................................. 13
Figure 2-5. 3C-SiC and 6H-SiC structure comparison; blue and black balls represent Si and C atoms, respectively [57]. .................................................................................................................... 15
Figure 2-6. Graphene grown on 3C-SiC/Si(111) surface after annealing at 1250 (a) STM image 25 × 52 nm2 (0.05 V, 0.1 nA), inset shows the FTs of the image, (b) STM image of 4 × 52 nm2 showing graphene honeycomb structure (-0.05 mV, 0.1 nA) [62]. ................................................... 15
Figure 2-7. Time dependence of normalized XPS C1s intensities from graphene overlayer on SiC/Si(111) [65]. ................................................................................................................................ 16
Figure 2-8. Schematic of the metal mediated epitaxial graphene growth on 3C-SiC/Si, (a) a thin film of 3C-SiC/Si can have (100) or (111) orientation, (b) metal catalyst layers (Cu and Ni) deposited, (c) annealing the sample to 1100 °C leads to graphitization, the intermixed layer can be later removed [66]. ..................................................................................................................................... 17
Figure 2-9. STM images showing the surface morphology after graphene growth: (a), (d) show the surface condition of the unpolished 250 nm thick 3C-SiC/Si(111); (b), (e) depict the surface morphology of unpolished 1 µm thick 3C-SiC/Si(111); (c), (f) are for the polished 1 µm thick 3C-SiC/Si(111) samples. Top images show a 5 × 5 µm2 area and bottom images represent a 1 × 1 µm2 area. All the images are taken with biased voltage of 2 V with various currents [69]. ..................... 17
Figure 2-10. Epitaxial graphene produced by Berger et al [72] (A) LEED pattern of the grown graphene, (B) AFM image indicating the terraces, (C) STM image of a monolayer epitaxial graphene (Vbias=-0.8 V, I=100 pA), (D) STM image of the interface reconstruction beneath grown graphene after lithography, (E) SEM image of the patterned epitaxial graphene, (F) EFM of the patterned epitaxial graphene [72]. ...................................................................................................... 18
Figure 2-11. Work done by Han et al [82], (a) AFM image of fabricated graphene nanoribbons contacted by metal electrodes, (b) SEM image of the fabricated device with parallel ribbons, (c) SEM image of the device fabricated with different orientation of the ribbons, (d)-(f) conductance of the ribbons with respect to gate voltage [82]. .................................................................................... 19
xii
Figure 2-12. Nanoscale graphene synthesis via chemical methods. (a) Yellow graphitic precursor and the final powder of produced graphene nanoribbons [90]. (b) A network of graphene ribbons dispersed in a solution imaged by SEM. (c) 60 nm-wide nanoribbon imaged via TEM [91]. .......... 20
Figure 2-13. (a) A schematic diagram demonstrating the process to fabricate graphene on SiC microbeams on Si via metal mediated growth. SEM images showing graphene covered 3C-SiC microstructures (b) bridges and (c) cantilevers [83]. ......................................................................... 21
Figure 2-14. (a) AFM topography image showing defects created in graphene on SiC sample after being exposed to moderate Ga+ ion dose - 104 ions/dot, (b) phase image, (c) line profile data from (a), (d) AFM topography image of the sample after being exposed to large Ga+ ion dose - 106 ions/dot, (e) phase image, (f) line profile of defects shown in (d) [96]. ............................................ 22
Figure 2-15. Schematic models of (a) buffer layer on SiC (b) epitaxial monolayer graphene; after hydrogen intercalation process they convert to (c) free-standing monolayer graphene (d) bilayer free-standing graphene, respectively [116]. ....................................................................................... 23
Figure 2-16. (a) C 1s and (b) Si 2p core level spectra showing a fully H-intercalated bilayer graphene sample (bottom spectra) and the hydrogen desorption process by annealing [116]. ......... 24
Figure 2-17. STM images (a) monolayer graphene, (b) after a small hydrogen exposure, (c) after complete intercalation process showing a complete coverage [117]. ................................................ 25
Figure 2-18. IMFP results for graphite comparing optical based calculation results, TPP-2M equation, experimental data using elastic peak electron spectroscopy by Lesiak et al [141] and Tanuma et al [130, 142]. .................................................................................................................... 26
Figure 2-19. (a) IMFP values for graphene samples derived from AES measurements and compared to the predictions by TPP-2 formula. (b) An average value of IMFP from all the graphene samples with different thicknesses [148]. ........................................................................................................ 27
Figure 3-1. (a) Schematic of the patterned 3C-SiC/Si(111) samples, (b) SEM image of a typical structure milled in 3C-SiC/Si(111). ................................................................................................... 40
Figure 3-2. STM images of 3C-SiC/Si(111) sample after graphene growth (a) without hydrogen etching, (b) with hydrogen etching procedure [7]. ............................................................................. 41
Figure 3-3. (a) Silicon atoms leaving the sample because of annealing at 1200-1300 °C, (b) a review of the steps taken to grow graphene in UHV. .................................................................................... 41
Figure 3-4. A typical FEI Quanta 3D FIB/SEM system [8]. ............................................................. 42
Figure 3-5. SEM image of the lift out procedure to prepare a TEM lamella using a Dual Beam system................................................................................................................................................. 43
Figure 3-6. UHV Omicron multiprobe system [5]. ............................................................................ 44
Figure 3-7. SiC sample during annealing by direct current heating in the analysis chamber of UHV Omicron system. From the left to the right, pointing at the sample, it is possible to see: (1) the terminal part of the X-ray source - DAR 400, (2) the electron analyzer - Sphera II -7/1 and (3) the SEM - SEM 20. .................................................................................................................................. 45
Figure 4-1. (a) STM image (Vbias= -1.4 V, I = 0.3 nA) of the surface after 40 minutes atomic hydrogen etching at 1000 ˚C, (b) XPS of the C1s region, showing the presence of three components attributed to the buffer layer, graphene (C-C) and SiC. ..................................................................... 58
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
xiii
Figure 4-2. (a) SEM image of the FIB milled structures, (b) HIM image showing the surface morphology after the graphene growth on the structures - the black square show the typical area used for roughness calculation, (c) HIM image showing the surface morphology after the graphene growth far from the structures, (d) EDX map indicating Ga implantation on the sample, where the green color represents Ga, (e) Raman spectra: blue line, acquired at the location of the blue circle in panel (b); green line, acquired on green circle in panel (c), (f) STM image (Vbias= -1.6 V, I = 0.9 nA) showing the morphology of surface after graphene growth far from the structures (panel (c)), and the quasi 6×6 periodicity due to the interface corrugation [46] (1.8 nm, marked by a red lozenge) in the inset (Vbias= -1.2 V, I = 0.8 nA)................................................................................. 59
Figure 4-3. Schematic diagram of the procedure leading to damage on nanostructures: (a) pristine thin film of 3C-SiC/Si (111), (b) milling structures by FIB, (c) high temperature annealing in UHV which leads to binding of the residuals to the surface and disturbs graphene growth. ...................... 60
Figure 4-4. Schematic of the modified procedure: (a) deposition of a silicon protective layer in UHV, (b) patterning by FIB, (c) graphene growth in UHV. .............................................................. 61
Figure 4-5. (a) XPS spectrum of Si 2p after silicon deposition, (b) STM image (Vbias= 2 V, I = 0.8 nA) on the silicon-protected sample after the second round of atomic hydrogen etching, (c) XPS spectrum of C1s after graphene growth on the silicon coated sample, (d) STM image (Vbias=2 V, I =0.8 nA) after graphene growth with a high resolution image in the inset showing the quasi 6×6 periodicity [46] (Vbias= -1 V, I = 0.3 nA). .......................................................................................... 63
Figure 4-6. (a) HIM image of the nanostructures after annealing at 1250 ˚C using the modified procedure with silicon mask - black square shows the typical area used for roughness calculation, (c) Raman spectra: blue line, acquired at the location of the blue circle in panel (a); green line shows the spectrum acquired ~ 3 µm away from the structures. ....................................................... 64
Figure 4-7. XPS survey spectrum of sample after (a) degassing, (b) atomic hydrogen etching, and (c) graphene growth. .......................................................................................................................... 73
Figure 4-8. SRIM simulation (Ga ions, 30 keV, 50000 particles) indicates that the Si layer does not alter the ions interactions or trajectories in the solid (a) without Si cap layer (b) with the Si cap layer. ................................................................................................................................................... 73
Figure 4-9. XPS spectrum of Si 2p: (a) after atomic hydrogen etching, (b) silicon deposition, (c) milling by FIB and second atomic hydrogen etching, (d) final graphene growth. ............................ 74
Figure 4-10. (a) SEM image showing the area covered by the clamp of the sample holder which was not coated by silicon. (b) Higher magnification HIM image showing that the surface is covered by the silicon thin film. (c) Atomic force microscopy (AFM) image taken from the area coated by Si indicating its uniform coverage at the micron scale, where only typical SiC features are present. The inset shows a higher-resolution image with a line profile taken on the region indicated by the yellow line; the variation in height across the terrace is less than 0.5 nm. AFM images were taken using an NT-MDT Solver Pro microscope equipped with an NT-MDT NSG30 cantilever and operated in non-contact mode. .............................................................................................................................. 74
Figure 5-1. C 1s core-level photoemission spectrum at 330 eV photon energy (a) as grown graphene (b) after hydrogen intercalation (c) after hydrogen desorption by annealing to 700 °C (d) after annealing to 850 °C. Si 2p core-level photoemission spectrum at 150 eV photon energy (e) as grown graphene (f) after hydrogen intercalation (g) after hydrogen desorption by annealing to 700 °C (h) after annealing to 850 °C. ....................................................................................................... 85
xiv
Figure 5-2. Core-level photoemission spectrum of (a) C 1s at 330 eV photon energy, (b) Si 2p at 150 eV photon energy. ....................................................................................................................... 86
Figure 5-3. LEED pattern (a) after graphene growth (b) after H intercalation. ................................. 87
Figure 5-4. C 1s NEXAFS spectrum for monolayer graphene (a) full spectrum (b) σ* peak region at normal incidence angle (c) π* peak region at grazing incidence angle. ............................................. 89
Figure 5-5. C 1s NEXAFS spectrum for bilayer and three-layers graphene (a) σ* peak region at normal incidence angle, for bilayer graphene sample (b) π* peak region at grazing incidence, for bilayer graphene sample (c) σ* peak region at normal incidence angle, for three-layer graphene sample (d) π* peak region at grazing incidence angle, for three-layer graphene sample. .................. 90
Figure 5-6. C 1s NEXAFS differential spectrum made of H-intercalation spectrum –as grown graphene one (a) π* peak region at grazing incidence angle, (b) σ* peak region at normal incidence angle. .................................................................................................................................................. 91
Figure 5-7. Graphical representation of H-intercalation (a) monolayer graphene formed with the buffer layer on SiC (b) free-standing bilayer graphene fabricated as a result of the H-intercalation. ............................................................................................................................................................ 92
Figure 5-8. C 1s core-level photoemission spectrum at 330 eV photon energy before and after H-intercalation (a) monolayer graphene (b) Bilayer graphene. ............................................................. 99
Figure 5-9. C 1s core-level photoemission spectrum at 330 eV photon energy for bilayer graphene after H-intercalation and being exposed to ambient conditions for 5 days. ....................................... 99
Figure 5-10. LEED pattern (a) after H-intercalation (b) after being exposed to ambient conditions for 5 days. ......................................................................................................................................... 100
Figure 5-11. C 1s NEXAFS spectrum after annealing to 1000 °C (a) full spectrum (b) σ* peak region at normal incidence angle (c) π* peak region at grazing incidence angle. ............................ 101
Figure 5-12. C 1s NEXAFS spectrum before and after H-intercalation (a) bilayer graphene (b) three-layer graphene sample. ........................................................................................................... 101
Figure 6-1. (a) Si 2p spectra for the SiC reference sample and the graphene/SiC sample indicating the intensity decrease. (b) Transmission with respect to the photon energies, points represent the experimental values fitted by an exponential equation. ................................................................... 109
Figure 6-2. PES C 1s spectrum of graphene/SiC measured with different photon energies. .......... 110
Figure 6-3. TEM image showing that the sample has the coverage of two graphene like layers (first layer corresponds to the buffer layer and the second one is graphene supported by the PES data).111
Figure 6-4. EAL values measured in the present work for monolayer graphene/buffer layer and bilayer free-standing graphene compared with EAL values measured using AES for exfoliated graphene transferred to SiO2 substrate by Xu et al [22] for monolayer graphene (ML) and averaged data over bilayer and three layer graphene. IMFP values for graphite estimated by TPP-2M model [11] and experimentally measured using EPES by Tanuma et al [40] are reported. The solid lines represent the fitting of the data using the modified Beth equation [18]. ......................................... 112
Figure 6-5. C 1s spectra of graphene/SiC sample with buffer layer. ............................................... 121
Figure 6-6. EALvariation for monolayer graphene plus buffer layer due to different layer spacing values [7, 8]. The solid lines represent the fitting of the data using the modified Beth equation. ... 122
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
xv
Figure 6-7. (a) A model representing monolayer graphene plus the buffer layer on SiC, (b) a model showing the effect of H-intercalation converting the sample to a bilayer free-standing graphene, (c) C 1s spectra indicating the elimination of the buffer layer components after H-intercalation. ....... 124
Figure 7-1. A possible configuration to fabricate a field effect transistor using the graphene grown on the SiC nanostructures. ............................................................................................................... 128
xvi
List of Tables
Table 3-1. Available pumps in the UHV multiprobe system ............................................................. 45
Table 4-1. Roughness calculated on 1×1 µm2 area STM images at different locations and on samples with and without the Si protective layer. ............................................................................. 64
Table 4-2. Raman data of graphene grown on 3C-SiC/Si. ................................................................. 75
Table 5-1. Dichroic ratio (DR) calculated for the samples before and after H-intercalation. ........... 92
Table 5-2. Fitting results for the core-level photoemission spectrum of binding energy (BE) position (±0.2 eV), full-width at half-maximum (FWHM), and relative intensity (peak areas). ................... 100
Table 6-1. Si 2p peak intensity change for the sample covered with graphene plus buffer layer (I) and the reference sample (I0). .......................................................................................................... 122
Table 6-2. Constant parameters of the modified version of the Bethe equation calculated for the fittings in Figure 6-4. ....................................................................................................................... 122
M. Amjadipour: Epitaxial graphene growth on 3C-SiC/Si(111): towards semiconducting graphene
xvii
List of Acronyms
UHV: Ultra-High Vacuum
FIB: Focused Ion Beam
STM: Scanning Tunneling Microscope
XPS: X-ray Photoelectron Spectroscopy
PES: Photoelectron Spectroscopy
AFM: Atomic Force Microscope
HIM: Helium Ion Microscope
SEM: Scanning Electron Microscope
TEM: Transmission Electron Microscope
CMP: Chemically Mechanically Polished
NEXAFS: Near Edge X-ray Absorption Fine Structure
LEED: Low Energy Electron Diffraction
IMFP: Inelastic Mean Free Path
GO: Graphene Oxide
CNTs: Carbon Nanotubes
CVD: Chemical Vapor Deposition
SRIM: Stopping and Range of Ions in Matter
ARPES: Angle-Resolved Photoemission Spectroscopy
LEEM: Low Energy Electron Microscopy
1
Chapter 1: Introduction
1.1 BACKGROUND
One of the fastest increasing trends in the modern world is the miniaturization of advanced
devices. The size of the smallest element in the electronic circuits (node) is rapidly reaching the
atomic scale, following the well-known prediction of “Moore’s law” [1]. Figure 1-1 shows how the
minimum size of transistor elements and interconnects has dramatically changed over years [2]. At
this point any increase in performance is most likely to come from the exploitation of the quantum
properties; it is known that two-dimensional (2D) or one dimensional (1D) materials show
extraordinary properties. Amongst these materials, graphene stands out due to its extremely high
mechanical strength [3], high electrical mobility [4], ease of functionalization [5], large surface-to-
mass ratio [6] and mass production capability [7].
Figure 1-1. Minimum feature size trend in semiconductor industries [2].
2
Graphene, due to its extraordinary properties, is rapidly becoming very appealing for a variety
of applications such as electronics and sensing [8-13]. Employing patterning techniques for
engineering graphene dimensions is emerging as a vital step towards its applications. A considerable
literature has grown up in order to explore different pathways for nanoscale graphene fabrication [14-
17].
There are several ways available to produce graphene, and the most common ones include
mechanical exfoliation [18], chemical reduction of graphene oxide (GO) [5], unzipping carbon
nanotubes (CNTs) [19], chemical vapor deposition (CVD) [20], and Si sublimation by annealing SiC
[21]. The latter technique is capable of wafer-size graphene production, which provides a solid
platform for the future use of graphene in the semiconductor industry, so in this research we have
used this growth procedure. Since bulk SiC is expensive, an alternative has also been explored in the
cheaper 3C-SiC/Si epitaxial system [22]. 3C-SiC/Si wafers can be produced in larger dimensions
compared to bulk SiC and are compatible with current semiconductor industry processes.
This thesis aims at exploring a different approach to fabricate nanoscale graphene on 3C-SiC
in ultra-high vacuum (UHV) and improving the properties of graphene by employing atomic
hydrogen intercalation to separate graphene from the SiC substrate. An improved value of the
electrons attenuation in epitaxial graphene was obtained using X-ray photoelectron spectroscopy at
different photon energies, providing an important tool to determine the number of graphene layers on
SiC.
1.2 OBJECTIVES
This thesis explores epitaxial graphene fabrication on 3C-SiC/Si(111) substrates. A range of
characterization techniques including scanning electron microscopy (SEM), transmission electron
microscopy (TEM), photoelectron spectroscopy (PES), near edge X-ray absorption fine structure
spectroscopy (NEXAFS), low energy electron diffraction (LEED), scanning tunneling microscopy
3
(STM) are employed. A particular focus of this work is to explore a different approach to fabricate
nanoscale graphene on 3C-SiC/Si(111) substrates and to improve its properties by atomic hydrogen
exposure. In addition, electron behavior inside graphene is explored.
The research objectives are:
1. Epitaxial graphene fabrication on FIB patterned 3C-SiC/Si(111) substrates,
2. Elimination of the buffer layer for epitaxial graphene on 3C-SiC/Si(111) substrates
using hydrogen intercalation,
3. Measuring electrons inelastic mean free path (IMFP) in epitaxial graphene on 3C-
SiC/Si(111) substrates.
1.3 SIGNIFICANCE AND CONTEXT
Epitaxial graphene growth on SiC has been recognized as a promising procedure to synthesize
graphene. Nanoscale graphene fabrication is vital for its application in electronics and sensing. The
effect of patterning SiC substrate prior to graphene growth has rarely been investigated before. This
thesis explores the possibility of patterning SiC substrate using FIB prior to the growth process aiming
at developing a new procedure to fabricate nanoscale graphene. 3C-SiC/Si(111) substrate was chosen
instead of bulk SiC to reduce the substrate cost making it appealing to industries. This procedure is
discussed in chapter 4. Later, the effectiveness of hydrogen intercalation procedure in eliminating the
buffer layer in epitaxial graphene on 3C-SiC/Si(111) substrates is investigated. The buffer layer
adversely affects graphene’s electronic properties. The success of the hydrogen intercalation process
is presented in Chapter 5.
Graphene is the subject of many electron-collection measurements such as PES, LEED, Auger
electron spectroscopy and electron microscopy making a precise understanding about electron
behavior inside graphene vital for accurate data analysis. Therefore, a direct measurement of the
4
electron effective attenuation length (EAL) as an estimation of IMFP in epitaxial graphene on 3C-
SiC/Si(111) is conducted and reported in Chapter 6.
1.4 THESIS OUTLINE
This PhD thesis is constructed as following:
Chapter 2 explores general information about graphene, and presents a literature review of the
topic. Different graphene synthesis methods are reported and in particular epitaxial graphene growth
on SiC is discussed in more detail as this is the method of choice for graphene fabrication in the
present work. Research progress regarding nanoscale graphene fabrication, hydrogen exposure effect
on epitaxial graphene, and inelastic mean free path (IMFP) in graphene is explored as well.
Chapter 3 presents detailed information about the experimental procedures and instruments
employed to conduct this research.
Chapter 4 presents a new procedure developed here to use focused ion beam (FIB) milling to
fabricate nanoscale graphene. It explores the effect of Ga ion milling of SiC on graphene growth and
the suggested method to prevent milling damages.
Chapter 5 provides information about use of hydrogen to eliminate the carbon rich layer
partially bonded to the SiC substrate at the interface of SiC and graphene commonly referred to as
the buffer layer. It demonstrates that the buffer layer can be fully eliminated using hydrogen
intercalation.
Chapter 6 presents electron effective attenuation length (EAL) measurement as an estimation
for inelastic mean free path (IMFP) in epitaxial graphene fabricated on SiC. It provides experimental
values estimating IMFP in graphene and demonstrates that the current theoretical and experimental
values for graphite under-predict the IMFP for graphene.
Chapter 7 summarizes the findings of this PhD project and recommends possible research
directions arising from this work.
5
1.5 REFERENCES
1. S. Thompson and S. Parthasarathy, Moore's law: the future of Si microelectronics. Materials Today, 2006. 9(6): p. 20-25.
2. M.T. Bohr, Nanotechnology goals and challenges for electronic applications. IEEE Transactions on Nanotechnology, 2002. 1(1): p. 56-62.
3. C. Lee, X. Wei, J.W. Kysar, and J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science, 2008. 321(5887): p. 385-388.
4. A.K. Geim and K.S. Novoselov, The rise of graphene. Nature Materials, 2007. 6(3): p. 183-191.
5. S. Park and R.S. Ruoff, Chemical methods for the production of graphenes. Nature Nanotechnology, 2009. 4(4): p. 217-224.
6. A. Peigney, C. Laurent, E. Flahaut, R. Bacsa, and A. Rousset, Specific surface area of carbon nanotubes and bundles of carbon nanotubes. Carbon, 2001. 39(4): p. 507-514.
7. F. Bonaccorso, A. Lombardo, T. Hasan, Z. Sun, L. Colombo, and A.C. Ferrari, Production and processing of graphene and 2d crystals. Materials Today, 2012. 15(12): p. 564-589.
8. Y.M. Lin, C. Dimitrakopoulos, K.A. Jenkins, D.B. Farmer, H.Y. Chiu, A. Grill, and P. Avouris, 100-GHz transistors from wafer-scale epitaxial graphene. Science, 2010. 327(5966): p. 662-662.
9. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, and W.A. De Heer, Electronic confinement and coherence in patterned epitaxial graphene. Science, 2006. 312(5777): p. 1191-1196.
10. M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu, C. Berger, and W.A. De Heer, Scalable templated growth of graphene nanoribbons on SiC. Nature Nanotechnology, 2010. 5(10): p. 727-731.
11. A. Celis, M. Nair, A. Taleb-Ibrahimi, E. Conrad, C. Berger, W. De Heer, and A. Tejeda, Graphene nanoribbons: fabrication, properties and devices. Journal of Physics D: Applied Physics, 2016. 49(14): p. 143001.
12. M. Terrones, A.R. Botello-Méndez, J. Campos-Delgado, F. López-Urías, Y.I. Vega-Cantú, F.J. Rodríguez-Macías, A.L. Elías, E. Muñoz-Sandoval, A.G. Cano-Márquez, J.-C. Charlier, and H. Terrones, Graphene and graphite nanoribbons: morphology, properties, synthesis, defects and applications. Nano Today, 2010. 5(4): p. 351-372.
13. Z. Li and F. Chen, Ion beam modification of two-dimensional materials: Characterization, properties, and applications. Applied Physics Reviews, 2017. 4(1): p. 011103.
14. R. Balog, B. Jørgensen, L. Nilsson, M. Andersen, E. Rienks, M. Bianchi, M. Fanetti, E. Lægsgaard, A. Baraldi, and S. Lizzit, Bandgap opening in graphene induced by patterned hydrogen adsorption. Nature Materials, 2010. 9(4): p. 315-319.
15. M. Dvorak, W. Oswald, and Z. Wu, Bandgap opening by patterning graphene. Scientific Reports, 2013. 3: p. 1-7.
16. J. Hicks, A. Tejeda, A. Taleb-Ibrahimi, M. Nevius, F. Wang, K. Shepperd, J. Palmer, F. Bertran, P. Le Fevre, and J. Kunc, A wide-bandgap metal-semiconductor-metal nanostructure made entirely from graphene. Nature Physics, 2013. 9(1): p. 49-54.
6
17. I. Palacio, A. Celis, M.N. Nair, A. Gloter, A. Zobelli, M. Sicot, D. Malterre, M.S. Nevius, W.A. De Heer, and C. Berger, Atomic structure of epitaxial graphene sidewall nanoribbons: flat graphene, miniribbons, and the confinement gap. Nano Letters, 2014. 15(1): p. 182-189.
18. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov, Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.
19. L. Jiao, L. Zhang, X. Wang, G. Diankov, and H. Dai, Narrow graphene nanoribbons from carbon nanotubes. Nature, 2009. 458(7240): p. 877-880.
20. S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, and Y.I. Song, Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 2010. 5(8): p. 574-578.
21. A. Van Bommel, J. Crombeen, and A. Van Tooren, LEED and Auger electron observations of the SiC (0001) surface. Surface Science, 1975. 48(2): p. 463-472.
22. A. Ouerghi, A. Kahouli, D. Lucot, M. Portail, L. Travers, J. Gierak, J. Penuelas, P. Jegou, A. Shukla, and T. Chassagne, Epitaxial graphene on cubic SiC (111)/Si (111) substrate. Applied Physics Letters, 2010. 96(19): p. 191910.
7
Chapter 2: Literature Review
2.1 GRAPHENE
Graphene is a two dimensional allotropic form of carbon, with a hexagonal lattice made of
carbon-carbon bonds; the lattice structure is shown in figure 2-1. The atoms are sp2 bonded, and they
are densely packed in a hexagonal lattice structure where the bond distance is 0.142 nm. Graphene’s
lattice can be described by two lattice vectors 𝐚𝐚𝟏𝟏and 𝐚𝐚𝟐𝟐 shown in figure 2-1a. These lattice vectors
can be described as:
𝐚𝐚𝟏𝟏 =a2
1,√3 (1)
𝐚𝐚𝟐𝟐 =a2−1,√3 (2)
where 𝑎𝑎 = |a1| = |a2| = 0.246 𝑛𝑛𝑛𝑛. The sp2 bond between A and B atoms in the graphene lattice
(figure 2-1) creates a strong interatomic coupling. Graphene lattice consists of two sublattices, shown
by red and black circles in figure 2-1a. All the atoms in the graphene sublattices are carbon atoms.
The similarity in graphene sublattice atoms is referred to as sublattice symmetry. This particular
symmetry makes graphene special compared to the other 2D materials such as boron nitride. The
hexagonal reciprocal lattice of graphene is shown in figure 2-1b by the red dots [1]. This reciprocal
lattice can be described by the vectors b1 and b2 shown in figure 2-1b. The first Brillouin zone of
graphene is shown in the figure marked by the blue hexagon. The center of the first Brillouin zone
and the corners of the hexagons are the high symmetry points denoted by Γ and Ki (i=+,-), but from
these six high symmetry points located on the corners, two points (K+ and K-) are different.
Furthermore graphene is the foundation of some of the other important carbon allotropes. It is the
basic building element of graphite and carbon nanotubes (CNTs) [2].
8
Figure 2-1. Graphene’s honeycomb structure (a) the ideal lattice structure of graphene with two sublattices A and B, (b)
reciprocal lattice of single layer graphene [1].
Graphene’s honeycomb lattice structure provides extraordinary properties such as remarkably
high electronic and thermal transport, exceptional mechanical strength and optical transparency [3,
4]. In terms of mechanical properties, graphene is the strongest material ever measured, with a tensile
strength of 130 GPa and elastic modulus of approximately 1 TPa [5]. The importance of graphene
has been proven by research carried over the past few years [6-10]. Further evidence showing the
significance of graphene is the amount of funding allocated to related research by government
agencies worldwide. For instance a funding of €1 billion over 10 years by the European Union, $350
million research support by South Korea and a £50 million funding by the United Kingdom [2].
Graphene is a single layer of carbon atoms, but two to ten layers are commonly referred to as
graphene as well [11]. Graphene is a zero-gap semimetal if it is made of less than two layers, with
one type of hole and one type of electron. When the number of graphene layers becomes three or
more, the electronic spectrum gets more complex; in this case several types of charge carriers appear.
After 10 layers or more it behaves like graphite. The Quantum Hall Effect (QHE), high carrier
mobility, ballistic electronic transport, and high electrical conductivity are also among the novel
properties of graphene. These fascinating properties can open up a number of new opportunities to
develop state-of-the-art nanoscale devises such as transistors and sensors [4, 7].
The first scientist who calculated the electronic band structure of graphene was Wallace in 1947
[12]. A simple tight binding approximation can be used to calculate the low energy electronic
9
structure of an infinite graphene lattice by solving the Schrödinger equation (3) where H and Ψ are
Hamiltonian matrix and the wavefunctions, respectively.
HΨ = E(k) Ψ (3)
It is possible to obtain the band structure of an isolated single graphene layer using ab initio
calculation (figure 2-2a). The most important part of this calculation is that near the K- point, the
energy dispersion is linear, and the E(k) bands intersect at k = 0 (figure 2-2b and c). These intersection
points are called the Dirac points. The Fermi surface consists of six cones near E=0 (figure 2-2c) [13].
Figure 2-2. (a) Graphene band structure calculated by ab initio, (b) schematic of the Fermi surface of graphene, (c) the two dimensional tight binding energy surface of graphene [13].
2.2 SYNTHESIS METHODS
Boehm et al [14, 15] firstly reported synthesis of a suspended monolayer graphite at 1962. Growing
monolayer graphite films on metallic surfaces was quite popular during 1970’s [16, 17]. There are a
10
number of methods reported for synthesising graphene, but the most commonly used techniques include
micromechanical exfoliation of highly oriented pyrolytic graphite (HOPG) [18], chemical reduction of
graphite oxide (GO) [19], unzipping of carbon nanotubes (CNTs) [20], chemical vapors deposition (CVD)
[21], and epitaxial growth on a SiC substrate [22, 23]. In this section, the first four techniques will be
discussed briefly, and the last method will be explored in detail, as this method is chosen for the graphene
growth in this work.
2.2.1 Micromechanical exfoliation
The mechanical exfoliation technique was introduced by Geim and Novoselov [24] and is based
on oxygen plasma etching of a highly oriented pyrolytic graphite (HOPG). The HOPG is covered by
a layer of photoresist, which is later baked in order to make way for HOPG to open up [17]. The risen
layers are then removed by using scotch tape and exposure to acetone. The removed flakes are then
captured on the surface of Si/SiO2. This technique is capable of producing graphene samples of up to
few 100 µm2. Although this method has been used for isolating graphene in laboratories, it cannot be
used for mass production because it is very time-consuming and expensive [2].
2.2.2 Chemical reduction of graphene oxide
Another method used to produce graphene is by chemical reduction of graphene oxide (GO)
[14, 15, 25]. This technique is based on a procedure commonly known as Hummer’s method [25]
including sonication of a mixture of graphite and de-ionized water followed by heating it in an oil
bath at the presence of hydrazine hydrate [19, 26]. Use of hydrazine leads to the formation of some
unwanted functional groups such as nitrogen on the graphene surface which adversely affects
graphene’s properties [27, 28].
2.2.3 Unzipping of carbon nanotubes
Unzipping of carbon nanotubes is also used for graphene synthesis. The first step in this
procedure is to suspend the carbon nanotubes in a sulphuric acid (H2SO4) solution for up to 12 hours.
Adding potassium permanganate (KMnO4) to the mixture and heating it to about 60 ˚C is the next
11
step. Finally, the mixture is quenched by being exposed to ice (which contains small amount of
hydrogen peroxide H2O2) and filtered by a polytetrafluoroethylene (PTFE) membrane [20]. This
process leads to synthesis of a long strip of nano-scale graphene which is useful in nanoelectronics
[29]. This procedure exposes some critical risks to the environment and human health as a result of
using toxic compounds such as H2O2 and KMnO4.
2.2.4 Chemical vapor deposition
Another methodology frequently used to grow graphene is chemical vapor deposition (CVD).
According to this technique the required carbon atoms for the growth are provided in a gas form in a
furnace kept at temperatures between 600 and 1200 ˚C [2], and normally graphene grows on a
substrate covered by a layer of catalyst (Cu, Ni, Pt). Although it is possible to produce large area
graphene using CVD, it commonly demands the transfer of graphene to another substrate for
applications adversely affecting its properties [30]. The transfer procedure includes a purification
process to eliminate the metal catalyst [30].
2.2.5 Epitaxial growth on SiC
Since 1975 when the graphitization of SiC (0001) was first studied by van Bommel et al [22],
growing epitaxial graphene by Si sublimation on SiC has been the subject of intense research [23, 30-
36]. The growth of graphene is obtained by heating the sample to high temperatures between 1100
and 1400 ˚C which causes Si to leave the surface while the remaining carbon atoms reorganize to
form graphene.
2.2.5.1 SiC crystal
The primitive structural element of SiC crystal is a hexagonal bilayer made of Si and C atoms.
These Si and C atoms are covalently bonded. Stacking variation of the bilayers results in having
different SiC polytypes. There are more than 200 polytypes reported for SiC [37]. These polytypes
are commonly labeled based on the number of bilayers staked in the same direction followed by a
letter representing the related Bravis lattice type: cubic (C), hexagonal (H) and rhombohedral (R)
12
such as 3C, 2H, 4H, 6H and 15 R. The only cubic polytype of SiC (3C) is formed when all the bilayers
are stacked with the same orientation in a zinc blende structure (figure 2-3). When the bilayers stack
in an opposite orientation, they create a wurtzite structure. This hexagonal polytype is labeled as 2H.
Another hexagonal polytype is 6H constructed from stacking three SiC bilayers linearly followed by
an orientation change (figure 2-3). For all the SiC polytypes, the Si-C bond length is approximately
1.89 Å and the neighboring Si or C atoms has a distance of about 3.08 Å. The SiC bilayer height is
about 2.5 Å [38]. These SiC polytypes can have C or Si termination.
Figure 2-3. Cross-sectional demonstration of 3C and 6H SiC polytypes. 3C polytype is shown parallel to (121) plane and 6H is parallel to (1120) plane. The cubic (3C) polytype has a linear stacking, and 6H polytype has six bilayers in which three of them are in opposite direction. Depending on the depth of the direction change, three different stacking
sequences can be formed: S1 (CACBABC), S2 (BCACBAB) and S3 (ABCACBA) [38].
2.2.5.2 Graphitization of SiC
Graphene growth using thermal decomposition of SiC can be done either in vacuum or under
atmospheric pressure condition. Bommel et al [22] demonstrated the formation of one graphitic layer
on SiC in UHV condition at 800 ˚C. Berger et al [23] fabricated a multilayer epitaxial graphene on
SiC via high temperature annealing in UHV and performed the first ever transport measurements on
epitaxial graphene on SiC. Emtsev et al [39] at 2009 explored epitaxial graphene growth in Ar
13
atmosphere and reported that Ar-assisted growth requires very high temperatures compared to UHV,
but it provides more control over the number of graphene layers. The Si sublimation rate decreases
due to Ar pressure on the surface providing some degree of control over the number of graphene
layers [39]. However, in case of growth in UHV, absence of any atmospheric pressure makes Si
sublimation very easy resulting in an uncontrolled growth. These two different techniques are
compared by Emtsev et al [39], and they concluded that graphene growth in Ar atmosphere leads to
larger crystal size and higher quality compared to UHV growth (figure 2-4). Another suggested way
to decrease Si sublimation rate is use of a Si flux during high temperature annealing [40].
Figure 2-4. AFM images of (a) initial surface of 6H-SiC, (b) graphene on 6H-SiC grown by annealing in UHV at 1280 ˚C, (c) graphene on 6H-SiC grown in a furnace with Ar atmosphere by annealing at 1650 ˚C [39].
De Heer et al introduced a new procedure called controlled confinement sublimation (CCS)
[41]. This method is based on confining SiC in a small carbon vessel in order to further control the
Si sublimation. They demonstrated that using CCS approach it is possible to grow defect-free and
uniform graphene. The most important feature of this method is the control of silicon vapor density
creating a near thermodynamic equilibrium, which is the key for high quality and uniform growth. It
has been also reported that using off axis SiC wafers improves graphene quality and produces larger
domains due to step bunching effects [33].
SiC has a hexagonal lattice structure, which is extremely favorable for graphene growth. SiC
lattices can have two surface terminations: Si-termination, corresponding to the (0001) surface and
C-termination corresponding to the (0001) surface. The surface termination can affect the final
14
structure of the grown graphene. The growth over the Si-terminated surface leads to formation of an
interface graphene-like layer which is partially bonded to the substrate. This interface layer is
commonly referred to as the buffer layer and adversely affects graphene’s electronic properties [31,
42]. The buffer layer can be eliminated using hydrogen intercalation process which will be discussed
later. On the other hand, the growth process is faster over the C-terminated surface and since there is
no buffer layer present at the interface to force the orientation, the grown graphene have a variety of
orientations which is not desired for most applications [43, 44].
2.2.5.3 Epitaxial graphene on 3C-SiC/Si(111)
The epitaxial growth of graphene on SiC by high temperature annealing is the only method
capable of producing wafer-scale graphene directly on a semiconducting substrate making it the best
synthesis approach for nanoelectronics purposes. However, bulk SiC wafers are still quite expensive
compared to Si substrates making them undesirable for commercial applications [33, 39]. An
alternative approach to address this issue is using cubic polytype of SiC (3C-SiC) grown on a Si
substrate instead of bulk SiC wafers [36, 45, 46].
Using 3C-SiC/Si wafer not only reduces the substrate cost, it can be produced in larger wafer
dimensions compared to bulk SiC wafers which is very appealing to semiconductor industries.
Although growing high quality 3C-SiC/Si thin films is challenging due to thermal and lattice
mismatch of Si and SiC, significant progress has been made towards producing a uniform and defect-
free film (~10 in) [47-49]. Different orientations of 3C-SiC substrate are available such as (100),
(110), (001), and (111) [36, 50-54]. Among these orientations, 3C-SiC(111) is the preferred one as it
has a hexagonal surface similar to 6H and 4H SiC making epitaxial graphene growth quite similar
[55]; the top four layers are identical to 6H (figure 2-5) [52]. 3C-SiC(111) is also free from anti-phase
boundaries, which can make graphene grow in multiple crystalline orientations [56]. In this thesis,
3C-SiC/Si(111) is used as the preferred substrate for graphene growth.
15
Figure 2-5. 3C-SiC and 6H-SiC structure comparison; blue and black balls represent Si and C atoms, respectively [57].
The heteroepitaxial growth of 3C-SiC on Si substrate is commonly performed in a horizontal
hot-wall CVD system at low pressure [58]. During the initial stage of the growth process a thin layer
of SiC (less than 10 nm) is formed on the Si substrate. At this stage, most of the Si atoms required for
the SiC growth are coming from the Si substrate and the C atoms are provided through decomposition
of a hydrocarbon precursor (commonly propane) at about 1000 ˚C. When the thickness of SiC layer
increases, the Si substrate cannot be used as a Si source, therefore, Si atoms are provided using silane
precursors. For further information regarding heteroepitaxial growth of 3C-SiC on Si substrate refer
to [59-61].
Figure 2-6. Graphene grown on 3C-SiC/Si(111) surface after annealing at 1250 (a) STM image 25 × 52 nm2 (0.05 V, 0.1 nA), inset shows the FTs of the image, (b) STM image of 4 × 52 nm2 showing graphene honeycomb structure (-0.05
mV, 0.1 nA) [62].
16
Ouerghi et al [50, 51, 62-64] is one of the pioneer groups working on epitaxial graphene
fabrication on SiC/Si thin films. They reported achieving a continuous graphene coverage over 3C-
SiC/Si surface and demonstrated that graphene is grown even on the step edges of the substrate (figure
2-6) [62]. Yazdi et al [55] compared the [36]epitaxial graphene growth on Si face of different SiC
polytypes including 3C, 4H and 6H. They reported strong similarity between epitaxial graphene
growth on 3C- and 4H-SiC [55]. Gupta et al [30, 65] further investigated the epitaxial graphene
growth on 3C-SiC/Si(111) substrate and reported a kinetic model demonstrating how graphene
growth develops with respect to annealing temperature and time (figure 2-7).
Figure 2-7. Time dependence of normalized XPS C1s intensities from graphene overlayer on SiC/Si(111) [65].
An alternative catalyst-based approach for graphitizing of 3C-SiC/Si has also been reported
[66, 67]. This method requires deposition of a metal layer commonly nickel, copper or cobalt on the
3C-SiC/Si wafer followed by annealing to 750 to 1200 °C (figure 2-8). Due to high temperature
annealing SiC reacts with the metallic layer creating a metal silicide causing C atoms to release into
the system. These C atoms reorganize and form graphene during cooling process [66]. This procedure
is capable of producing wafer-scale and good quality graphene, however, it lacks high surface quality
and leads to formation of a rough surface [68].
17
Figure 2-8. Schematic of the metal mediated epitaxial graphene growth on 3C-SiC/Si, (a) a thin film of 3C-SiC/Si can have (100) or (111) orientation, (b) metal catalyst layers (Cu and Ni) deposited, (c) annealing the sample to 1100 °C
leads to graphitization, the intermixed layer can be later removed [66].
The surface quality of 3C-SiC/Si thin film has a significant effect on the quality of the grown
graphene [69, 70]. Gupta et al [69] demonstrated that polishing the 3C-SiC/Si substrate improves the
quality and domain size of epitaxial graphene (figure 2-9). Growing epitaxial graphene on a thicker
SiC layer (1µm compared to 250 nm thick layer) results in lower defect densities [69].
Figure 2-9. STM images showing the surface morphology after graphene growth: (a), (d) show the surface condition of the unpolished 250 nm thick 3C-SiC/Si(111); (b), (e) depict the surface morphology of unpolished 1 µm thick 3C-
SiC/Si(111); (c), (f) are for the polished 1 µm thick 3C-SiC/Si(111) samples. Top images show a 5 × 5 µm2 area and bottom images represent a 1 × 1 µm2 area. All the images are taken with biased voltage of 2 V with various currents
[69].
2.3 NANOSCALE GRAPHENE FABRICATION
Fabrication of nanoscale graphene is a necessity for a number of applications such as electronics
and sensing [9, 71-75]. The fact that graphene is a zero-bandgap semimetal is a critical challenge for
its applications in electronics as semiconducting industries rely on a sizeable bandgap [76, 77]. One
18
of the suggested ways to create a bandgap for graphene is through quantum confinement by
fabricating graphene nanoribbons [78-81]. Nanoscale graphene fabrication approaches can be
classified to top-down, bottom-up, and combined methods [73]. Top-down methods involve
fabricating nanoscale graphene from a large sheet of graphene via patterning or chemical methods
[82]. Bottom-up approaches involve building nanoscale graphene using small building blocks in a
chemical reactor. Combined methods include patterning substrate and epitaxial growth over the
patterned substrate [74, 83].
Patterning is a key step for nanoscale graphene fabrication, and can be performed using a variety
of techniques such as electron beam lithography [82], chemical etching [84, 85], nanolithography via
scanning tunneling microscopy (STM) [86, 87], and focused ion beam (FIB) milling [88, 89]. Berger
et al [72] explored the effect of patterning epitaxial graphene on the electronic confinement (figure
2-10). Their work relied on patterning epitaxial grown graphene on single crystal SiC using
nanolithography methods. They showed evidence of quantum confinement in graphene nanoribbons
and its effect on altering graphene’s electrical properties [72].
Figure 2-10. Epitaxial graphene produced by Berger et al [72] (A) LEED pattern of the grown graphene, (B) AFM image indicating the terraces, (C) STM image of a monolayer epitaxial graphene (Vbias=-0.8 V, I=100 pA), (D) STM
19
image of the interface reconstruction beneath grown graphene after lithography, (E) SEM image of the patterned epitaxial graphene, (F) EFM of the patterned epitaxial graphene [72].
Han et al [82] transferred mechanically exfoliated graphene sheets onto a SiO2/Si substrate and
fabricated lithographically patterned graphene ribbons. They then analyzed their electronic properties
and showed that it is possible to create and engineer energy gaps in graphene nanoribbons by
controlling the width [82]. The graphene nanoribbons can be seen in figure 2-11. For further
information about this technique refer to [82].
Figure 2-11. Work done by Han et al [82], (a) AFM image of fabricated graphene nanoribbons contacted by metal electrodes, (b) SEM image of the fabricated device with parallel ribbons, (c) SEM image of the device fabricated with
different orientation of the ribbons, (d)-(f) conductance of the ribbons with respect to gate voltage [82].
Chemical methods have been used to fabricate nanoscale graphene [90]. Such methods require
graphitic precursors and suitable solutions to create graphene nanoribbons in desired dimensions [90].
These ribbons are commonly synthesized in a solution or in a powder (figure 2-12) [90, 91]. Using
20
CNTs as graphitic precursors and employing a chemical agent to unzip their walls is another
technique to fabricate nanoscale graphene. Tao et al [92] produced graphene nanoribbons by
unzipping CNTs and studied their electronic properties by scanning tunneling spectroscopy (STS).
This method allows production of a variety of graphene nanoribbons with different ribbons’ length,
width, and chirality [92].
Figure 2-12. Nanoscale graphene synthesis via chemical methods. (a) Yellow graphitic precursor and the final powder of produced graphene nanoribbons [90]. (b) A network of graphene ribbons dispersed in a solution imaged by SEM. (c)
60 nm-wide nanoribbon imaged via TEM [91].
In 2010, Cai et al [93] introduced a bottom-up procedure to fabricate graphene nanoribbons
with atomic precision. In this method, monomeric precursors such as DBBA react on hot catalytic
metal surfaces (~400 ˚C) and create graphene nanoribbons [93]. Improving their method in a
successive paper they presented a heterojunction made of graphene nanoribbons [94]. However, their
method has difficulties due to the variation of the bandgap resulting from the sensitivity of the gap
energy to the ribbons’ edge termination and dimension. In addition, a too narrow graphene nanoribbon
is not capable of transmitting large currents and the transfer process required for the ribbons to be
used in devices is very damaging [95].
Sprinkle et al [73] firstly suggested a procedure combining the top-down and bottom-up
approaches. They initially patterned some artificial trenches on bulk 4H/6H SiC substrates and later
they fabricated sidewall graphene nanoribbons via high temperature annealing [73]. Cunning et al
[83] followed the same approach by fabricating SiC micro-structures at the wafer-level by
21
photolithography and graphitization on this substrate through metal-catalyst-aided method (figure 2-
13).
Figure 2-13. (a) A schematic diagram demonstrating the process to fabricate graphene on SiC microbeams on Si via metal mediated growth. SEM images showing graphene covered 3C-SiC microstructures (b) bridges and (c) cantilevers
[83].
Coating and removal of a resist layer for electron beam lithography is challenging and adversely
affects graphene quality. Employing STM for nanostructuring is very time consuming and
incompatible with industrial procedures. On the other hand, FIB is a flexible and easy to use
instrument for patterning a variety of materials with nanoscale precision. Therefore, a number of
attempts have been made to employ FIB for nanoscale graphene fabrication [96-99]. The typical
process includes the transfer of CVD-grown or mechanically cleaved graphene onto a SiO2 substrate
followed by structuring [88, 97, 100-102]. Prével et al [96] investigated the effect of Ga+ exposure as
a result of FIB patterning on epitaxial graphene grown on bulk 6H-SiC (figure 2-14). Despite
observing some degree of unintended graphene amorphization, they suggested FIB as a promising
method for patterning graphene [96]. Furthermore, Zhang et al [99] explored the optimization of Ga+
22
ion beam conditions such as beam current, acceleration voltage, dwell time, and ion dose. They
demonstrated the possibility of graphene patterning in the 15 nm range using their optimum beam
settings [99]. Wang et al [103] studied the damage caused to FIB patterned multilayer graphene using
Raman spectroscopy. They observed defects appearing in graphene crystal as a result of Ga+ ion
exposure concluding that dwell time has a critical role in defect creation [103].
Figure 2-14. (a) AFM topography image showing defects created in graphene on SiC sample after being exposed to moderate Ga+ ion dose - 104 ions/dot, (b) phase image, (c) line profile data from (a), (d) AFM topography image of the sample after being exposed to large Ga+ ion dose - 106 ions/dot, (e) phase image, (f) line profile of defects shown in (d)
[96].
Some of the common ion-beam related damage caused to solids include contamination,
amorphization, and swelling. Such damage is dominantly caused by redeposition of etched atoms or
implantation of the ions into the solid [101, 103-106]. The discussion about the effectiveness of using
FIB milling for graphene nanostructuring is ongoing. Therefore, in this thesis, the use of FIB milling
on manipulating the SiC substrate dimension to grow graphene over FIB-patterned SiC
nanostructures is investigated.
2.4 FREE-STANDING EPITAXIAL GRAPHENE ON SIC
High temperature annealing of SiC converts its topmost layer to an insulating carbon-rich layer,
commonly referred to as the buffer layer, and graphene grows over this layer [30, 42, 57, 65, 68, 107].
23
About one-third of carbon atoms in the buffer layer are covalently bound to the SiC substrate [108-
110]. The buffer layer causes significant doping (~1×1013 cm-2) to graphene [31, 111-114], adversely
affecting graphene’s carrier mobility [109, 115]. Therefore, elimination of the buffer layer is
beneficial for epitaxial graphene electronic transport applications.
The elimination of the buffer layer can be performed through breaking the carbon bonds to
silicon within the topmost layer of SiC. A hydrogen intercalation process is suggested by Riedl et al
[116] to convert the buffer layer to a graphene layer. In this process the sample is annealed at ~ 600
˚C in a furnace with molecular hydrogen atmosphere. They demonstrated that hydrogen penetrates
between the buffer layer and the substrate breaking the backbonds between carbon and Si; it also
saturates the dangling Si bonds at topmost layer of SiC substrate. This makes the carbon atoms in the
buffer layer create a pure sp2 bond converting to a graphene layer [116]. Using this procedure they
converted a monolayer graphene with the buffer layer grown on a bulk SiC(0001) to a free-standing
bilayer graphene (figure 2-15) [116]. They investigated the process using LEED, XPS, LEEM and
ARPES. They reported that annealing the H-intercalated sample at 1000 ˚C results in hydrogen
desorption converting free-standing bilayer graphene to a monolayer graphene with the buffer layer
(figure 2-16). Their results indicate that after annealing to about 800 ˚C the buffer-layer-related
components (S1 and S2) strongly appear in the C 1s spectrum.
Figure 2-15. Schematic models of (a) buffer layer on SiC (b) epitaxial monolayer graphene; after hydrogen intercalation process they convert to (c) free-standing monolayer graphene (d) bilayer free-standing graphene, respectively [116].
24
Watcharinyanon et al [117] repeated the hydrogen intercalation process with atomic hydrogen
and demonstrated its effectiveness in eliminating the buffer layer. They also demonstrated that the
intercalation process on a monolayer graphene leads to the formation of (√3 × √3) bilayer graphene
islands which slowly expand and cover the whole surface (figure 2-17) [117]. Yu et al [118] reported
the significant success of hydrogen intercalation procedure in improving electrical transport
properties of graphene on 4H-SiC. They also demonstrated the lower quality SiC crystal results in
more graphene/substrate interactions [118]. The hydrogen intercalation process leads to improving
the performance of field-effect transistors fabricated from epitaxial graphene on SiC [119-121].
Figure 2-16. (a) C 1s and (b) Si 2p core level spectra showing a fully H-intercalated bilayer graphene sample (bottom spectra) and the hydrogen desorption process by annealing [116].
Sforzini et al [122] evaluated the interaction level between graphene and its substrate by
calculating and measuring adsorption height of hydrogen-intercalated quasi-free-standing monolayer
graphene. They concluded that hydrogen intercalation of graphene on SiC leads to the lowest
interaction between graphene and its substrate compared to a range of other substrates such as Ir, Ni,
Re and Co, making SiC an ideal substrate for graphene synthesis [122].
25
Figure 2-17. STM images (a) monolayer graphene, (b) after a small hydrogen exposure, (c) after complete intercalation process showing a complete coverage [117].
2.5 INELASTIC MEAN FREE PATH IN EPITAXIAL GRAPHENE
Inelastic mean free path (IMFP) is the average distance that an electron moves through a
material before undergoing any energy-loss scattering event [123]. Having an accurate understanding
of the IMFP for materials is crucial for a number of measurement methods including photoelectron
diffraction [124], photoelectron spectroscopy (PES) [125], and Auger electron spectroscopy [126].
A number of theoretical models have been reported to calculate the IMFP in various materials
[127-133]. Tanuma et al reported a series of work calculating IMFPs for a variety of elements in
energy range of 50-2000 eV [127-130, 134-137]. Their calculations are based on an algorithm
developed by Penn [138] using optical data collected from synchrotron radiation studies [139]. They
also introduced a universal equation called Tanuma, Powell, and Penn (TPP-2M) to estimate the
IMFP in different materials in the 50-2000 eV energy range (equations 4-8) [140]. The parameters in
TPP-2M model are calculated from material properties such as atomic weight, density, and number
of valence electrons per atom [134, 140].
𝜆𝜆 = 𝐸𝐸/(𝐸𝐸𝑝𝑝2(𝛽𝛽 𝑙𝑙𝑛𝑛(𝛾𝛾𝐸𝐸) − (𝐶𝐶 𝐸𝐸) + ⁄ (𝐷𝐷 𝐸𝐸2)))⁄ 𝑐𝑐, where (4)
𝛽𝛽 = −0.1 + 0.944𝐸𝐸𝑝𝑝2 + 𝐸𝐸𝑔𝑔2−1 2⁄
+ 0.069𝜌𝜌0.1, (5)
𝐶𝐶 = 1.97 − 0.91 𝑈𝑈, (6)
𝐷𝐷 = 53.4 − 20.8 𝑈𝑈, (7)
26
and 𝑈𝑈 = 𝑁𝑁𝑣𝑣 𝜌𝜌 𝑀𝑀⁄ , (8)
where λ is the IMFP, E is the electron energy, Ep is the free-electron plasmon energy, Eg is the band-
gap energy, ρ is the density, Nv is the number of valence electrons per atom and M is the atomic or
molecular weight. Figure 2-18 compares the TPP-2M formula estimation for graphite with the
calculated values based on the optical data and the experimental data measured using elastic peak
electron spectroscopy [141, 142]. From the figure it can be seen that the TPP-2M model matches the
experimental values whereas the calculations based on optical data show a significant variation.
Figure 2-18. IMFP results for graphite comparing optical based calculation results, TPP-2M equation, experimental data using elastic peak electron spectroscopy by Lesiak et al [141] and Tanuma et al [130, 142].
Experimental measurement of IMFPs can be conducted using low-energy electron diffraction
(LEED) [143], X-ray absorption fine structure (XAFS) spectroscopy [144], and photoelectron
spectroscopy (PES) [145]. IMFP estimation is commonly performed using overlayer-film method by
depositing or growing a thin layer of material of interest on a substrate, and then comparing the
substrate peak intensity change as a function of thickness of the deposited layer [146]. Having a
precise estimate of the thickness profoundly affects the accuracy of this method. The deposited or
grown film homogeneity and any possible contamination can significantly affect the measurements.
27
Recently, graphene [3, 6, 36, 72, 147] has been studied by a variety of electron-collection-based
instrumental measurements such as PES, LEED, Auger electron spectroscopy and electron
microscopy [30, 72, 148-150]. Precise understanding about IMFP is vital for accurate data
interpretation and analysis in such techniques. However, the significant challenge associated with
IMFP estimation of carbon polymorphs has consistently been reported [127-130, 134]. Tanuma et al
found that their experimental data was about 50% higher than their theoretical estimations (figure 2-
18) [142]. Xu et al [148] studied the IMFP in exfoliated graphene transferred to SiO2/Si substrate
using Auger electron spectroscopy in the 50-500 eV energy range, and used Raman spectroscopy to
estimated graphene film thickness. They found no significant difference between their experimental
values and the theoretical predictions for graphite (figure 2-19).
Figure 2-19. (a) IMFP values for graphene samples derived from AES measurements and compared to the predictions by TPP-2 formula. (b) An average value of IMFP from all the graphene samples with different thicknesses [148].
In case of graphene, the IMFP characterization becomes even more challenging due to the
dominance of interface effects and anisotropies [129]. Yet, not much is known about how IMFPs
change in moving from bulk to 2D materials [148] which is an emerging challenge for 2D materials.
Much more research is needed to have a clear understanding about IMFP in graphene.
2.6 LITERATURE REVIEW SUMMARY
Graphene patterning is vital for future applications. FIB milling is suggested as a flexible and
easy procedure for nanoscale patterning of graphene which may cause unintentional damage to the
28
surface through Ga implantation. Patterning is commonly performed after growing graphene and
transferring it to another substrate. Some important questions can therefore be: what happens if one
fabricates the nanostructures first and grow graphene later? Also, how effective FIB is for this
procedure? Is it possible to grow graphene over FIB patterned SiC substrate? All of these questions
are explored and discussed in chapter 4.
Epitaxial graphene on 3C-SiC/Si thin film holds potential for commercial applications of
graphene in electronics. However, the presence of a buffer layer at the interface of SiC and graphene
adversely affects graphene’s properties. Therefore, hydrogen intercalation is suggested to eliminate
this layer on bulk SiC. The question arises whether this procedure works similarly on 3C-SiC/Si thin
film compared to bulk SiC? How effective it is for eliminating the buffer layer on thin films? Is the
buffer layer recoverable by high temperature annealing? Is the free-standing graphene fabricated
using this procedure stable in air? All of these questions are discussed and addressed in chapter 5.
Epitaxial graphene on SiC is exposed to a number of electron-based measurements such as PES,
LEED, Auger electron spectroscopy and electron microscopy. A precise data analysis and
interpretation of these measurements critically depends on having an accurate understanding of
electron IMFP in graphene. There are a number of work available for measuring and estimating the
IMFP for graphite, but very little is known about the effects of interface and anisotropies when
moving from a bulk graphite to a 2D graphene. How much different is IMFP in graphite compared to
2D graphene? Are the theoretical models able to provide a reasonable prediction of IMFP in
graphene? How is the IMFP different in the buffer layer compared to a free-standing graphene layer?
All of these questions are discussed in chapter 6.
2.7 REFERENCES
1. G.M. Rutter, Atomic scale properties of epitaxial graphene grown on silicon carbide (0001). 2008: Doctoral Dissertation, Georgia Institute of Technology.
2. H. Tetlow, J.P. de Boer, I. Ford, D. Vvedensky, J. Coraux, and L. Kantorovich, Growth of epitaxial graphene: theory and experiment. Physics Reports, 2014. 542(3): p. 195-295.
29
3. A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, and A.K. Geim, The electronic properties of graphene. Reviews of Modern Physics, 2009. 81(1): p. 109-162.
4. C. Soldano, A. Mahmood, and E. Dujardin, Production, properties and potential of graphene. Carbon, 2010. 48(8): p. 2127-2150.
5. C. Lee, X. Wei, J.W. Kysar, and J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science, 2008. 321(5887): p. 385-388.
6. A.K. Geim and K.S. Novoselov, The rise of graphene. Nature Materials, 2007. 6(3): p. 183-191.
7. K.S. Novoselov, V.I. Falko, L. Colombo, P.R. Gellert, M.G. Schwab, and K. Kim, A roadmap for graphene. Nature, 2012. 490(7419): p. 192-200.
8. V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, and S. Seal, Graphene based materials: past, present and future. Progress in Materials Science, 2011. 56(8): p. 1178-1271.
9. M. Terrones, A.R. Botello-Méndez, J. Campos-Delgado, F. López-Urías, Y.I. Vega-Cantú, F.J. Rodríguez-Macías, A.L. Elías, E. Muñoz-Sandoval, A.G. Cano-Márquez, J.-C. Charlier, and H. Terrones, Graphene and graphite nanoribbons: morphology, properties, synthesis, defects and applications. Nano Today, 2010. 5(4): p. 351-372.
10. F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, and V. Pellegrini, Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science, 2015. 347(6217): p. 1246501.
11. J.A. Venables, G.D.T. Spiller, and M. Hanbucken, Nucleation and growth of thin films. Reports on Progress in Physics, 1984. 47(4): p. 399.
12. P.R. Wallace, The band theory of graphite. Physical Review, 1947. 71(9): p. 622. 13. J.R. Hass, Structural characterization of epitaxial graphene on silicon carbide. 2008: Georgia
Institute of Technology. 14. H. Boehm, A. Clauss, G. Fischer, and U. Hofmann. Surface properties of extremely thin
graphite lamellae. in Proceedings of the fifth conference on carbon. 1962. Pergamon Press New York.
15. H. Boehm, A. Clauss, G. Fischer, and U. Hofmann, Dünnste kohlenstoff-folien. Zeitschrift Für Naturforschung B, 1962. 17(3): p. 150-153.
16. N. Gall, E. Rut'Kov, and A.Y. Tontegode, Influence of surface carbon on the formation of silicon-refractory metal interfaces. Thin Solid Films, 1995. 266(2): p. 229-233.
17. N. Gall, E. Rut'Kov, and A.Y. Tontegode, Two dimensional graphite films on metals and their intercalation. International Journal of Modern Physics B, 1997. 11(16): p. 1865-1911.
18. K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, and A.K. Geim, Two-dimensional atomic crystals. Proceedings of the National Academy of Sciences of the United States of America, 2005. 102(30): p. 10451.
19. S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, and R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon, 2007. 45(7): p. 1558-1565.
20. D.V. Kosynkin, A.L. Higginbotham, A. Sinitskii, J.R. Lomeda, A. Dimiev, B.K. Price, and J.M. Tour, Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature, 2009. 458: p. 872.
30
21. Y. Yao, Z. Li, Z. Lin, K.-S. Moon, J. Agar, and C. Wong, Controlled growth of multilayer, few-layer, and single-layer graphene on metal substrates. The Journal of Physical Chemistry C, 2011. 115(13): p. 5232-5238.
22. A. Van Bommel, J. Crombeen, and A. Van Tooren, LEED and Auger electron observations of the SiC (0001) surface. Surface Science, 1975. 48(2): p. 463-472.
23. C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A.N. Marchenkov, E.H. Conrad, and P.N. First, Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics. The Journal of Physical Chemistry B, 2004. 108(52): p. 19912-19916.
24. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov, Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.
25. W.S. Hummers and R.E. Offeman, Preparation of Graphitic Oxide. Journal of the American Chemical Society, 1958. 80(6): p. 1339-1339.
26. J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, and P.E. Sheehan, Reduced graphene oxide molecular sensors. Nano Letters, 2008. 8(10): p. 3137-3140.
27. D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R.D. Piner, S. Stankovich, I. Jung, D.A. Field, C.A. Ventrice, and R.S. Ruoff, Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon, 2009. 47(1): p. 145-152.
28. I. Jung, D. Dikin, S. Park, W. Cai, S.L. Mielke, and R.S. Ruoff, Effect of water vapor on electrical properties of individual reduced graphene oxide sheets. The Journal of Physical Chemistry C, 2008. 112(51): p. 20264-20268.
29. H. Santos, L. Chico, and L. Brey, Carbon nanoelectronics: unzipping tubes into graphene ribbons. Physical Review Letters, 2009. 103(8): p. 086801.
30. B. Gupta, M. Notarianni, N. Mishra, M. Shafiei, F. Iacopi, and N. Motta, Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
31. P. Mallet, F. Varchon, C. Naud, L. Magaud, C. Berger, and J.-Y. Veuillen, Electron states of mono-and bilayer graphene on SiC probed by scanning-tunneling microscopy. Physical Review B, 2007. 76(4): p. 041403.
32. K.V. Emtsev, F. Speck, T. Seyller, L. Ley, and J.D. Riley, Interaction, growth, and ordering of epitaxial graphene on SiC (0001) surfaces: A comparative photoelectron spectroscopy study. Physical Review B, 2008. 77(15): p. 155303.
33. A. Ouerghi, M.G. Silly, M. Marangolo, C. Mathieu, M. Eddrief, M. Picher, F. Sirotti, S. El Moussaoui, and R. Belkhou, Large-area and high-quality epitaxial graphene on off-axis SiC wafers. ACS Nano, 2012. 6(7): p. 6075-6082.
34. C. Riedl, U. Starke, J. Bernhardt, M. Franke, and K. Heinz, Structural properties of the graphene-SiC (0001) interface as a key for the preparation of homogeneous large-terrace graphene surfaces. Physical Review B, 2007. 76(24): p. 245406.
35. B. Gupta, Placidi, E., Hogan, C., Mishra, N., Iacopi, F., & Motta, N., The transition from 3C SiC (111) to graphene captured by ultra high vacuum scanning tunneling microscopy. Carbon, 2015. 91: p. 378-385.
31
36. V.Y. Aristov, G. Urbanik, K. Kummer, D.V. Vyalikh, O.V. Molodtsova, A.B. Preobrajenski, A.A. Zakharov, C. Hess, T. Hänke, and B. Buchner, Graphene synthesis on cubic SiC/Si wafers. Perspectives for mass production of graphene-based electronic devices. Nano Letters, 2010. 10(3): p. 992-995.
37. N.W. Jepps and T.F. Page, Polytypic transformations in silicon carbide. Progress in Crystal Growth and Characterization, 1983. 7(1): p. 259-307.
38. J. Schardt, J. Bernhardt, U. Starke, and K. Heinz, Atomic structure of hexagonal 6H– and 3C–SiC surfaces. Surface Review and Letters, 1998. 05(01): p. 181-186.
39. K.V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G.L. Kellogg, L. Ley, J.L. McChesney, T. Ohta, S.A. Reshanov, and J. Röhrl, Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials, 2009. 8(3): p. 203-207.
40. R. Tromp and J. Hannon, Thermodynamics and kinetics of graphene growth on SiC (0001). Physical review letters, 2009. 102(10): p. 106104.
41. W.A. De Heer, C. Berger, M. Ruan, M. Sprinkle, X. Li, Y. Hu, B. Zhang, J. Hankinson, and E. Conrad, Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide. Proceedings of the National Academy of Sciences, 2011. 108(41): p. 16900-16905.
42. F. Varchon, R. Feng, J. Hass, X. Li, B.N. Nguyen, C. Naud, P. Mallet, J.-Y. Veuillen, C. Berger, and E.H. Conrad, Electronic structure of epitaxial graphene layers on SiC: effect of the substrate. Physical Review Letters, 2007. 99(12): p. 126805.
43. N. Blanc, F. Jean, A.V. Krasheninnikov, G. Renaud, and J. Coraux, Strains induced by point defects in graphene on a metal. Physical Review Letters, 2013. 111(8): p. 085501.
44. I. Forbeaux, J.-M. Themlin, A. Charrier, F. Thibaudau, and J.-M. Debever, Solid-state graphitization mechanisms of silicon carbide 6H–SiC polar faces. Applied Surface Science, 2000. 162: p. 406-412.
45. A. Severino, C. Bongiorno, N. Piluso, M. Italia, M. Camarda, M. Mauceri, G. Condorelli, M. Di Stefano, B. Cafra, and A. La Magna, High-quality 6inch (111) 3C-SiC films grown on off-axis (111) Si substrates. Thin Solid Films, 2010. 518(6): p. S165-S169.
46. F. Hirokazu, M. Yu, H. Hiroyuki, S. Eiji, and S. Maki, Epitaxial growth processes of graphene on silicon substrates. Japanese Journal of Applied Physics, 2010. 49(1S): p. 01AH03.
47. F. Iacopi, G. Walker, L. Wang, L. Malesys, S. Ma, B.V. Cunning, and A. Iacopi, Orientation-dependent stress relaxation in hetero-epitaxial 3C-SiC films. Applied Physics Letters, 2013. 102(1): p. 011908.
48. L. Wang, S. Dimitrijev, J. Han, A. Iacopi, L. Hold, P. Tanner, and H.B. Harrison, Growth of 3C–SiC on 150-mm Si (100) substrates by alternating supply epitaxy at 1000 C. Thin Solid Films, 2011. 519(19): p. 6443-6446.
49. C. Wang, N. Huang, H. Zhuang, Z. Zhai, B. Yang, L. Liu, and X. Jiang, Growth of large-scale heteroepitaxial 3C-SiC films and nanosheets on silicon substrates by microwave plasma enhanced CVD at higher powers. Surface and Coatings Technology, 2016. 299: p. 96-103.
50. A. Ouerghi, M. Marangolo, R. Belkhou, S. El Moussaoui, M. Silly, M. Eddrief, L. Largeau, M. Portail, B. Fain, and F. Sirotti, Epitaxial graphene on 3C-SiC (111) pseudosubstrate: Structural and electronic properties. Physical Review B, 2010. 82(12): p. 125445.
32
51. A. Ouerghi, M. Ridene, A. Balan, R. Belkhou, A. Barbier, N. Gogneau, M. Portail, A. Michon, S. Latil, and P. Jegou, Sharp interface in epitaxial graphene layers on 3 C-SiC (100)/Si (100) wafers. Physical Review B, 2011. 83(20): p. 205429.
52. R. Takahashi, H. Handa, S. Abe, K. Imaizumi, H. Fukidome, A. Yoshigoe, Y. Teraoka, and M. Suemitsu, Low-energy-electron-diffraction and X-ray-phototelectron-spectroscopy studies of graphitization of 3C-SiC (111) thin film on Si (111) substrate. Japanese Journal of Applied Physics, 2011. 50(7R): p. 070103.
53. M. Suemitsu and H. Fukidome, Epitaxial graphene on silicon substrates. Journal of Physics D: Applied Physics, 2010. 43(37): p. 374012.
54. A.N. Chaika, O.V. Molodtsova, A.A. Zakharov, D. Marchenko, J. Sánchez-Barriga, A. Varykhalov, I.V. Shvets, and V.Y. Aristov, Continuous wafer-scale graphene on cubic-SiC(001). Nano Research, 2013. 6(8): p. 562-570.
55. G.R. Yazdi, R. Vasiliauskas, T. Iakimov, A. Zakharov, M. Syväjärvi, and R. Yakimova, Growth of large area monolayer graphene on 3C-SiC and a comparison with other SiC polytypes. Carbon, 2013. 57: p. 477-484.
56. R. Yakimova, C. Virojanadara, D. Gogova, M. Syväjärvi, D. Siche, K. Larsson, and L.I. Johansson, Analysis of the Formation Conditions for Large Area Epitaxial Graphene on SiC Substrates. Materials Science Forum, 2010. 645: p. 565-568.
57. B. Gupta, E. Placidi, C. Hogan, N. Mishra, F. Iacopi, and N. Motta, The transition from 3C SiC (111) to graphene captured by ultra high vacuum scanning tunneling microscopy. Carbon, 2015. 91: p. 378-385.
58. M. Zielinski, M. Portail, S. Roy, T. Chassagne, C. Moisson, S. Kret, and Y. Cordier, Elaboration of (111) oriented 3C–SiC/Si layers for template application in nitride epitaxy. Materials Science and Engineering: B, 2009. 165(1): p. 9-14.
59. S. Nishino, J.A. Powell, and H.A. Will, Production of large‐area single‐crystal wafers of cubic SiC for semiconductor devices. Applied Physics Letters, 1983. 42(5): p. 460-462.
60. S. Nishino, H. Suhara, H. Ono, and H. Matsunami, Epitaxial growth and electric characteristics of cubic SiC on silicon. Journal of Applied Physics, 1987. 61(10): p. 4889-4893.
61. M. Zielinski, A. Leycuras, S. Ndiaye, and T. Chassagne, Stress relaxation during the growth of 3C-SiC∕Si thin films. Applied Physics Letters, 2006. 89(13): p. 131906.
62. A. Ouerghi, R. Belkhou, M. Marangolo, M. Silly, S. El Moussaoui, M. Eddrief, L. Largeau, M. Portail, and F. Sirotti, Structural coherency of epitaxial graphene on 3C–SiC (111) epilayers on Si (111). Applied Physics Letters, 2010. 97(16): p. 161905.
63. A. Ouerghi, A. Kahouli, D. Lucot, M. Portail, L. Travers, J. Gierak, J. Penuelas, P. Jegou, A. Shukla, and T. Chassagne, Epitaxial graphene on cubic SiC (111)/Si (111) substrate. Applied Physics Letters, 2010. 96(19): p. 191910.
64. N. Gogneau, A. Balan, M. Ridene, A. Shukla, and A. Ouerghi, Control of the degree of surface graphitization on 3C-SiC (100)/Si (100). Surface Science, 2012. 606(3): p. 217-220.
65. F. Zarotti, B. Gupta, F. Iacopi, A. Sgarlata, M. Tomellini, and N. Motta, Time evolution of graphene growth on SiC as a function of annealing temperature. Carbon, 2016. 98: p. 307-312.
33
66. F. Iacopi, N. Mishra, B.V. Cunning, D. Goding, S. Dimitrijev, R. Brock, R.H. Dauskardt, B. Wood, and J. Boeckl, A catalytic alloy approach for graphene on epitaxial SiC on silicon wafers. Journal of Materials Research, 2015. 30(05): p. 609-616.
67. Z.-Y. Juang, C.-Y. Wu, C.-W. Lo, W.-Y. Chen, C.-F. Huang, J.-C. Hwang, F.-R. Chen, K.-C. Leou, and C.-H. Tsai, Synthesis of graphene on silicon carbide substrates at low temperature. Carbon, 2009. 47(8): p. 2026-2031.
68. N. Mishra, J. Boeckl, N. Motta, and F. Iacopi, Graphene growth on silicon carbide: a review. Physica Status Solidi (a), 2016. 213(9): p. 2277-2289.
69. B. Gupta, I. Di Bernardo, P. Mondelli, A. Della Pia, M. Betti, F. Iacopi, C. Mariani, and N. Motta, Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
70. N. Mishra, L. Hold, A. Iacopi, B. Gupta, N. Motta, and F. Iacopi, Controlling the surface roughness of epitaxial SiC on silicon. Journal of Applied Physics, 2014. 115(20): p. 203501.
71. Y.M. Lin, C. Dimitrakopoulos, K.A. Jenkins, D.B. Farmer, H.Y. Chiu, A. Grill, and P. Avouris, 100-GHz transistors from wafer-scale epitaxial graphene. Science, 2010. 327(5966): p. 662-662.
72. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, and W.A. De Heer, Electronic confinement and coherence in patterned epitaxial graphene. Science, 2006. 312(5777): p. 1191-1196.
73. M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu, C. Berger, and W.A. De Heer, Scalable templated growth of graphene nanoribbons on SiC. Nature Nanotechnology, 2010. 5(10): p. 727-731.
74. A. Celis, M. Nair, A. Taleb-Ibrahimi, E. Conrad, C. Berger, W. De Heer, and A. Tejeda, Graphene nanoribbons: fabrication, properties and devices. Journal of Physics D: Applied Physics, 2016. 49(14): p. 143001.
75. Z. Li and F. Chen, Ion beam modification of two-dimensional materials: Characterization, properties, and applications. Applied Physics Reviews, 2017. 4(1): p. 011103.
76. K. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M. Katsnelson, I. Grigorieva, S. Dubonos, and A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene. Nature, 2005. 438(7065): p. 197-200.
77. I. Meric, M.Y. Han, A.F. Young, B. Ozyilmaz, P. Kim, and K.L. Shepard, Current saturation in zero-bandgap, top-gated graphene field-effect transistors. Nature Nanotechnology, 2008. 3(11): p. 654-659.
78. I. Palacio, A. Celis, M.N. Nair, A. Gloter, A. Zobelli, M. Sicot, D. Malterre, M.S. Nevius, W.A. De Heer, and C. Berger, Atomic structure of epitaxial graphene sidewall nanoribbons: flat graphene, miniribbons, and the confinement gap. Nano Letters, 2014. 15(1): p. 182-189.
79. Y.-W. Son, M.L. Cohen, and S.G. Louie, Energy gaps in graphene nanoribbons. Physical Review Letters, 2006. 97(21): p. 216803.
80. K. Wakabayashi and M. Sigrist, Zero-conductance resonances due to flux states in nanographite ribbon junctions. Physical Review Letters, 2000. 84(15): p. 3390.
81. K. Wakabayashi, Y. Takane, M. Yamamoto, and M. Sigrist, Edge effect on electronic transport properties of graphene nanoribbons and presence of perfectly conducting channel. Carbon, 2009. 47(1): p. 124-137.
34
82. M.Y. Han, B. Özyilmaz, Y. Zhang, and P. Kim, Energy band-gap engineering of graphene nanoribbons. Physical Review Letters, 2007. 98(20): p. 206805.
83. B.V. Cunning, M. Ahmed, N. Mishra, A.R. Kermany, B. Wood, and F. Iacopi, Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers. Nanotechnology, 2014. 25(32): p. 325301.
84. L.C. Campos, V.R. Manfrinato, J.D. Sanchez-Yamagishi, J. Kong, and P. Jarillo-Herrero, Anisotropic etching and nanoribbon formation in single-layer graphene. Nano Letters, 2009. 9(7): p. 2600-2604.
85. S.S. Datta, D.R. Strachan, S.M. Khamis, and A.C. Johnson, Crystallographic etching of few-layer graphene. Nano Letters, 2008. 8(7): p. 1912-1915.
86. P. Nemes-Incze, L. Tapasztó, G.Z. Magda, Z. Osváth, G. Dobrik, X. Jin, C. Hwang, and L. Biró, Graphene nanoribbons with zigzag and armchair edges prepared by scanning tunneling microscope lithography on gold substrates. Applied Surface Science, 2014. 291: p. 48-52.
87. L. Tapasztó, G. Dobrik, P. Lambin, and L.P. Biró, Tailoring the atomic structure of graphene nanoribbons by scanning tunnelling microscope lithography. Nature Nanotechnology, 2008. 3(7): p. 397-401.
88. M.C. Lemme, D.C. Bell, J.R. Williams, L.A. Stern, B.W. Baugher, P. Jarillo-Herrero, and C.M. Marcus, Etching of graphene devices with a helium ion beam. ACS Nano, 2009. 3(9): p. 2674-2676.
89. B.S. Archanjo, B. Fragneaud, L. Gustavo Cançado, D. Winston, F. Miao, C. Alberto Achete, and G. Medeiros-Ribeiro, Graphene nanoribbon superlattices fabricated via He ion lithography. Applied Physics Letters, 2014. 104(19): p. 193114.
90. T.H. Vo, M. Shekhirev, D.A. Kunkel, M.D. Morton, E. Berglund, L. Kong, P.M. Wilson, P.A. Dowben, A. Enders, and A. Sinitskii, Large-scale solution synthesis of narrow graphene nanoribbons. Nature Communications, 2014. 5: p. 3189.
91. X. Jia, M. Hofmann, V. Meunier, B.G. Sumpter, J. Campos-Delgado, J.M. Romo-Herrera, H. Son, Y.-P. Hsieh, A. Reina, J. Kong, M. Terrones, and M.S. Dresselhaus, Controlled formation of sharp zigzag and armchair edges in graphitic nanoribbons. Science, 2009. 323(5922): p. 1701.
92. C. Tao, L. Jiao, O.V. Yazyev, Y.C. Chen, J. Feng, X. Zhang, R.B. Capaz, J.M. Tour, A. Zettl, and S.G. Louie, Spatially resolving edge states of chiral graphene nanoribbons. Nature Physics, 2011. 7(8): p. 616-620.
93. J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A.P. Seitsonen, M. Saleh, and X. Feng, Atomically precise bottom-up fabrication of graphene nanoribbons. Nature, 2010. 466(7305): p. 470-473.
94. J. Cai, C.A. Pignedoli, L. Talirz, P. Ruffieux, H. Söde, L. Liang, V. Meunier, R. Berger, R. Li, X. Feng, K. Müllen, and R. Fasel, Graphene nanoribbon heterojunctions. Nature Nanotechnology, 2014. 9(11): p. 896-900.
95. M. Dvorak, W. Oswald, and Z. Wu, Bandgap opening by patterning graphene. Scientific Reports, 2013. 3: p. 1-7.
96. B. Prével, J.-M. Benoit, L. Bardotti, P. Mélinon, A. Ouerghi, D. Lucot, E. Bourhis, and J. Gierak, Nanostructuring graphene on SiC by focused ion beam: effect of the ion fluence. Applied Physics Letters, 2011. 99(8): p. 083116.
35
97. B. Archanjo, A. Barboza, B. Neves, L. Malard, E. Ferreira, J. Brant, E. Alves, F. Plentz, V. Carozo, and B. Fragneaud, The use of a Ga+ focused ion beam to modify graphene for device applications. Nanotechnology, 2012. 23(25): p. 255305.
98. N. Kalhor, S.A. Boden, and H. Mizuta, Sub-10nm patterning by focused He-ion beam milling for fabrication of downscaled graphene nano devices. Microelectronic Engineering, 2014. 114: p. 70-77.
99. Y. Zhang, C. Hui, R. Sun, K. Li, K. He, X. Ma, and F. Liu, A large-area 15 nm graphene nanoribbon array patterned by a focused ion beam. Nanotechnology, 2014. 25(13): p. 135301.
100. V. Iberi, A.V. Ievlev, I. Vlassiouk, S. Jesse, S.V. Kalinin, D.C. Joy, A.J. Rondinone, A. Belianinov, and O.S. Ovchinnikova, Graphene engineering by neon ion beams. Nanotechnology, 2016. 27(12): p. 125302.
101. D.C. Bell, M.C. Lemme, L.A. Stern, J.R. Williams, and C.M. Marcus, Precision cutting and patterning of graphene with helium ions. Nanotechnology, 2009. 20(45): p. 455301.
102. X. Wu, H. Zhao, D. Yan, and J. Pei, Investigation on gallium ions impacting monolayer graphene. AIP Advances, 2015. 5(6): p. 067171.
103. Q. Wang, Y. Shao, D. Ge, Q. Yang, and N. Ren, Surface modification of multilayer graphene using Ga ion irradiation. Journal of Applied Physics, 2015. 117(16): p. 165303.
104. D. Fox, Y. Zhou, A. O’Neill, S. Kumar, J.J. Wang, J. Coleman, G. Duesberg, J. Donegan, and H. Zhang, Helium ion microscopy of graphene: beam damage, image quality and edge contrast. Nanotechnology, 2013. 24(33): p. 335702.
105. Y. Xu, K. Zhang, C. Brüsewitz, X. Wu, and H.C. Hofsäss, Investigation of the effect of low energy ion beam irradiation on mono-layer graphene. AIP Advances, 2013. 3(7): p. 072120.
106. Z. Liao, T. Zhang, M. Gall, A. Dianat, R. Rosenkranz, R. Jordan, G. Cuniberti, and E. Zschech, Lateral damage in graphene carved by high energy focused gallium ion beams. Applied Physics Letters, 2015. 107(1): p. 013108.
107. S. Goler, C. Coletti, V. Piazza, P. Pingue, F. Colangelo, V. Pellegrini, K.V. Emtsev, S. Forti, U. Starke, F. Beltram, and S. Heun, Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene. Carbon, 2013. 51: p. 249-254.
108. U. Starke and C. Riedl, Epitaxial graphene on SiC (0001) and: from surface reconstructions to carbon electronics. Journal of Physics: Condensed Matter, 2009. 21(13): p. 134016.
109. A. Mattausch and O. Pankratov, Ab initio study of graphene on SiC. Physical Review Letters, 2007. 99(7): p. 076802.
110. K.V. Emtsev, F. Speck, T. Seyller, L. Ley, and J.D. Riley, Interaction, growth, and ordering of epitaxial graphene on SiC0001 surfaces: A comparative photoelectron spectroscopy study. Physical Review B, 2008. 77(15): p. 155303.
111. T. Ohta, A. Bostwick, T. Seyller, K. Horn, and E. Rotenberg, Controlling the electronic structure of bilayer graphene. Science, 2006. 313(5789): p. 951-954.
112. C. Riedl, A. Zakharov, and U. Starke, Precise in situ thickness analysis of epitaxial graphene layers on SiC (0001) using low-energy electron diffraction and angle resolved ultraviolet photoelectron spectroscopy. Applied Physics Letters, 2008. 93(3): p. 033106.
36
113. K.V. Emtsev, A.A. Zakharov, C. Coletti, S. Forti, and U. Starke, Ambipolar doping in quasifree epitaxial graphene on SiC (0001) controlled by Ge intercalation. Physical Review B, 2011. 84(12): p. 125423.
114. U. Starke, S. Forti, K. Emtsev, and C. Coletti, Engineering the electronic structure of epitaxial graphene by transfer doping and atomic intercalation. Mrs Bulletin, 2012. 37(12): p. 1177-1186.
115. K. Emtsev, F. Speck, T. Seyller, L. Ley, and J.D. Riley, Interaction, growth, and ordering of epitaxial graphene on SiC 0001 surfaces: A comparative photoelectron spectroscopy study. Physical Review B, 2008. 77(15): p. 155303.
116. C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, and U. Starke, Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation. Physical Review Letters, 2009. 103(24): p. 246804.
117. S. Watcharinyanon, C. Virojanadara, J. Osiecki, A. Zakharov, R. Yakimova, R. Uhrberg, and L.I. Johansson, Hydrogen intercalation of graphene grown on 6H-SiC (0001). Surface Science, 2011. 605(17): p. 1662-1668.
118. C. Yu, Q. Liu, J. Li, W. Lu, Z. He, S. Cai, and Z. Feng, Preparation and electrical transport properties of quasi free standing bilayer graphene on SiC (0001) substrate by H intercalation. Applied Physics Letters, 2014. 105(18): p. 183105.
119. J.A. Robinson, M. Hollander, M. LaBella, K.A. Trumbull, R. Cavalero, and D.W. Snyder, Epitaxial graphene transistors: enhancing performance via hydrogen intercalation. Nano Letters, 2011. 11(9): p. 3875-3880.
120. M. Winters, O. Habibpour, I.G. Ivanov, J. Hassan, E. Janzén, H. Zirath, and N. Rorsman, Assessment of H-intercalated graphene for microwave FETs through material characterization and electron transport studies. Carbon, 2015. 81: p. 96-104.
121. C. Yu, Z. He, J. Li, X. Song, Q. Liu, S. Cai, and Z. Feng, Quasi-free-standing bilayer epitaxial graphene field-effect transistors on 4H-SiC (0001) substrates. Applied Physics Letters, 2016. 108(1): p. 013102.
122. J. Sforzini, L. Nemec, T. Denig, B. Stadtmüller, T.L. Lee, C. Kumpf, S. Soubatch, U. Starke, P. Rinke, V. Blum, F.C. Bocquet, and F.S. Tautz, Approaching truly freestanding graphene: the structure of hydrogen-intercalated graphene on 6H-SiC(0001). Physical Review Letters, 2015. 114(10): p. 106804.
123. C.J. Powell and A. Jablonski, Surface sensitivity of X-ray photoelectron spectroscopy. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009. 601(1): p. 54-65.
124. F. Hillebrecht, H. Rose, T. Kinoshita, Y. Idzerda, G. van der Laan, R. Denecke, and L. Ley, Photoelectron diffraction in magnetic linear dichroism. Physical Review Letters, 1995. 75(15): p. 2883.
125. P. Cumpson, Elastic scattering corrections in AES and XPS. III. Behaviour of electron transport mean free path in solids for kinetic energies in the range 100 eV< E< 400 eV. Surface and Interface Analysis, 1997. 25(6): p. 447-453.
126. W.S.M. Werner, W. Smekal, H. Störi, H. Winter, G. Stefani, A. Ruocco, F. Offi, R. Gotter, A. Morgante, and F. Tommasini, Emission-depth-selective Auger photoelectron coincidence spectroscopy. Physical Review Letters, 2005. 94(3): p. 038302.
37
127. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths for 31 materials. Surface and Interface Analysis, 1988. 11(11): p. 577-589.
128. H. Shinotsuka, S. Tanuma, C. Powell, and D. Penn, Calculations of electron inelastic mean free paths. X. Data for 41 elemental solids over the 50 eV to 200 keV range with the relativistic full Penn algorithm. Surface and Interface Analysis, 2015. 47(9): p. 871-888.
129. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. IX. Data for 41 elemental solids over the 50 eV to 30 keV range. Surface and Interface Analysis, 2011. 43(3): p. 689-713.
130. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. Surface and Interface Analysis, 2005. 37(1): p. 1-14.
131. H.T. Nguyen-Truong, Low-energy electron inelastic mean free path in materials. Applied Physics Letters, 2016. 108(17): p. 172901.
132. H.T. Nguyen-Truong, Analytical formula for high-energy electron inelastic mean free path. The Journal of Physical Chemistry C, 2015. 119(41): p. 23627-23631.
133. W.S.M. Werner, K. Glantschnig, and C. Ambrosch-Draxl, Optical constants and inelastic electron-scattering data for 17 elemental metals. Journal of Physical and Chemical Reference Data, 2009. 38(4): p. 1013-1092.
134. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. Surface and Interface Analysis, 1994. 21(3): p. 165-176.
135. H. Bethe, Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie. Annalen der Physik, 1930. 397(3): p. 325-400.
136. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. III. Data for 15 inorganic compounds over the 50–2000 eV range. Surface and Interface Analysis, 1991. 17(13): p. 927-939.
137. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electorn inelastic mean free paths. II. Data for 27 elements over the 50–2000 eV range. Surface and Interface Analysis, 1991. 17(13): p. 911-926.
138. D.R. Penn, Electron mean-free-path calculations using a model dielectric function. Physical Review B, 1987. 35(2): p. 482-486.
139. H.J. Hagemann, W. Gudat, and C. Kunz, Optical constants from the far infrared to the x-ray region: Mg, Al, Cu, Ag, Au, Bi, C, and Al2O3. Journal of the Optical Society of America, 1975. 65(6): p. 742-744.
140. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths (IMFPs) VI. analysis of the gries inelastic scattering model and predictive IMFP equation. Surface and Interface Analysis, 1997. 25(1): p. 25-35.
141. B. Lesiak, A. Jablonski, Z. Prussak, and P. Mrozek, Experimental determination of the inelastic mean free path of electrons in solids. Surface Science, 1989. 223(1): p. 213-232.
142. S. Tanuma, T. Shiratori, T. Kimura, K. Goto, S. Ichimura, and C.J. Powell, Experimental determination of electron inelastic mean free paths in 13 elemental solids in the 50 to 5000 eV energy range by elastic‐peak electron spectroscopy. Surface and Interface Analysis, 2005. 37(11): p. 833-845.
143. J. Rundgren, Electron inelastic mean free path, electron attenuation length, and low-energy electron-diffraction theory. Physical Review B, 1999. 59(7): p. 5106-5114.
38
144. C.T. Chantler and J.D. Bourke, X-ray spectroscopic measurement of photoelectron inelastic mean free paths in molybdenum. The Journal of Physical Chemistry Letters, 2010. 1(15): p. 2422-2427.
145. I. Lindau and W.E. Spicer, The probing depth in photoemission and auger-electron spectroscopy. Journal of Electron Spectroscopy and Related Phenomena, 1974. 3(5): p. 409-413.
146. P.W. Palmberg and T.N. Rhodin, Auger electron spectroscopy of fcc metal surfaces. Journal of Applied Physics, 1968. 39(5): p. 2425-2432.
147. Q. Wang, R. Kitaura, S. Suzuki, Y. Miyauchi, K. Matsuda, Y. Yamamoto, S. Arai, and H. Shinohara, Fabrication and in situ transmission electron microscope characterization of freestanding graphene nanoribbon devices. ACS Nano, 2016. 10(1): p. 1475-1480.
148. M. Xu, D. Fujita, J. Gao, and N. Hanagata, Auger electron spectroscopy: a rational method for determining thickness of graphene films. ACS Nano, 2010. 4(5): p. 2937-2945.
149. E. Rollings, G.H. Gweon, S.Y. Zhou, B.S. Mun, J.L. McChesney, B.S. Hussain, A.V. Fedorov, P.N. First, W.A. De Heer, and A. Lanzara, Synthesis and characterization of atomically thin graphite films on a silicon carbide substrate. Journal of Physics and Chemistry of Solids, 2006. 67(9): p. 2172-2177.
150. M. Amjadipour, A. Tadich, J.J. Boeckl, J. Lipton-Duffin, J. MacLeod, F. Iacopi, and N. Motta, Quasi free-standing epitaxial graphene fabrication on 3C–SiC/Si(111). Nanotechnology, 2018. 29(14): p. 145601.
39
Chapter 3: Methodology
In this chapter, different experimental procedures and instruments employed to synthesize
graphene are presented and discussed.
3.1 EXPERIMENTAL DATA
3.1.1 3C-SiC sample specification
A polished epitaxial 3C-SiC(111) wafer with thickness of 1 µm grown on a Si(111) substrate
has been used for the graphene growth [1]. This wafer was purchased from Novasic, France. Polishing
3C-SiC surface improves graphene quality, therefore, a polished sample was used [2-4]. The
polishing procedure was performed by Novasic using a chemical and mechanical polishing method
(CMP).
3.1.2 Sample Preparation
Samples were diced to 12×2 mm2 to be compatible with the sample holder dimension. All of
the samples were cleaned by sonication at 40 kHz for 10 minutes in three different liquids (acetone,
isopropanol, and deionised water). The sonication helps to remove the contamination present on the
sample prior to introducing the sample to UHV system.
3.1.3 Patterning
Patterning 3C-SiC/Si(111) was performed using FEI Quanta 200 3D Dual Beam FIB/SEM
system. This instrument is further discussed below. FIB allows the fabrication of arbitrary structure
shapes with nanoscale precision. Different dimensions and shapes were milled with a variety of
depths up to 800 nm, which is the thickness of the SiC film. The typical dimension of the patterned
structures was five structures with width of 500 nm and length of 3 µm (figure 3-1). All of the
patterning was done by 30 keV acceleration voltage, 50% beam overlap and 1 µs dwell time. A
40
number of beam currents were explored and used for milling, but the majority of the structures were
milled by 0.1 nA.
Figure 3-1. (a) Schematic of the patterned 3C-SiC/Si(111) samples, (b) SEM image of a typical structure milled in 3C-SiC/Si(111).
3.1.4 Epitaxial Graphene Growth
After inserting the sample into UHV system, the sample was left for a few hours at 600 ˚C to
degas. The degassing procedure is effective in removing contaminants such as water vapor, hydrogen,
and other gas molecules trapped on the sample [5]. Annealing for degassing purposes should be
performed at a temperature in which there is no chance of chemical or structural modification of the
surface. The temperature used commonly for degassing the 3C-SiC/Si sample is in the range of 500-
600 ˚C.
To further improve the surface condition, atomic hydrogen etching was employed. Hydrogen
etching removes native oxides from the surface and helps to flatten the surface [6]. As atomic
hydrogen selectively etches Si atoms from the surface, it leaves a homogenous carbon rich layer
useful to control the growth rate and to improve graphene quality [7]. In this thesis, the atomic
hydrogen etching procedure was conducted at P ~ 5×10-6 mbar, Tsample = 1000 ± 15 °C and 40 W
power using an EFM-H atomic hydrogen source (FOCUS GmbH). Figure 3-2 shows the success of
atomic hydrogen etching procedure in producing a uniform graphene coverage [7].
41
Figure 3-2. STM images of 3C-SiC/Si(111) sample after graphene growth (a) without hydrogen etching, (b) with hydrogen etching procedure [7].
During the final annealing for graphene growth, the sample was heated up to 1200-1300 °C for
5-10 minutes. At such high temperatures silicon-carbon bonds break and silicon atoms leave the
surface; the remaining carbon atoms reconstruct and form graphene. This procedure is demonstrated
in figure 3-3.
Figure 3-3. (a) Silicon atoms leaving the sample because of annealing at 1200-1300 °C, (b) a review of the steps taken to grow graphene in UHV.
3.2 INSTRUMENTATION
3.2.1 Focused Ion Beam (FIB)
The milling has been done using a FEI Quanta 200 3D Dual Beam FIB/SEM system (figure 3-
4). These instruments are equipped with two different beams: an electron beam which is normally
used for observation and imaging, and a Ga+ ion beam for structuring and milling purposes. The ion
beam can also be used for imaging, but exposure of the sample to Ga+ ions causes surface damage
42
and Ga poisoning. Therefore, it is not desirable for imaging applications. The electron beam is
produced using a tungsten filament source and the ion beam is formed by a Ga liquid metal ion source.
For this study a Ga+ ion beam was used for milling with a beam energy of 30 keV. Different beam
currents were used with 1 µs dwell time and beam overlap of 50%.
Figure 3-4. A typical FEI Quanta 3D FIB/SEM system [8].
The instrument has different settings to define the milling depth of different materials, usually
calibrated by the manufacturer. However, the instrument does not have pre-set rates for SiC.
Therefore, it was necessary to calibrate the milling depth for SiC by measuring the actual milled depth
based on specific ion beam settings. SEM imaging was performed with 10-20 keV beam energy and
a variety of beam currents.
FIB/SEM dual beam systems have been also used for TEM sample preparation. This is a
complex procedure which starts by cutting a thin lamella using FIB and lifting it out using a needle
(figure 3-5). The lamella then was welded to a copper grid using Pt deposition. The transferred sample
then was thinned to about 100 nm thickness to make it transparent to electrons.
43
Figure 3-5. SEM image of the lift out procedure to prepare a TEM lamella using a Dual Beam system.
3.2.2 Omicron UHV System
The epitaxial graphene growth has been conducted using an Omicron multiprobe UHV system
with a base pressure of 4×10-11 mbar. This machine has three chambers: prechamber (airlock),
analysis chamber (main chamber), and a variable temperature (VT) chamber (figure 3-6). The
prechamber (airlock) is for introducing a sample into the vacuum. It performs as a middle stage to
drop the pressure from atmospheric to 10-7 mbar range. This chamber has a N2 supply for venting.
After reaching the desired pressure in the prechamber, the sample is transferred to the analysis
chamber by a magnetic arm.
44
Figure 3-6. UHV Omicron multiprobe system [5].
The analysis chamber is the largest chamber of the machine and can achieve a base pressure of
4×10-11 mbar. However due to the frequent sample introduction the pressure is commonly in the 10-
10 mbar range. The analysis chamber is also equipped with XPS, SEM, Ar+ sputtering system, atomic
hydrogen source (EFM-H) and e-beam evaporator (EFM 3). The EFM-H atomic hydrogen source
was employed in this project for atomic hydrogen etching. An electron beam evaporator (EFM 3) was
used for silicon deposition. In this evaporator the silicon rod (evaporant) is located in the focus of an
e-beam system. By monitoring the ion flux current, it is possible to control the deposition rate (0.01
to 1 Å/min). The sample can be heated in the manipulator by direct current heating (up to 1600˚C),
resisting heating (up to 600 ˚C) or electron bombardment (up to 1000 ˚C). Figure 3-7 shows the
sample heated to about 1000 ˚C in the analysis chamber.
The VT chamber is mainly used for scanning probe microscopy, operating in a temperature
range between 50 and 500 K. It contains three different microscopes: STM, AFM, and Q-plus (a
45
combination of STM and AFM). It also has a gas dozer attached to it which can be used to introduce
small amount of gasses to the sample during microscopy.
Figure 3-7. SiC sample during annealing by direct current heating in the analysis chamber of UHV Omicron system. From the left to the right, pointing at the sample, it is possible to see: (1) the terminal part of the X-ray source - DAR
400, (2) the electron analyzer - Sphera II -7/1 and (3) the SEM - SEM 20.
The vacuum is obtained and maintained with four kind of pumps, working in different pressure
ranges (Table 3-1). A gate valve between the analysis and the VT chamber reduces contamination in
the VT chamber. The analysis and VT chambers are equipped with separate ion and Ti pumps.
Table 3-1. Available pumps in the UHV multiprobe system
Pump Type Pumping Range
Rotary Pump 1 bar to 10-3 mbar
Turbo Pump 10-2 to 10-10 mbar
Ion Getters 10-4 to 10-11 mbar
Titanium Sublimation Pump 10-6 to 10-11 mbar
46
An optical pyrometer (IRCON Ultimax UX-20P with emissivity = 0.9) was used to measure
the sample temperature.
3.2.3 X-ray Photoelectron Spectroscopy (XPS)
XPS is an excellent tool for characterizing the chemical composition and the thickness of
graphene on SiC. This technique is based on the photoelectric effect, i.e. extracting an electron bound
to an atom using photons. By measuring the kinetic energy (K.E) of the emitted electrons, it is possible
to obtain the binding energy (B.E.). Binging energy is an element specific property and can be used
to identify elements on the sample. The binding energy of the ejected electron can be calculated by
K. E. = Eph − ∅XPS − B. E. (9)
where ∅XPS is the work function of the analyzer and Eph is the photon energy. Electrons with the
typical kinetic energy in XPS (0-1200 eV) cannot travel more than 10 nm from the surface because
they strongly interact with the other electrons in the solid and lose their energy [9]. This makes XPS
a surface sensitive method.
For this study a non-monochromatized XPS source (DAR 400, Omicron Nanotechnology) has
been used. This source is capable of producing Mg Kα (1253.6 eV) or Al Kα (1486.6 eV) lines with
a power of 300 W. The incident angle for its setting is 65˚ and the photoelectrons are collected at take
–off angle of 90˚. In this thesis, Mg Kα was used. In order to analyze the data CasaXPS [10] and Igor
Pro [11] software have been used. Shirley-type background subtraction was conducted to analyze Si
and C high resolution data. Gaussian-Lorentzian line shape was used for curve fitting of the data.
Error calculation based on Monte Carlo approximation was also performed using CasaXPS software
[10].
47
3.2.4 Synchrotron Radiation
Synchrotron-based XPS measurements were conducted at the soft X-ray beamline at the
Australian Synchrotron. The measurements were performed using a SPECS Phoibos 150
hemispherical analyzer, operating at a pass energy of 10 eV.
3.2.5 Scanning Tunneling Microscopy (STM)
Since 1981 when STM was invented [12], it has been attracting a lot of interest for studying
semiconductors. STM is capable of not only imaging the surface with atomic resolution, but also is a
great tool for performing electronic properties measurements [13]. STM employes a probe tip which
is commonly made of W or Pt-Ir alloy. The tip is drived using three perpendicular piezo electric
transducers installed in three different axis: x, y and z. The piezo transducers located in x and y axis
move the tip on the sample’s surface plane, and the one installed in z axis helps the tip to move in
this direction and reveals the surface topography. Depending on the voltage applied to the transducers,
they expand or contract accordingly; this movement enables the tip to move across the sample and
scan the surface.
In my study I used STM installed in the VT chamber of the Omicron multiprobe system. This
microscope does not have three separate piezos, but it takes advantage of a piezo tube. This piezo
tube can move around 1 µm in z axis and around 12 µm in the x and y axes. A bias voltage (V) is set
between the tip and surface. The tunneling current (I) also can be set to define the movement of the
tip on the sample. The current is related to the bias voltage and the distance between the tip and the
sample by:
𝐈𝐈 ∝ 𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕(−𝟐𝟐𝟐𝟐𝟐𝟐) (10)
where k defines the wave vector associated with the particles crossing the tunneling barrier and is
equal to 2mφh ; m represents the mass of the electron and 𝜑𝜑 represents the height of the barrier.
This barrier is made of the vacuum between the tip and sample [13]. In our microscope the tip is
48
grounded, and the bias voltage can be considered the sample voltage. When we expose a positive
voltage, the electrons are tunneling from the tip to the sample and vice-versa. In order to achieve
atomic resolution imaging with STM, it should be very well isolated from any vibration. Therefore,
there is a damping system installed on the microscope to decrease the noise caused by vibration. In
order to analyze the images Gwiddyon and SPIP software are used [14, 15].
In order to be able to perform STM, a very stable and flat surface, extremely sharp tip, very
good vibrational damping and sophisticated electronics are needed. Despite the fact that STM is a
very challenging instrument, it has been used frequently for studying graphene [16]. STM can be used
to investigate the morphology, continuity and structure of the grown graphene [17, 18].
3.2.6 Raman Spectroscopy
Since 1928 when Raman spectroscopy was discovered, it has been attracting a lot of interest
within a wide range of research [19]. In particular, it is commonly used for characterizing graphene
[20-25]. Raman spectroscopy is a great tool for identifying molecular species. It is based on
illumination of a monochromatic laser light to a sample and analyzing the scattered light. The
scattered light contains information about the excitation or de-excitation of vibrational levels in a
solid or a molecule. These vibrational levels are considered as fingerprints for different molecules,
and they are very sensitive to changes in geometry or bonding of the sample. In this thesis, A
Renishaw Raman microscope with λ=532 nm laser light at room temperature with 10% laser power
(approximately 7 mW) and an X50 objective was employed.
3.2.7 Electron and Ion Microscopy
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been
employed for this research. The different SEM instruments used are: FE-SEM (JEOL JSM-7001F),
FEI Quanta 200 3D Dual Beam FIB/SEM and FEI SCIOS Dual Beam system. SEM was mainly used
for imaging purposes at 20 keV acceleration voltage and a variety of beam currents. A JEOL JEM-
49
2100 TEM was also employed. TEM has been widely used to study graphene as TEM is the only
technique capable of showing suspended graphene on a substrate [26-35].
Helium ion microscopy was employed for imaging using a Carl Zeiss Orion NanoFab HIM.
HIM uses a He ion beam for imaging. This instrument is capable of patterning with a very high
precision (a few nanometer resolution), however, it is very time consuming and challenging.
Comparing to SEM, HIM provides a higher resolution (up to few nanometers) making it a great tool
to study surfaces. For image processing and analysis ImageJ software [36] was used.
3.3 REFRENCES
1. J. Meziere, M. Ucar, E. Blanquet, M. Pons, P. Ferret, and L. Di Cioccio, Modeling and simulation of SiC CVD in the horizontal hot-wall reactor concept. Journal of Crystal Growth, 2004. 267(3): p. 436-451.
2. B. Gupta, I. Di Bernardo, P. Mondelli, A. Della Pia, M. Betti, F. Iacopi, C. Mariani, and N. Motta, Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
3. M. Portail, A. Michon, S. Vezian, D. Lefebvre, S. Chenot, E. Roudon, M. Zielinski, T. Chassagne, A. Tiberj, and J. Camassel, Growth mode and electric properties of graphene and graphitic phase grown by argon–propane assisted CVD on 3C–SiC/Si and 6H–SiC. Journal of Crystal Growth, 2012. 349(1): p. 27-35.
4. M. Zielinski, M. Portail, S. Roy, T. Chassagne, C. Moisson, S. Kret, and Y. Cordier, Elaboration of (111) oriented 3C–SiC/Si layers for template application in nitride epitaxy. Materials Science and Engineering: B, 2009. 165(1): p. 9-14.
5. B. Gupta, Epitaxial graphene growth on 3C SiC by Si sublimation in UHV. 2015, Queensland University of Technology.
6. A. Sandin, J.J. Rowe, and D.B. Dougherty, Improved graphene growth in UHV: pit-free surfaces by selective Si etching of SiC (0001)–Si with atomic hydrogen. Surface Science, 2013. 611: p. 25-31.
7. P. Mondelli, B. Gupta, M.G. Betti, C. Mariani, J. Lipton-Duffin, and N. Motta, High quality epitaxial graphene by hydrogen-etching of 3C-SiC(111) thin-film on Si(111). Nanotechnology, 2017. 28(11): p. 115601.
8. A.J. Bushby, K.M.Y. P'ng, R.D. Young, C. Pinali, C. Knupp, and A.J. Quantock, Imaging three-dimensional tissue architectures by focused ion beam scanning electron microscopy. Nature Protocols, 2011. 6: p. 845.
9. P. Van der Heide, X-ray photoelectron spectroscopy: an introduction to principles and practices. 2011: John Wiley & Sons.
10. CasaXPS. http://www.casaxps.com/. 11. Igor Pro. https://www.wavemetrics.com/products/igorpro/igorpro.htm.
50
12. G. Binnig and H. Rohrer, Scanning tunneling microscopy. Surface Science, 1983. 126(1-3): p. 236-244.
13. C.J. Chen, Introduction to scanning tunneling microscopy. Vol. 2. 1993: Oxford University Press New York.
14. Gwyddion. http://gwyddion.net/download.php. 15. SPIP. http://www.imagemet.com/index.php?main=download. 16. M. Ridene, J. Girard, L. Travers, C. David, and A. Ouerghi, STM/STS investigation of edge
structure in epitaxial graphene. Surface Science, 2012. 606(15): p. 1289-1292. 17. S. Goler, C. Coletti, V. Piazza, P. Pingue, F. Colangelo, V. Pellegrini, K.V. Emtsev, S. Forti,
U. Starke, and F. Beltram, Revealing the atomic structure of the buffer layer between SiC (0001) and epitaxial graphene. Carbon, 2013. 51: p. 249-254.
18. Y. Zhang, T. Gao, Y. Gao, S. Xie, Q. Ji, K. Yan, H. Peng, and Z. Liu, Defect-like structures of graphene on copper foils for strain relief investigated by high-resolution scanning tunneling microscopy. Acs Nano, 2011. 5(5): p. 4014-4022.
19. J. Ferraro, K. Nakamoto, and C. Brown, Introductory raman spectroscopy. 2003, Academic Press. p. 267-293.
20. T. Perova, J. Wasyluk, S. Kukushkin, A. Osipov, N. Feoktistov, and S. Grudinkin, Micro-Raman mapping of 3C-SiC thin films grown by solid–gas phase epitaxy on Si (111). Nanoscale Research Letters, 2010. 5(9): p. 1507-1511.
21. L. Malard, M. Pimenta, G. Dresselhaus, and M. Dresselhaus, Raman spectroscopy in graphene. Physics Reports, 2009. 473(5): p. 51-87.
22. R. Nishinakagawa, K. Matsuda, T. Arai, A. Sawada, and T. Terashima, Raman spectroscopy investigations of chemically derived zigzag edge graphene nanoribbons. AIP Advances, 2013. 3(9): p. 092111.
23. Z. Ni, W. Chen, X. Fan, J. Kuo, T. Yu, A. Wee, and Z. Shen, Raman spectroscopy of epitaxial graphene on a SiC substrate. Physical Review B, 2008. 77(11): p. 115416.
24. A.C. Ferrari, Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects. Solid State Communications, 2007. 143(1): p. 47-57.
25. Z. Zhi, H. Ying, S. Xin-Yan, and L. Xing-Hua, Raman spectrum of epitaxial graphene grown on ion beam illuminated 6H-SiC (0001). Chinese Physics Letters, 2014. 31(11): p. 116801.
26. F. Iacopi, N. Mishra, B.V. Cunning, D. Goding, S. Dimitrijev, R. Brock, R.H. Dauskardt, B. Wood, and J. Boeckl, A catalytic alloy approach for graphene on epitaxial SiC on silicon wafers. Journal of Materials Research, 2015. 30(05): p. 609-616.
27. M. Suemitsu and H. Fukidome, Epitaxial graphene on silicon substrates. Journal of Physics D: Applied Physics, 2010. 43(37): p. 374012.
28. W. Norimatsu, J. Takada, and M. Kusunoki, Formation mechanism of graphene layers on SiC (000 1) in a high-pressure argon atmosphere. Physical Review B, 2011. 84(3): p. 035424.
29. S. Sambonsuge, S. Jiao, H. Nagasawa, H. Fukidome, S.N. Filimonov, and M. Suemitsu, Formation of qualified epitaxial graphene on Si substrates using two-step heteroexpitaxy of C-terminated 3C-SiC (-1-1-1) on Si (110). Diamond and Related Materials, 2016. 67: p. 51-53.
51
30. W. Norimatsu and M. Kusunoki, Formation process of graphene on SiC (0001). Physica E: Low-Dimensional Systems and Nanostructures, 2010. 42(4): p. 691-694.
31. M. Kusunoki, W. Norimatsu, J. Bao, K. Morita, and U. Starke, Growth and features of epitaxial graphene on SiC. Journal of the Physical Society of Japan, 2015. 84(12): p. 121014.
32. W. Norimatsu and M. Kusunoki, Growth of graphene from SiC 0001 surfaces and its mechanisms. Semiconductor Science and Technology, 2014. 29(6): p. 064009.
33. W. Norimatsu and M. Kusunoki, Selective formation of ABC-stacked graphene layers on SiC (0001). Physical Review B, 2010. 81(16): p. 161410.
34. W. Norimatsu and M. Kusunoki, Structural features of epitaxial graphene on SiC 0 0 0 1 surfaces. Journal of Physics D: Applied Physics, 2014. 47(9): p. 094017.
35. W. Norimatsu and M. Kusunoki, Transitional structures of the interface between graphene and 6H–SiC (0001). Chemical Physics Letters, 2009. 468(1): p. 52-56.
36. ImageJ. https://imagej.nih.gov/ij/index.html.
52
Chapter 4: Graphene Growth on FIB Patterned 3C-SiC Nanostructures on Si (111)
4.1 ABSTRACT
Epitaxial growth of graphene on SiC is a scalable procedure which does not require any
further transfer step, making this an ideal platform for graphene nanostructure fabrication. Focused
ion beam (FIB) is a very promising tool for exploring the reduction of the lateral dimension of
graphene on SiC to the nanometer scale. However, exposure of graphene to the Ga+ beam causes
significant surface damage through amorphization and contamination, preventing epitaxial
graphene growth. In this paper we demonstrate that combining a protective silicon layer with FIB
patterning implemented prior to graphene growth can significantly reduce the damage associated
with FIB milling. Using this approach, we successfully achieve graphene growth over 3C-SiC/Si
FIB patterned nanostructures.
Keywords: epitaxial graphene; focused ion beam (FIB); nanofabrication; graphene nanostructures.
53
Nanotechnology, 2017. 28(34): p. 345602. DOI: https://doi.org/10.1088/1361-6528/aa752e
Epitaxial Graphene Growth on FIB Patterned 3C-SiC Nanostructures on Si
(111): Reducing Milling Damage
Mojtaba Amjadipour1, Jennifer MacLeod1, Josh Lipton-Duffin2, Francesca Iacopi3, and Nunzio
Motta1*
1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, QLD, Australia.
2 Central Analytical Research Facility, Institute for Future Environments, Science and
Engineering Faculty, Queensland University of Technology, QLD, Australia.
3 School of Computing and Communications, Faculty of Engineering and Information
Technology, University of Technology Sydney, NSW, Australia.
* Corresponding author. Tel: + 61731385104. E-mail: [email protected]
54
4.2 INTRODUCTION
Nano-scale patterning of graphene is emerging as a potential key to a variety of applications
such as sensing and electronics [1-6]. Patterning can be achieved with different techniques such as
electron beam lithography [7], nanolithography via atomic force microscopy (AFM) [8, 9] or
scanning tunneling microscopy (STM) [10, 11], chemical etching [12, 13], and focused ion beam
(FIB) milling [14, 15]. Electron beam lithography requires coating and removal of a resist layer,
which is challenging and can adversely affect the quality of graphene. Nanolithography using
AFM or STM is very time consuming which makes it incompatible with industrial processes.
Flexibility and ease of use for a variety of materials have made FIB a powerful tool for patterning
at nanoscale resolution. Therefore, in the past few years, a number of attempts have been made to
fabricate nanostructures of graphene using FIB [16-19]. Typical fabrication of graphene
nanostructures using FIB involves the transfer of the CVD grown or mechanically cleaved
graphene layer onto a silicon dioxide substrate, followed by patterning of the graphene [14, 17,
20-22]. In this work we explore the possibility of patterning the SiC substrate by FIB before
graphene growth, such that the versatile graphene direct growth and maskless patterning capability
on silicon demonstrated by Cunning et al [23] could be extended to nanoscale features. This is
expected to be a pathway towards the use of FIB milling for graphene nanofabrication.
Graphitization of SiC was first reported by van Bommel et al [24], and growing graphene on
SiC substrate has emerged as an attractive and facile platform for graphene production [25-27].
However, single crystal SiC wafers are very expensive and limited in size, therefore the use of
cubic polytype SiC (3C-SiC) grown on silicon substrates was explored as a cheaper alternative
[28-30]. Although growing smooth and uniform 3C-SiC on a silicon substrate is challenging and
recently concerns have been raised about degradation of SiC and Si interface on the transport
55
properties of graphene [31], there have been considerable improvements in this field [32-35].
Using 3C-SiC/Si not only decreases the cost of graphene production and makes it more desirable
for industries, but also it makes it possible to grow graphene over a semiconducting substrate,
thereby, avoiding the requirement of graphene transfer for applications [36, 37].
A number of approaches have been investigated in the pursuit of nano-patterned
graphene/SiC [3, 16, 23, 38, 39]. Sprinkle et al [3] initially patterned some trenches on single
crystalline 4H/6H-SiC using photolithography, and they managed to grow graphene on SiC facets
by high temperature annealing [4, 38]. More recently, Cunning et al [23] reported a pathway for
fabricating SiC micro-structures at the wafer-level through photolithography, on which they
achieved metal catalyst aided graphitization. Prével et al [16] studied the effect of Ga+ ions
exposure to graphene grown on bulk 6H-SiC, and they concluded that FIB is a promising method
for engineering graphene, but it leads to some degree of unintended amorphization of graphene.
Furthermore, Zhi et al [39] reported that Ga+ ion bombardment of 6H-SiC leads to growth of
defective graphene, and suggested that by use of a H2 etching procedure the defects can be
decreased [39]. More research is needed to explore the capability of FIB milling for graphene
nanostructure fabrication on SiC substrate.
Amorphization, contamination, and swelling are some of the common types of ion-beam
related damage caused to solids; these are mainly caused by redeposition of etched atoms or
implantation of the ions into the solid [21, 40-43]. In the present paper we study the damage caused
to graphene by the FIB process and we demonstrate the efficacy of using a silicon layer to protect
the surface prior to milling. Use of this protective layer allows graphene to be grown on FIB-
patterned 3C-SiC/Si nanostructures via high temperature annealing in ultra-high vacuum (UHV).
The growth process has been investigated by using X-ray photoelectron spectroscopy (XPS),
56
scanning tunneling microscopy (STM), scanning electron microscopy (SEM) with Energy
Dispersive X-ray spectroscopy (EDX), helium ion microscopy (HIM) and Raman spectroscopy.
4.3 EXPERIMENTAL DETAILS
A 1 µm 3C-SiC (111) epilayer grown on Si (111) was purchased from NOVASIC (France).
The sample was polished using a combination of chemical and mechanical steps (StepSiC® by
NOVASIC (France)). The polishing process reduced the surface roughness to less than 1 nm [44].
The substrate is P-doped with a resistivity of 1-10 Ω-cm. The wafer was diced to the size of 12×2
mm2 and each sample was cleaned by 10 minutes sonication in acetone, isopropanol and deionized
water, before introducing it into the FIB for the milling procedure and into the UHV system for
graphene growth.
An FEI Quanta 200 3D Dual Beam system was used to pattern the substrate by FIB with Ga+
ions. All patterns were fabricated with a beam energy of 30 keV, a dwell time of 1 µs, a beam
overlap of 50 %, and a beam current of 0.1 nA.
Graphene growth was performed in a UHV Omicron Multiprobe system (base pressure
5×10-11 mbar). The sample was degassed at 600 °C for few hours and the final growth was
performed by annealing at 1250 ± 15 °C. An optical pyrometer (IRCON Ultimax UX-20P with
emissivity = 0.9) was used to measure the sample temperature. The roughness was calculated by
analyzing (1×1) µm2 area STM images through the image metrology software (SPIP).
In all experiments hydrogen etching was used to decrease contamination and to flatten the
surface using an EFM-H atomic hydrogen source (FOCUS GmbH) at P ~ 5×10-6 mbar, Tsample =
1000 ± 15 °C and 40 W power.
57
An electron beam evaporator (EFM 3, FOCUS GmbH) was used to deposit a Si layer on the
sample from a high purity 2 mm, n-doped (P), ρ = 10-100 Ω-cm, Si rod (Hans Holm GmbH). In
our configuration typical parameters to obtain a 1 nm/h deposition on the sample are: HV = 700
V, Iem = 33 mA.
A Carl Zeiss Orion NanoFab HIM was used for imaging, and a FE-SEM (JEOL JSM-7001F)
was used for EDX study.
The sample surface was analyzed in situ using XPS and STM before and after graphene
growth. Ex situ characterization has been conducted using SEM, HIM and Raman Spectroscopy.
The Raman measurements were performed at room temperature using a Renishaw Raman
microscope with laser wavelength λ = 532 nm.
4.4 RESULTS AND DISCUSSIONS
4.4.1 FIB patterned 3C-SiC/Si (111): effects on graphene growth
Atomic hydrogen etching was employed to clean and flatten the surface in order to achieve
better graphene quality. Hydrogen etching removes silicon from the 3C-SiC surface and produces
a uniform carbon-rich layer [45], which helps in improving the quality of the epitaxial graphene.
XPS results show the effectiveness of hydrogen etching in eliminating oxygen species (Figure 4-
7, supporting information). The STM image in Figure 4-1a also shows the quality of the surface
after 40 minutes hydrogen etching. Flat SiC terraces are clearly observable in the image.
After each step of the epitaxial growth process (degassing, atomic hydrogen etching, and
final annealing), the surface chemical composition was analyzed by XPS. The XPS C1s spectrum
(Figure 4-1b) is fitted by three components: SiC at ~ 283 eV, graphene at ~284.7 eV, and the buffer
layer at ~285.8 eV. The buffer layer is a carbon rich layer at the interface between graphene and
SiC, which is bonded to the substrate and does not possess graphene properties [46]. These results
58
show that graphene has grown on the sample. By comparing the intensity ratio of the graphene (C-
C) peak to the SiC peak in Figure 4-1b, following ref [29, 47], we estimate that ~7 graphene layers
were obtained on this sample (see supporting information).
Figure 4-1. (a) STM image (Vbias= -1.4 V, I = 0.3 nA) of the surface after 40 minutes atomic hydrogen etching at
1000 ˚C, (b) XPS of the C1s region, showing the presence of three components attributed to the buffer layer, graphene (C-C) and SiC.
Figure 4-2a shows an SEM image taken from the sample after milling structures to a
maximum depth of ~ 500 nm and before graphene growth. HIM images taken after final 10 minutes
annealing at 1250 ˚C show a rough surface around the structures (Figure 4-2b), whereas in areas
not exposed to the Ga+ beam steps and flat terraces are clearly observable (Figure 4-2c). This
suggests that the FIB milling process causes substantial surface damage, which significantly
disturbs the epitaxial graphene growth on the nanostructures. We believe the main reason for the
damage is the redeposition of milled SiC from the trenches, which has Ga implanted in it. This is
consistent with the presence of Ga on the nanostructures in the EDX map (Figure 4-2d). These
milled residuals appear to chemically bind to the substrate during the graphene growth process,
creating the clusters visible on the nanostructures.
59
Figure 4-2. (a) SEM image of the FIB milled structures, (b) HIM image showing the surface morphology after the graphene growth on the structures - the black square show the typical area used for roughness calculation, (c) HIM image showing the surface morphology after the graphene growth far from the structures, (d) EDX map indicating Ga implantation on the sample, where the green color represents Ga, (e) Raman spectra: blue line, acquired at the location of the blue circle in panel (b); green line, acquired on green circle in panel (c), (f) STM image (Vbias= -1.6 V, I = 0.9 nA) showing the morphology of surface after graphene growth far from the structures (panel (c)), and the quasi 6×6 periodicity due to the interface corrugation [46] (1.8 nm, marked by a red lozenge) in the inset (Vbias= -1.2
V, I = 0.8 nA).
Raman spectroscopy (Figure 4-2e) was used to assess the quality of grown graphene. Four
peaks are commonly reported in the Raman spectrum as the fingerprint of graphene: D at ~1350
cm-1, G at ~1580 cm-1, 2D at ~2720 cm-1 and D+G at ~2950 cm-1 [48]. The G peak originates from
sp2 carbon-carbon bonding [48], and the D peak comes from defects within the graphene structure,
which are common in graphene grown on SiC and increase substantially in graphene grown on
SiC/Si thin films [44, 50, 51]. The intensity ratio of G and D peaks (IG/ID) is commonly used to
assess the quality of graphene, and for graphene grown on SiC(111)/Si(111) is typically in the
range of 0.5 – 1 [29, 44, 50]. The Raman spectrum acquired on the structures (Figure 4-2b – blue
circle) shows a merging of D and G peaks (Figure 4-2e – blue line), consistent with amorphization
60
of C layer; while Raman spectrum acquired far from the area exposed to the Ga+ ion beam (Figure
4-2c – green circle) shows clearly the signature bands of graphene (Figure 4-2e – green line) [42,
52]. STM images taken on areas far from the structures also shows the SiC terraces covered with
graphene after the final annealing (Figure 4-2f), with a quasi 6 × 6 pattern (unit cell of 1.8 nm
marked by a red lozenge) due to the interface corrugation [46, 53]. A schematic diagram
demonstrating the redeposition of the milled residuals of SiC/Ga leading to the rough surface of
3C-SiC/Si (111) is depicted in Figure 4-3.
Figure 4-3. Schematic diagram of the procedure leading to damage on nanostructures: (a) pristine thin film of 3C-SiC/Si (111), (b) milling structures by FIB, (c) high temperature annealing in UHV which leads to binding of the
residuals to the surface and disturbs graphene growth.
4.4.2 Si cap layer on SiC/Si (111): reducing FIB milling damage
The damage caused by Ga+ milling can be prevented using a Si cap layer that can be
eliminated prior the graphene growth in UHV (Figure 4-9, supporting information). This layer has
a double effect: a) it isolates the surface of the SiC nanostructures during patterning from the
redeposited SiC/Ga residuals, b) it traps the residuals which are then removed during the growth
process. Si is an appropriate cap layer as it is not expected to modify the chemistry of SiC, and can
be removed easily by heating the sample at 1000-1100 C, which is the temperature range normally
used for atomic hydrogen etching before graphene growth. The silicon overlayer also conveniently
61
compensates for excessive Si loss from the bulk during the SiC annealing for graphene growth,
and helps in controlling the number of graphene layers and improving the graphene quality [26].
A Stopping and Range of Ions in Matter (SRIM) simulation [54] indicates that the Si layer does
not alter the ions’ interactions or trajectories in the solid (Figure 4-8, supporting information).
Before silicon deposition the sample was cleaned ex situ and in situ as described in the
experimental details. After degassing and hydrogen etching in UHV a silicon layer was deposited
at room temperature (Figure 4-4a). Afterwards, the sample was milled by FIB (Figure 4-4b) and
then introduced to UHV for graphene growth (Figure 4-4c).
Figure 4-4. Schematic of the modified procedure: (a) deposition of a silicon protective layer in UHV, (b) patterning
by FIB, (c) graphene growth in UHV.
Following the deposition of the protective layer, the Si 2p XPS spectrum (Figure 4-5a)
exhibits a peak at ~100 eV, corresponding to pure Si, in addition to the expected SiC contribution.
This peak disappears after hydrogen etching in preparation for the graphene growth (Figure 4-9,
supporting information), confirming that the silicon mask can be easily removed during the normal
graphene growth procedure without the need for any supplementary steps, and has no chemical
effects on the substrate.
62
The thickness of the deposited silicon layer can be calculated by analyzing the XPS data
using a simple formula based on the differential cross sections and the inelastic mean free paths of
electrons, as suggested in ref [29] for graphene/SiC. By applying a similar procedure to the data
shown in Figure 4-5a, we calculated the thickness of the Si layer as ~ 2 nm (see supporting
information).
The sample was also studied by STM in order to find whether the silicon mask interferes
with graphene growth. By comparing the surface condition after removal of the silicon mask
(Figure 4-5b) to a surface prepared without a mask layer (Figure 4-1a), we observe similar features
and no significant difference in roughness (Table 1). This suggests that the surface is not adversely
affected by the use and removal of the silicon mask. This is further supported by our XPS data,
which show no difference between the masked and unmasked surface following hydrogen etching
(Figure 4-9, supporting information).
63
Figure 4-5. (a) XPS spectrum of Si 2p after silicon deposition, (b) STM image (Vbias= 2 V, I = 0.8 nA) on the
silicon-protected sample after the second round of atomic hydrogen etching, (c) XPS spectrum of C1s after graphene growth on the silicon coated sample, (d) STM image (Vbias=2 V, I =0.8 nA) after graphene growth with a high
resolution image in the inset showing the quasi 6×6 periodicity [46] (Vbias= -1 V, I = 0.3 nA).
After annealing at 1250 ˚C, successful growth of graphene is confirmed by XPS and STM
(Figure 4-5c and d). HIM imaging on the nanostructures (Figure 4-6a) does not reveal any visible
damage on the surface, which appears similar to the area not exposed to the Ga+ ion beam.
Comparison of the average roughness calculated for samples after graphene growth without and
with the protecting layer indicates the significant improvement, from 6.00 nm to 0.49 nm (Table
4-1). Raman spectra (blue line in Figure 4-6b) acquired on the nanostructures (blue circle in Figure
4-6a), confirms the presence of graphene (Table 4-1). Having a significant D peak is common for
64
graphene grown via Si sublimation on SiC/Si thin films [35, 51, 55]. One reported reason for
having a mediocre quality graphene on SiC/Si thin films is the significant epitaxial lattice-
mismatch (~ 20 %) between SiC and Si crystals which originates defects in the SiC film [34].
However, defective graphene is reported to be desirable for doping purposes which is useful for
tuning graphene’s properties and developing advanced devices such as high performance energy
storage devices [56-59].
Figure 4-6. (a) HIM image of the nanostructures after annealing at 1250 ˚C using the modified procedure with silicon mask - black square shows the typical area used for roughness calculation, (c) Raman spectra: blue line,
acquired at the location of the blue circle in panel (a); green line shows the spectrum acquired ~ 3 µm away from the structures.
Table 4-1. Roughness calculated on 1×1 µm2 area STM images at different locations and on samples with and without the Si protective layer.
Sample Condition Roughness (nm) at different distance from the structures
~ 1 µm > 5 µm
After H etching 0.42 0.46
After H etching with Si protective layer 0.41 0.32
After graphene growth 6.00 0.80
After graphene growth with Si protective layer 0.49 0.73
65
4.5 CONCLUSIONS
By using a combination of HIM, STM and Raman spectroscopy we demonstrated a pathway
to fabricate spatially-constrained graphene on FIB-patterned 3C-SiC nanostructures by UHV
annealing. On bare SiC, FIB milling leads to beam-related damage that prevents graphene growth
on or near the milled structures. Depositing a cap layer of silicon prior to milling reduces the beam-
related damage and allows for the growth of graphene on the nanostructures. This protective layer
can be easily removed during the normal UHV growth process, resulting in epitaxial graphene, as
proven by Raman spectroscopy. This approach paves the way for nanoscale engineering of
graphene on SiC using FIB.
4.6 ACKNOWLEDGMENTS
The authors acknowledge the support of the Queensland Government through the Q-CAS
Collaborative Science Fund 2016. The data reported in this paper were obtained at the Central
Analytical Research Facility (CARF) operated by the Institute for Future Environments (QUT).
Access to CARF is supported by generous funding from the Science and Engineering Faculty
(QUT). This work was performed in part at the Queensland node of the Australian National
Fabrication Facility (ANFF) - a company established under the National Collaborative Research
Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers.
Dr Bharati Gupta and Mr Jonathan Bradford are kindly acknowledged for their help and support
in this research. We gratefully thank the technical support of Dr. Annalena Wolff, Dr. Peter Hines,
and Dr. Llew Rintoul from the Central Analytical Research Facility of the Institute of Future
Environments at QUT.
66
4.7 REFERENCES
1. Y.M. Lin, C. Dimitrakopoulos, K.A. Jenkins, D.B. Farmer, H.Y. Chiu, A. Grill, and P. Avouris, 100-GHz transistors from wafer-scale epitaxial graphene. Science, 2010. 327(5966): p. 662-662.
2. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, and W.A. De Heer, Electronic confinement and coherence in patterned epitaxial graphene. Science, 2006. 312(5777): p. 1191-1196.
3. M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu, C. Berger, and W.A. De Heer, Scalable templated growth of graphene nanoribbons on SiC. Nature Nanotechnology, 2010. 5(10): p. 727-731.
4. A. Celis, M. Nair, A. Taleb-Ibrahimi, E. Conrad, C. Berger, W. De Heer, and A. Tejeda, Graphene nanoribbons: fabrication, properties and devices. Journal of Physics D: Applied Physics, 2016. 49(14): p. 143001.
5. M. Terrones, A.R. Botello-Méndez, J. Campos-Delgado, F. López-Urías, Y.I. Vega-Cantú, F.J. Rodríguez-Macías, A.L. Elías, E. Muñoz-Sandoval, A.G. Cano-Márquez, J.-C. Charlier, and H. Terrones, Graphene and graphite nanoribbons: morphology, properties, synthesis, defects and applications. Nano Today, 2010. 5(4): p. 351-372.
6. Z. Li and F. Chen, Ion beam modification of two-dimensional materials: Characterization, properties, and applications. Applied Physics Reviews, 2017. 4(1): p. 011103.
7. M.Y. Han, B. Özyilmaz, Y. Zhang, and P. Kim, Energy band-gap engineering of graphene nanoribbons. Physical Review Letters, 2007. 98(20): p. 206805.
8. L. Weng, L. Zhang, Y.P. Chen, and L.P. Rokhinson, Atomic force microscope local oxidation nanolithography of graphene. Applied Physics Letters, 2008. 93(9): p. 093107.
9. S. Masubuchi, M. Ono, K. Yoshida, K. Hirakawa, and T. Machida, Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope. Applied Physics Letters, 2009. 94(8): p. 082107.
10. P. Nemes-Incze, L. Tapasztó, G.Z. Magda, Z. Osváth, G. Dobrik, X. Jin, C. Hwang, and L. Biró, Graphene nanoribbons with zigzag and armchair edges prepared by scanning tunneling microscope lithography on gold substrates. Applied Surface Science, 2014. 291: p. 48-52.
11. L. Tapasztó, G. Dobrik, P. Lambin, and L.P. Biró, Tailoring the atomic structure of graphene nanoribbons by scanning tunnelling microscope lithography. Nature Nanotechnology, 2008. 3(7): p. 397-401.
12. L.C. Campos, V.R. Manfrinato, J.D. Sanchez-Yamagishi, J. Kong, and P. Jarillo-Herrero, Anisotropic etching and nanoribbon formation in single-layer graphene. Nano Letters, 2009. 9(7): p. 2600-2604.
13. S.S. Datta, D.R. Strachan, S.M. Khamis, and A.C. Johnson, Crystallographic etching of few-layer graphene. Nano Letters, 2008. 8(7): p. 1912-1915.
67
14. M.C. Lemme, D.C. Bell, J.R. Williams, L.A. Stern, B.W. Baugher, P. Jarillo-Herrero, and C.M. Marcus, Etching of graphene devices with a helium ion beam. ACS Nano, 2009. 3(9): p. 2674-2676.
15. B.S. Archanjo, B. Fragneaud, L. Gustavo Cançado, D. Winston, F. Miao, C. Alberto Achete, and G. Medeiros-Ribeiro, Graphene nanoribbon superlattices fabricated via He ion lithography. Applied Physics Letters, 2014. 104(19): p. 193114.
16. B. Prével, J.-M. Benoit, L. Bardotti, P. Mélinon, A. Ouerghi, D. Lucot, E. Bourhis, and J. Gierak, Nanostructuring graphene on SiC by focused ion beam: effect of the ion fluence. Applied Physics Letters, 2011. 99(8): p. 083116.
17. B. Archanjo, A. Barboza, B. Neves, L. Malard, E. Ferreira, J. Brant, E. Alves, F. Plentz, V. Carozo, and B. Fragneaud, The use of a Ga+ focused ion beam to modify graphene for device applications. Nanotechnology, 2012. 23(25): p. 255305.
18. N. Kalhor, S.A. Boden, and H. Mizuta, Sub-10nm patterning by focused He-ion beam milling for fabrication of downscaled graphene nano devices. Microelectronic Engineering, 2014. 114: p. 70-77.
19. Y. Zhang, C. Hui, R. Sun, K. Li, K. He, X. Ma, and F. Liu, A large-area 15 nm graphene nanoribbon array patterned by a focused ion beam. Nanotechnology, 2014. 25(13): p. 135301.
20. V. Iberi, A.V. Ievlev, I. Vlassiouk, S. Jesse, S.V. Kalinin, D.C. Joy, A.J. Rondinone, A. Belianinov, and O.S. Ovchinnikova, Graphene engineering by neon ion beams. Nanotechnology, 2016. 27(12): p. 125302.
21. D.C. Bell, M.C. Lemme, L.A. Stern, J.R. Williams, and C.M. Marcus, Precision cutting and patterning of graphene with helium ions. Nanotechnology, 2009. 20(45): p. 455301.
22. X. Wu, H. Zhao, D. Yan, and J. Pei, Investigation on gallium ions impacting monolayer graphene. AIP Advances, 2015. 5(6): p. 067171.
23. B.V. Cunning, M. Ahmed, N. Mishra, A.R. Kermany, B. Wood, and F. Iacopi, Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers. Nanotechnology, 2014. 25(32): p. 325301.
24. A. Van Bommel, J. Crombeen, and A. Van Tooren, LEED and Auger electron observations of the SiC (0001) surface. Surface Science, 1975. 48(2): p. 463-472.
25. A. Ouerghi, A. Kahouli, D. Lucot, M. Portail, L. Travers, J. Gierak, J. Penuelas, P. Jegou, A. Shukla, and T. Chassagne, Epitaxial graphene on cubic SiC (111)/Si (111) substrate. Applied Physics Letters, 2010. 96(19): p. 191910.
26. W.A. De Heer, C. Berger, M. Ruan, M. Sprinkle, X. Li, Y. Hu, B. Zhang, J. Hankinson, and E. Conrad, Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide. Proceedings of the National Academy of Sciences, 2011. 108(41): p. 16900-16905.
27. A. Ouerghi, M.G. Silly, M. Marangolo, C. Mathieu, M. Eddrief, M. Picher, F. Sirotti, S. El Moussaoui, and R. Belkhou, Large-area and high-quality epitaxial graphene on off-axis SiC wafers. ACS Nano, 2012. 6(7): p. 6075-6082.
68
28. A. Severino, C. Bongiorno, N. Piluso, M. Italia, M. Camarda, M. Mauceri, G. Condorelli, M. Di Stefano, B. Cafra, and A. La Magna, High-quality 6inch (111) 3C-SiC films grown on off-axis (111) Si substrates. Thin Solid Films, 2010. 518(6): p. S165-S169.
29. B. Gupta, Notarianni, M., Mishra, N., Shafiei, M., Iacopi, F., & Motta, N., Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
30. F. Zarotti, B. Gupta, F. Iacopi, A. Sgarlata, M. Tomellini, and N. Motta, Time evolution of graphene growth on SiC as a function of annealing temperature. Carbon, 2016. 98: p. 307-312.
31. A. Pradeepkumar, N. Mishra, A.R. Kermany, J.J. Boeckl, J. Hellerstedt, M.S. Fuhrer, and F. Iacopi, Catastrophic degradation of the interface of epitaxial silicon carbide on silicon at high temperatures. Applied Physics Letters, 2016. 109(1): p. 011604.
32. F. Iacopi, G. Walker, L. Wang, L. Malesys, S. Ma, B.V. Cunning, and A. Iacopi, Orientation-dependent stress relaxation in hetero-epitaxial 3C-SiC films. Applied Physics Letters, 2013. 102(1): p. 011908.
33. L. Wang, S. Dimitrijev, J. Han, A. Iacopi, L. Hold, P. Tanner, and H.B. Harrison, Growth of 3C–SiC on 150-mm Si (100) substrates by alternating supply epitaxy at 1000 C. Thin Solid Films, 2011. 519(19): p. 6443-6446.
34. S. Sambonsuge, S. Jiao, H. Nagasawa, H. Fukidome, S.N. Filimonov, and M. Suemitsu, Formation of qualified epitaxial graphene on Si substrates using two-step heteroexpitaxy of C-terminated 3C-SiC (-1-1-1) on Si (110). Diamond and Related Materials, 2016. 67: p. 51-53.
35. P. Mondelli, B. Gupta, M.G. Betti, C. Mariani, J. Lipton-Duffin, and N. Motta, High quality epitaxial graphene by hydrogen-etching of 3C-SiC(111) thin-film on Si(111). Nanotechnology, 2017. 28(11): p. 115601.
36. K.V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G.L. Kellogg, L. Ley, J.L. McChesney, T. Ohta, S.A. Reshanov, and J. Röhrl, Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials, 2009. 8(3): p. 203-207.
37. P. Sutter, Epitaxial graphene: how silicon leaves the scene. Nature Materials, 2009. 8(3): p. 171-172.
38. I. Palacio, A. Celis, M.N. Nair, A. Gloter, A. Zobelli, M. Sicot, D. Malterre, M.S. Nevius, W.A. De Heer, and C. Berger, Atomic structure of epitaxial graphene sidewall nanoribbons: flat graphene, miniribbons, and the confinement gap. Nano Letters, 2014. 15(1): p. 182-189.
39. Z. Zhi, H. Ying, S. Xin-Yan, and L. Xing-Hua, Raman spectrum of epitaxial graphene grown on ion beam illuminated 6H-SiC (0001). Chinese Physics Letters, 2014. 31(11): p. 116801.
40. D. Fox, Y. Zhou, A. O’Neill, S. Kumar, J.J. Wang, J. Coleman, G. Duesberg, J. Donegan, and H. Zhang, Helium ion microscopy of graphene: beam damage, image quality and edge contrast. Nanotechnology, 2013. 24(33): p. 335702.
69
41. Y. Xu, K. Zhang, C. Brüsewitz, X. Wu, and H.C. Hofsäss, Investigation of the effect of low energy ion beam irradiation on mono-layer graphene. AIP Advances, 2013. 3(7): p. 072120.
42. Z. Liao, T. Zhang, M. Gall, A. Dianat, R. Rosenkranz, R. Jordan, G. Cuniberti, and E. Zschech, Lateral damage in graphene carved by high energy focused gallium ion beams. Applied Physics Letters, 2015. 107(1): p. 013108.
43. Q. Wang, Y. Shao, D. Ge, Q. Yang, and N. Ren, Surface modification of multilayer graphene using Ga ion irradiation. Journal of Applied Physics, 2015. 117(16): p. 165303.
44. B. Gupta, I. Di Bernardo, P. Mondelli, A. Della Pia, M. Betti, F. Iacopi, C. Mariani, and N. Motta, Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
45. A. Sandin, J.J. Rowe, and D.B. Dougherty, Improved graphene growth in UHV: pit-free surfaces by selective Si etching of SiC (0001)–Si with atomic hydrogen. Surface Science, 2013. 611: p. 25-31.
46. U. Starke and C. Riedl, Epitaxial graphene on SiC (0001) and: from surface reconstructions to carbon electronics. Journal of Physics: Condensed Matter, 2009. 21(13): p. 134016.
47. E. Rollings, G.-H. Gweon, S. Zhou, B. Mun, J. McChesney, B. Hussain, A. Fedorov, P. First, W. De Heer, and A. Lanzara, Synthesis and characterization of atomically thin graphite films on a silicon carbide substrate. Journal of Physics and Chemistry of Solids, 2006. 67(9): p. 2172-2177.
48. A.C. Ferrari, Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects. Solid State Communications, 2007. 143(1): p. 47-57.
49. L. Malard, M. Pimenta, G. Dresselhaus, and M. Dresselhaus, Raman spectroscopy in graphene. Physics Reports, 2009. 473(5): p. 51-87.
50. N. Gogneau, A. Balan, M. Ridene, A. Shukla, and A. Ouerghi, Control of the degree of surface graphitization on 3C-SiC (100)/Si (100). Surface Science, 2012. 606(3): p. 217-220.
51. M. Suemitsu, S. Jiao, H. Fukidome, Y. Tateno, I. Makabe, and T. Nakabayashi, Epitaxial graphene formation on 3C-SiC/Si thin films. Journal of Physics D: Applied Physics, 2014. 47(9): p. 094016.
52. N.F.W. Thissen, R.H.J. Vervuurt, J.J.L. Mulders, J.W. Weber, W.M.M. Kessels, and A.A. Bol, The effect of residual gas scattering on Ga ion beam patterning of graphene. Applied Physics Letters, 2015. 107(21): p. 213101.
53. W. Chen, H. Xu, L. Liu, X. Gao, D. Qi, G. Peng, S.C. Tan, Y. Feng, K.P. Loh, and A.T.S. Wee, Atomic structure of the 6H–SiC (0001) nanomesh. Surface Science, 2005. 596(1): p. 176-186.
54. J.F. Ziegler and J.P. Biersack, The stopping and range of ions in matter, in Treatise on Heavy-Ion Science. 1985, Springer. p. 93-129.
70
55. N. Mishra, J.J. Boeckl, A. Tadich, R.T. Jones, P.J. Pigram, M. Edmonds, M.S. Fuhrer, B.M. Nichols, and F. Iacopi, Solid source growth of graphene with Ni–Cu catalysts: towards high quality in situ graphene on silicon. Journal of Physics D: Applied Physics, 2017. 50(9): p. 095302.
56. Y. Liu, Y. Shen, L. Sun, J. Li, C. Liu, W. Ren, F. Li, L. Gao, J. Chen, and F. Liu, Elemental superdoping of graphene and carbon nanotubes. Nature Communications, 2016. 7.
57. Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai, and L. Qu, Nitrogen-doped graphene quantum dots with oxygen-rich functional groups. Journal of the American Chemical Society, 2011. 134(1): p. 15-18.
58. X. Wang, G. Sun, P. Routh, D.-H. Kim, W. Huang, and P. Chen, Heteroatom-doped graphene materials: syntheses, properties and applications. Chemical Society Reviews, 2014. 43(20): p. 7067-7098.
59. C. Rao, K. Gopalakrishnan, and A. Govindaraj, Synthesis, properties and applications of graphene doped with boron, nitrogen and other elements. Nano Today, 2014. 9(3): p. 324-343.
71
Nanotechnology, 2017. 28(34): p. 345602. DOI: https://doi.org/10.1088/1361-6528/aa752e
4.8 SUPPORTING INFORMATION
Epitaxial Graphene Growth on FIB Patterned 3C-SiC Nanostructures on Si
(111): Reducing Milling Damage
Mojtaba Amjadipour1, Jennifer MacLeod1, Josh Lipton-Duffin2, Francesca Iacopi3, and Nunzio
Motta1
1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, QLD, Australia.
2 Central Analytical Research Facility, Institute for Future Environments, Science and Engineering
Faculty, Queensland University of Technology, QLD, Australia.
3 School of Computing and Communications, Faculty of Engineering and Information Technology,
University of Technology Sydney, NSW, Australia.
Corresponding author. Tel: + 61731385104. E-mail: [email protected]
72
4.8.1 Determination of graphene layer thickness
Using XPS data and the fitted components it is possible to calculate the number of graphene
layers grown on the sample. By comparing the intensity ratio of C1s photoelectrons in graphene peak
(NG) to SiC peak (NSiC) as a reference peak, graphene thickness t can be calculated using equation 11
[1]:
NG
NSiC=
T ( EG)ρ′ CG λ′ (EG)[1 − exp −tλ′ (EG)]
T ( ESiC) ρ CSiC λ (ESiC) exp −tλ (ESiC)]
. F
(11)
where T is the analyzer transmission function, E is the kinetic energy of photoelectrons associated
with a specific peak, and C represents the differential cross section (dσ dΩ⁄ ); ρ stands for atomic
density of the element and λ is the electrons inelastic mean free path in the corresponding element.
F represents the geometrical correction factor and superscript ′ shows the parameters are related to
graphene as opposed to the reference peak (SiC). The same procedure has been used for calculating
the thickness of deposited Si layer by considering the intensity of the Si2p in Si peak and SiC peak.
73
Figure 4-7. XPS survey spectrum of sample after (a) degassing, (b) atomic hydrogen etching, and (c) graphene growth.
4.8.2 SRIM visualizations
Figure 4-8. SRIM simulation (Ga ions, 30 keV, 50000 particles) indicates that the Si layer does not alter the ions interactions or trajectories in the solid (a) without Si cap layer (b) with the Si cap layer.
74
4.8.3 XPS data collected at Si 2p
Figure 4-9. XPS spectrum of Si 2p: (a) after atomic hydrogen etching, (b) silicon deposition, (c) milling by FIB and
second atomic hydrogen etching, (d) final graphene growth.
4.8.4 Homogeneity of the silicon film at various length scales
Figure 4-10. (a) SEM image showing the area covered by the clamp of the sample holder which was not coated by silicon. (b) Higher magnification HIM image showing that the surface is covered by the silicon thin film. (c) Atomic force microscopy (AFM) image taken from the area coated by Si indicating its uniform coverage at the micron scale, where only typical SiC features are present. The inset shows a higher-resolution image with a line profile taken on the
region indicated by the yellow line; the variation in height across the terrace is less than 0.5 nm. AFM images were taken using an NT-MDT Solver Pro microscope equipped with an NT-MDT NSG30 cantilever and operated in non-
contact mode.
75
4.8.5 Compilation of Raman data Table 4-2. Raman data of graphene grown on 3C-SiC/Si.
Sample Type
D band G band
Position Area FWHM Position Area FWHM IG/ID
With Si mask on structures 1347 79314 64 1599 57654 66 0.73
With Si mask ~ 3 µm away from structures 1357 36454 52 1599 29741 56 0.82
Without Si mask away from structures 1359 25333 48 1599 21164 32 0.83
The best quality graphene fabricated by [2] 1359 1210000 53 1592 1080000 66 0.89
Graphene on 3C-SiC(111)/Si(111) by [3] - - - - - - 0.85
4.8.6 REFERENCES
1. Gupta, B., Notarianni, M., Mishra, N., Shafiei, M., Iacopi, F., & Motta, N., Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
2. Gupta, B., Di Bernardo, I., Mondelli, P., Della Pia, A., Betti, M., Iacopi, F., et al., Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
3. Sambonsuge, S., Jiao, S., Nagasawa, H., Fukidome, H., Filimonov, S.N., and Suemitsu, M., Formation of qualified epitaxial graphene on Si substrates using two-step heteroexpitaxy of C-terminated 3C-SiC (-1-1-1) on Si (110). Diamond and Related Materials, 2016. 67: p. 51-53.
76
Chapter 5: Free-Standing Graphene on 3C-SiC/Si(111)
5.1 ABSTRACT
Growing graphene on SiC thin films on Si is a cheaper alternative to the growth on bulk SiC,
and for this reason it has been recently intensively investigated. Here we study the effect of
hydrogen intercalation on epitaxial graphene obtained by high temperature annealing on 3C-
SiC/Si(111) in ultra-high vacuum (UHV). Using a combination of core-level photoelectron
spectroscopy (PES), low energy electron diffraction (LEED), and near-edge X-ray absorption fine
structure (NEXAFS) we find that hydrogen saturates the Si atoms at the topmost layer of the
substrate, leading to free-standing graphene on 3C-SiC/Si(111). The intercalated hydrogen fully
desorbs after heating the sample at 850 °C and the buffer layer appears again, similar to what has
been reported for bulk SiC. However, the NEXAFS analysis sheds new light on the effect of
77
hydrogen intercalation, showing an improvement of graphene’s flatness after annealing in atomic
H at 600˚C. These results provide new insight into free-standing graphene fabrication on SiC/Si
thin films.
Keywords: epitaxial graphene; H-intercalation; free-standing graphene.
78
Nanotechnology, 2018. 29(14): p. 145601. DOI: https://doi.org/10.1088/1361-6528/aaab1a
Quasi Free-Standing Epitaxial Graphene Fabrication on 3C-SiC/Si(111)
Mojtaba Amjadipour1, Anton Tadich2, John J Boeckl3, Josh Lipton-Duffin4, Jennifer MacLeod1,
Francesca Iacopi5, and Nunzio Motta1*
1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, QLD, Australia.
2 Australian Synchrotron, 800 Blackburn Road, Clayton, 3168 VIC, Australia.
3 Materials and Manufacturing Directorate, Air Force Research Laboratories, Wright-Patterson
AFB, 45433 OH, United States of America.
4 Central Analytical Research Facility, Institute for Future Environments, Science and
Engineering Faculty, Queensland University of Technology, QLD, Australia.
5 School of Computing and Communications, Faculty of Engineering and Information
Technology, University of Technology Sydney, NSW, Australia.
* Corresponding author. Tel: + 61731385104. E-mail: [email protected]
79
5.2 INTRODUCTION
Graphene grown on SiC, either bulk or thin films, is a promising platform for applications
in electronics and sensing [1-5]. The growth of graphene directly on SiC eliminates the transfer
step that is required to move graphene obtained by mechanical exfoliation or by CVD synthesis
on metal to an insulating or semiconducting substrate [6, 7]. Compared to bulk SiC, SiC/Si thin
films provide a cheaper and more versatile alternative for graphene fabrication as well as a better
integration with the standard microelectronic fabrication protocols [8-17]. After annealing SiC to
high temperature its topmost layer converts to an insulating carbon-rich layer with a 6√3 × 6√3
R30° structure, and graphene grows on top of this layer by progressive sublimation of Si atoms [4,
18-21]. About 30% of carbon atoms in the interface layer (commonly called the buffer layer) are
covalently bound to the substrate [22-24]. The presence of the buffer layer is suggested as the
reason that significant doping (~1× 1013 cm-2) is observed in graphene grown on SiC [25-29],
which adversely affects the carrier mobility [23, 30]. Therefore, elimination of the buffer layer is
necessary for growing graphene for device fabrication.
Breaking the carbon bonds to silicon within the topmost layer of SiC and saturating them
with some other species eliminates the buffer layer. Riedl et al [31] demonstrated that hydrogen
intercalation can perform this function and decouple graphene from the substrate by penetrating
between the buffer layer and the substrate and saturating the dangling Si bonds. This allows the
carbon atoms in the buffer layer to establish a pure sp2 bond which converts it to a graphene layer
[31]. In this way it is possible to produce free-standing bilayer graphene from monolayer graphene
grown on a bulk SiC (0001) [31]. Watcharinyanon et al [32] demonstrated by scanning tunneling
microscopy (STM) that hydrogen intercalation on monolayer graphene results in the creation of
(√3 × √3) bilayer graphene islands which gradually expand and fully cover the surface [32].
80
Sforzini et al [33] further investigated the hydrogen intercalation of graphene grown on bulk 6H-
SiC (0001) by measuring and calculating its adsorption height to evaluate the amount of interaction
between graphene and its substrate. They concluded that free-standing graphene fabricated on SiC
using H-intercalation has the lowest interaction with its substrate compared to a range of reported
substrates such as Ir, Ni, Re and Co, suggesting that SiC is an ideal platform for graphene
fabrication [33]. Furthermore, Yu et al [34] studied the effect of hydrogen intercalation on the
electrical transport properties of graphene, demonstrating that this procedure is very effective in
improving the mobility of graphene fabricated on bulk 4H-SiC (0001). They also investigated the
effect of SiC crystal properties and reported that lower crystal quality leads to more interaction
between graphene and the substrate [34]. It has been also demonstrated that H-intercalation
improves the performance of field-effect transistors fabricated from epitaxial graphene on SiC [35-
37]. Some of these studies also reported that the intercalation mechanism is reversible through
annealing [31, 32].
So far the fabrication of free-standing graphene has been considered only on bulk SiC [31-
34, 38-43]; due to the technological importance of SiC/Si we explored the production of free-
standing graphene by hydrogen intercalation on 3C-SiC thin films on Si(111). We combine, for
the first time, synchrotron radiation near-edge X-ray absorption fine structure (NEXAFS) with
core-level photoelectron spectroscopy (PES) and low energy electron diffraction (LEED), with the
aim of obtaining a clear picture of the intercalation process on graphene grown on 3C-SiC/Si(111).
5.3 EXPERIMENTAL DETAILS
A 1 µm thick 3C-SiC(111) layer grown on Si(111) was obtained from NOVASIC (France).
Chemical and mechanical polishing was performed on the samples (StepSiC® by NOVASIC
(France)), which reduced the surface roughness to ~ 1 nm [44]. The 3C-SiC/Si thin film is an
81
(111)-oriented epitaxial film of SiC grown on a single crystal of P-doped Si(111) ( 1-10 Ω-cm),
with a miscut of 0.5˚ and a typical terrace size of ~ 50 nm [17]. The wafer was cut to the size of
12×2 mm2. Prior to introduction into the UHV system for graphene growth each sample was
cleaned by 10 minutes sonication in each of acetone, isopropanol and deionized water,
respectively.
Synchrotron-based PES measurements were performed at the soft x-ray beamline at the
Australian Synchrotron. Samples were introduced into the UHV system at the beamline and
annealed at 400 °C for several hours in order to remove contaminants. An optical pyrometer
(IRCON Ultimax UX-20P with emissivity = 0.9) was used to measure the sample temperature.
PES measurements were performed using a SPECS Phoibos 150 hemispherical analyser, operating
at a pass energy of 10 eV.
An EFM-H atomic hydrogen source (FOCUS GmbH) was employed for atomic hydrogen
exposure operating at P ~ 5×10-6 mbar and 40 W power. Atomic hydrogen etching prior to
graphene growth is performed for all of the experiments assisting with elimination of
contamination and improving the surface flatness [17, 45, 46].
The epitaxial graphene growth procedure we employed includes: inserting the sample into
UHV system, degassing the sample for several hours at ~ 400 °C to eliminate contamination,
atomic hydrogen etching for about 30 minutes in order to further clean the sample and flatten the
surface, and finally annealing for graphene growth at temperatures ranging from 1200 - 1250 °C
for 5 - 10 minutes. The number of grown graphene layers is dependent on annealing time and
temperature; for further information in this regard refer to [20, 47]. Based on the results of this
growth process on similar substrates we expect terrace widths to be in a range of hundreds of
82
nanometers [46]. The H-intercalation process was performed on the grown graphene for 30
minutes at ~ 600 °C.
5.4 RESULTS AND DISCUSSIONS
5.4.1 PES
To investigate the chemical bonding changes during the H-intercalation experiment, the
sample was studied using core-level photoelectron spectroscopy (PES), which is based on
measuring the energy distribution of the electrons emitted as a result of X-ray irradiation [48]. PES
is a surface sensitive technique and is very powerful in providing information about the chemical
state of the atoms present in the sample [49]. Figure 5-1 shows C 1s and Si 2p core level spectrum
measured during intercalation at photon energies of 330 eV and 150 eV, respectively. The C1s
spectrum are fitted with different components: Si-C at ~ 283.6 eV, graphene at ~ 284.7, and buffer
layer related components S1 and S2 at ~ 285 eV and ~ 285.6 eV, respectively [18, 50, 51]. The Si
2p spectrum are fitted with two spin-orbit split doublets (Si 2p3/2 and Si 2p1/2) for Si-C at ~ 101.3
eV, Si-H at ~ 101 eV, buffer layer at ~ 100.7 eV and Si at ~ 99.4 eV; the energies are given for Si
2p3/2 and the splitting of the Si 2p3/2 and Si 2p1/2 components is 0.6 eV [31, 52]. The fits use a
combination of Gaussian and Lorentzian line shapes (Voigt), and the background was subtracted
using the Shirley procedure [52]; the graphene component was fitted with an asymmetric peak.
Figure 5-1a shows the PES spectrum for the as-grown graphene, which approximately
corresponds to a three-layer graphene sample; the graphene thickness was calculated using an
equation based on the differential cross sections and the inelastic mean free paths of electrons
suggested by Rollings et al [18, 53]. Here cross section of an atom refers to the number of electrons
excited per unit time divided by the number of incident photons per unit time per unit area and the
inelastic mean free path of electrons is the average distance an electron can travel into a solid
83
before scattering. Peaks (S1 and S2) related to the buffer layer are apparent in the C 1s region for
the as-grown epitaxial graphene (Figure 5-1a). After exposure to atomic hydrogen, these peaks
completely disappear, indicating that H-intercalation has removed the interface layer (Figure 5-
1b) [31, 32]. The Si 2p spectrum confirms the presence of Si-H bonds under the graphene layer
(Figure 5-1f), which originates from Si atoms bonded to H as a result of H exposure [21, 31]. These
are mainly the Si atoms at the topmost layer of the SiC substrate saturated by hydrogen bonding.
The buffer layer component in the Si 2p spectrum also disappears after H-intercalation (Figure 5-
1e and f).
A closer look at the spectrum reveals that after intercalation the graphene-related peak in C
1s region is shifted by ~ 0.1 eV (Table 5-2, supporting information) towards lower binding energies
(Figure 5-1b and 2a). The SiC component in Si 2p region is also shifted by ~ 0.4 eV to lower
binding energies (Figure 5-1f and 2b). These shifts in binding energies are known to occur as a
result of band bending induced by the hydrogen termination of the surface of SiC [31, 54]. The
shift observed here after H-intercalation of graphene on SiC/Si thin film is smaller than the
previously-reported value for graphene on bulk 4H/6H SiC by Riedl et al (~0.4 for graphene
component and 1 eV for SiC one) [31]. We believe the main reason for the difference in observed
band bending is the different crystal structure, and therefore different intrinsic electronic structure,
of the SiC substrates in the present and previous work (3C in the present case vs. 4H/6H in the
previous work). Furthermore, for our 3C-SiC/Si epitaxial thin films we expect that the limited
domain size and relatively high defect density may have a nontrivial effect on the electronic
properties.
After atomic H exposure, the intensity of the graphene peak increases, indicating that the
carbon atoms in the buffer layer, originally linked to the Si atoms, joined the graphene layer
84
(Fig.1b). The comparison of the peak intensity further confirms this, as the ratio of graphene-
related spectral weight to silicon carbide spectral weight, R = Igr+IS1+IS2ISiC
≅ 12 , remains constant
through the intercalation. In this expression, Igr, IS1, IS2, and ISiC are intensities of the components
related to graphene, S1, S2, and SiC in C1s spectrum, respectively. Therefore, as a result of H-
intercalation a ~ three-layer graphene converts into a ~ four-layer graphene. The effect of H-
intercalation in monolayer and bilayer graphene samples has been reported in the supporting
information (Figure 5-8).
85
Figure 5-1. C 1s core-level photoemission spectrum at 330 eV photon energy (a) as grown graphene (b) after hydrogen intercalation (c) after hydrogen desorption by annealing to 700 °C (d) after annealing to 850 °C. Si 2p
core-level photoemission spectrum at 150 eV photon energy (e) as grown graphene (f) after hydrogen intercalation (g) after hydrogen desorption by annealing to 700 °C (h) after annealing to 850 °C.
86
By annealing the H-treated sample in vacuum to 700 °C, the buffer layer is partially restored
(Figure 1c and g), due to the fact that the Si-H bonds are not stable at temperatures over 700 °C
[31, 32]. After further annealing the H-intercalated sample to 850 °C, the signatures of buffer layer
fully recover to their original intensity (Figure 5-1d and h). The shift in binding energies observed
as a result of H-intercalation are also recovered after annealing to 850 °C (Figure 5-2). H-
intercalated samples were further studied after being exposed to air for 5 days and no significant
change was observed (refer to supporting information). This indicates that the free-standing
graphene prepared by this procedure is stable in air which is similar to free-standing graphene
fabricated on bulk SiC [31, 32].
Figure 5-2. Core-level photoemission spectrum of (a) C 1s at 330 eV photon energy, (b) Si 2p at 150 eV photon
energy.
5.4.2 LEED
Figure 5-3 shows the LEED patterns of the ~3 layer graphene sample before and after H
intercalation. LEED is widely used for investigating the surface structure and is based on observing
the electrons diffracted from the surface during bombardment by a low-energy electron beam
(typically in a range of 20-200 eV) [55]. Each of the three primary SiC spots are surrounded by
six less intense superstructure spots that originate from the buffer layer (Figure 5-3a) [7]. These
87
spots originating from the 6√3 × 6√3 interface layer vanish after exposure to hydrogen (Figure
5-3b), suggesting the removal of the buffer layer. The LEED patterns of the sample are unchanged
after being exposed to air for 5 days (refer to supporting information).
Figure 5-3. LEED pattern (a) after graphene growth (b) after H intercalation.
5.4.3 NEXAFS
The unoccupied electronic states of graphene grown on a 3C-SiC/Si thin film were studied
using near-edge X-ray absorption fine structure (NEXAFS) to further understand how the
hydrogen intercalation affects the interface layer. Figure 5-4 shows the carbon K-edge NEXAFS
spectrum from monolayer epitaxial graphene grown on 3C-SiC/Si thin film. NEXAFS was
acquired with two different incident angles of the synchrotron light with respect to the surface
normal: 0° (normal) and 70° (grazing). The NEXAFS spectra are characterized by two main peaks
at 285.3 and 291.6 eV which correspond to transitions from the C1s core level to π* and σ* empty
states respectively [9].
Varying the incident angle modifies the probability for different transitions for C1s. 1s - π*
has higher probability for the electric field vector perpendicular to the molecular plane, and 1s - σ*
is maximized for a parallel field [9]. As a consequence, the intensity of the σ* peak significantly
88
decreases at grazing angles, and the π* transition has the opposite behavior. NEXAFS was
conducted on three different samples before and after H-intercalation with different numbers of
graphene layers: monolayer, bilayer and ~ 3 graphene layers.
Figure 5-4 shows C1s NEXAFS spectrum for monolayer graphene before and after H-
intercalation. The spectrum for the H-intercalated sample in the σ* region is sharper (Figure 5-4b)
due to the lower interaction of the graphene layer with the substrate, which decreases its level of
doping [56]. The intensity (calculated from the area under the peak) of the π* band increases after
H intercalation, which is expected, due to the increase in sp2 hybridization after the bonds are
broken. Hydrogen mainly breaks the bonds between the buffer layer and the topmost layer of SiC
substrate and saturates the dangling bonds at the interface. This changes the sp3 nature of the bonds
and converts them to sp2 of graphene, causing the intensity increase in the σ* and π* region of the
NEXAFS spectrum. The π* region shows also the appearance of a shoulder at higher photon
energies (~287 eV), which can be linked to carbon atoms bonded to hydrogen atoms at the edges
of SiC terraces as a byproduct of hydrogen intercalation (Figure 5-7) [57-60].
89
Figure 5-4. C 1s NEXAFS spectrum for monolayer graphene (a) full spectrum (b) σ* peak region at normal
incidence angle (c) π* peak region at grazing incidence angle.
Figure 5-5 shows NEXAFS data acquired from a bilayer and three layer graphene samples
before and after H-intercalation. The σ* region for the bilayer sample shows that the peak becomes
sharper after intercalation, although this effect is less evident than in the monolayer case (compare
Figure 5-4b and 5a). The increase in intensity of the π* peak between H-intercalated and as grown
samples is also observable, but to a lower extent compared to the monolayer sample (compare
Figure 5-4c and 5b). The shoulder at 287 eV is still observable, but with lower intensity. For the
three-layer graphene sample a significantly lower intensity change can be noted in both the σ* and
in π* peaks as a result of H-intercalation (Figure 5-5c and d), and the shoulder at higher photon
energies in π* region is hardly noticeable (Figure 5-5d).
90
Figure 5-5. C 1s NEXAFS spectrum for bilayer and three-layers graphene (a) σ* peak region at normal incidence
angle, for bilayer graphene sample (b) π* peak region at grazing incidence, for bilayer graphene sample (c) σ* peak region at normal incidence angle, for three-layer graphene sample (d) π* peak region at grazing incidence angle, for
three-layer graphene sample.
In order to enhance the details of this effect we calculated the differential spectra by
subtracting the normalized NEXAFS spectrum before and after H-intercalation for the monolayer,
bilayer and three-layer graphene (Figure 5-6). As expected the change in intensity as a result of
the intercalation is larger for the thinnest graphene sample, and by increasing the number of
graphene layers the intensity change decreases significantly. In particular, the shoulder appearing
at 287 eV is hardly noticeable for the sample with three graphene layers, indicating that this feature
is definitely related to the interface between first graphene layer and SiC surface, in agreement
with our interpretation which assigns this feature to carbon atoms at the topmost layer of the
substrate, bonded to hydrogen atoms [58].
91
Figure 5-6. C 1s NEXAFS differential spectrum made of H-intercalation spectrum –as grown graphene one (a) π* peak region at grazing incidence angle, (b) σ* peak region at normal incidence angle.
In order to further study the H-intercalation process, a dichroic ratio DR = Iϴ1− Iϴ2Iϴ1+ Iϴ2
is
calculated, where Iϴ1and Iϴ2show the π* peak region intensity which is extrapolated by integrating
the area under the peak for normal incident angle (ϴ1 = 0) and grazing incident angle (ϴ2 = 90).
The dichroic ratio is a measure for assessing the alignment of graphene, and is expected to equal
to -1 for a completely flat graphene and becomes 0 for a randomly oriented sample [58]. H-
intercalation improves the DR and makes the graphene flatter (Table 5-1 and Figure 5-7). This
increase in DR is smaller for the sample with more graphene layers (from -0.068 for monolayer to
-0.026 for the three-layer one). This is due to the fact that most of the improvement in the
orientation of graphene stems from elimination of the buffer layer after H-intercalation, and by
increasing the number of the graphene layer, the contribution of the buffer layer to the signal
becomes less significant.
92
Table 5-1. Dichroic ratio (DR) calculated for the samples before and after H-intercalation.
Graphene condition
DR for As-grown
DR for H-intercalated DR change
~ 1 layer -0.542 -0.610 -0.068
~ 2layers -0.545 -0.596 -0.051
~ 3 layers -0.630 -0.663 -0.026
Figure 5-7. Graphical representation of H-intercalation (a) monolayer graphene formed with the buffer layer on SiC (b) free-standing bilayer graphene fabricated as a result of the H-intercalation.
5.5 CONCLUSIONS
We demonstrated that hydrogen intercalation can eliminate the buffer layer at the interface
between graphene and 3C-SiC/Si(111) thin films, producing free-standing graphene. The carbon
atoms initially contained in the buffer layer create a new graphene layer and convert monolayer
graphene into bilayer (Figure 5-7). NEXAFS spectra indicate an increase in intensity of π* and σ*
peaks, confirming a reduction of the substrate’s effect on graphene. The change in intensity due to
H-intercalation in the π* and σ* regions decreases by increasing the number of graphene layers,
indicating that the substrate contribution becomes less noticeable by NEXAFS for thicker
graphene samples. Moreover, the free-standing graphene fabricated on the 3C-SiC/Si(111) thin
film is stable in air. Finally, we demonstrated that the intercalation procedure is a reversible
process, and that hydrogen desorbs as a result of heating the sample to 850 °C, resulting in the
reformation of the buffer layer (Figure 5-7).
93
5.6 ACKNOWLEDGMENTS
The authors acknowledge the support of the Queensland Government through the Q-CAS
Collaborative Science Fund 2016. This research was undertaken on the Soft X-ray Beamline at the
Australian Synchrotron, part of ANSTO. The authors acknowledge the support of the Australian
Synchrotron and ANSTO. Prof Patrick Soukiassian and Dr Neeraj Mishra are kindly
acknowledged for their help and support in this research.
5.7 REFERENCES
1. A. Van Bommel, J. Crombeen, and A. Van Tooren, LEED and Auger electron observations of the SiC (0001) surface. Surface Science, 1975. 48(2): p. 463-472.
2. A. Celis, M. Nair, A. Taleb-Ibrahimi, E. Conrad, C. Berger, W. De Heer, and A. Tejeda, Graphene nanoribbons: fabrication, properties and devices. Journal of Physics D: Applied Physics, 2016. 49(14): p. 143001.
3. M. Beshkova, L. Hultman, and R. Yakimova, Device applications of epitaxial graphene on silicon carbide. Vacuum, 2016. 128(Supplement C): p. 186-197.
4. N. Mishra, J. Boeckl, N. Motta, and F. Iacopi, Graphene growth on silicon carbide: a review. Physica Status Solidi (a), 2016. 213(9): p. 2277-2289.
5. B.V. Cunning, M. Ahmed, N. Mishra, A.R. Kermany, B. Wood, and F. Iacopi, Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers. Nanotechnology, 2014. 25(32): p. 325301.
6. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, and W.A. De Heer, Electronic confinement and coherence in patterned epitaxial graphene. Science, 2006. 312(5777): p. 1191-1196.
7. C. Riedl, U. Starke, J. Bernhardt, M. Franke, and K. Heinz, Structural properties of the graphene-SiC (0001) interface as a key for the preparation of homogeneous large-terrace graphene surfaces. Physical Review B, 2007. 76(24): p. 245406.
8. M. Suemitsu, Y. Miyamoto, H. Handa, and A. Konno, Graphene formation on a 3C-SiC(111) thin film grown on Si(110) substrate. e-Journal of Surface Science and Nanotechnology, 2009. 7: p. 311-313.
9. V.Y. Aristov, G. Urbanik, K. Kummer, D.V. Vyalikh, O.V. Molodtsova, A.B. Preobrajenski, A.A. Zakharov, C. Hess, T. Hänke, and B. Buchner, Graphene synthesis on cubic SiC/Si wafers. Perspectives for mass production of graphene-based electronic devices. Nano Letters, 2010. 10(3): p. 992-995.
10. A. Ouerghi, R. Belkhou, M. Marangolo, M. Silly, S. El Moussaoui, M. Eddrief, L. Largeau, M. Portail, and F. Sirotti, Structural coherency of epitaxial graphene on 3C–SiC (111) epilayers on Si (111). Applied Physics Letters, 2010. 97(16): p. 161905.
94
11. Y. Miyamoto, H. Handa, E. Saito, A. Konno, Y. Narita, M. Suemitsu, H. Fukidome, T. Ito, K. Yasui, H. Nakazawa, and T. Endoh, Raman-scattering spectroscopy of epitaxial graphene formed on SiC film on Si substrate. e-Journal of Surface Science and Nanotechnology, 2009. 7: p. 107-109.
12. M. Suemitsu and H. Fukidome, Epitaxial graphene on silicon substrates. Journal of Physics D: Applied Physics, 2010. 43(37): p. 374012.
13. F. Hirokazu, M. Yu, H. Hiroyuki, S. Eiji, and S. Maki, Epitaxial growth processes of graphene on silicon substrates. Japanese Journal of Applied Physics, 2010. 49(1S): p. 01AH03.
14. A. Severino, C. Bongiorno, N. Piluso, M. Italia, M. Camarda, M. Mauceri, G. Condorelli, M. Di Stefano, B. Cafra, and A. La Magna, High-quality 6inch (111) 3C-SiC films grown on off-axis (111) Si substrates. Thin Solid Films, 2010. 518(6): p. S165-S169.
15. L. Wang, S. Dimitrijev, J. Han, A. Iacopi, L. Hold, P. Tanner, and H.B. Harrison, Growth of 3C–SiC on 150-mm Si (100) substrates by alternating supply epitaxy at 1000 C. Thin Solid Films, 2011. 519(19): p. 6443-6446.
16. F. Iacopi, G. Walker, L. Wang, L. Malesys, S. Ma, B.V. Cunning, and A. Iacopi, Orientation-dependent stress relaxation in hetero-epitaxial 3C-SiC films. Applied Physics Letters, 2013. 102(1): p. 011908.
17. M. Amjadipour, M. MacLeod, J. Lipton-Duffin, F. Iacopi, and N. Motta, Epitaxial graphene growth on FIB patterned 3C-SiC nanostructures on Si (111): reducing milling damage. Nanotechnology, 2017. 28(34): p. 345602.
18. B. Gupta, M. Notarianni, N. Mishra, M. Shafiei, F. Iacopi, and N. Motta, Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
19. B. Gupta, E. Placidi, C. Hogan, N. Mishra, F. Iacopi, and N. Motta, The transition from 3C SiC (111) to graphene captured by Ultra High Vacuum Scanning Tunneling Microscopy. Carbon, 2015. 91: p. 378-385.
20. F. Zarotti, B. Gupta, F. Iacopi, A. Sgarlata, M. Tomellini, and N. Motta, Time evolution of graphene growth on SiC as a function of annealing temperature. Carbon, 2016. 98: p. 307-312.
21. S. Goler, C. Coletti, V. Piazza, P. Pingue, F. Colangelo, V. Pellegrini, K.V. Emtsev, S. Forti, U. Starke, F. Beltram, and S. Heun, Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene. Carbon, 2013. 51: p. 249-254.
22. U. Starke and C. Riedl, Epitaxial graphene on SiC (0001) and: from surface reconstructions to carbon electronics. Journal of Physics: Condensed Matter, 2009. 21(13): p. 134016.
23. A. Mattausch and O. Pankratov, Ab initio study of graphene on SiC. Physical Review Letters, 2007. 99(7): p. 076802.
24. K.V. Emtsev, F. Speck, T. Seyller, L. Ley, and J.D. Riley, Interaction, growth, and ordering of epitaxial graphene on SiC0001 surfaces: A comparative photoelectron spectroscopy study. Physical Review B, 2008. 77(15): p. 155303.
95
25. T. Ohta, A. Bostwick, T. Seyller, K. Horn, and E. Rotenberg, Controlling the electronic structure of bilayer graphene. Science, 2006. 313(5789): p. 951-954.
26. C. Riedl, A. Zakharov, and U. Starke, Precise in situ thickness analysis of epitaxial graphene layers on SiC (0001) using low-energy electron diffraction and angle resolved ultraviolet photoelectron spectroscopy. Applied Physics Letters, 2008. 93(3): p. 033106.
27. K.V. Emtsev, A.A. Zakharov, C. Coletti, S. Forti, and U. Starke, Ambipolar doping in quasifree epitaxial graphene on SiC (0001) controlled by Ge intercalation. Physical Review B, 2011. 84(12): p. 125423.
28. P. Mallet, F. Varchon, C. Naud, L. Magaud, C. Berger, and J.-Y. Veuillen, Electron states of mono-and bilayer graphene on SiC probed by scanning-tunneling microscopy. Physical Review B, 2007. 76(4): p. 041403.
29. U. Starke, S. Forti, K. Emtsev, and C. Coletti, Engineering the electronic structure of epitaxial graphene by transfer doping and atomic intercalation. Mrs Bulletin, 2012. 37(12): p. 1177-1186.
30. K. Emtsev, F. Speck, T. Seyller, L. Ley, and J.D. Riley, Interaction, growth, and ordering of epitaxial graphene on SiC 0001 surfaces: A comparative photoelectron spectroscopy study. Physical Review B, 2008. 77(15): p. 155303.
31. C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, and U. Starke, Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation. Physical Review Letters, 2009. 103(24): p. 246804.
32. S. Watcharinyanon, C. Virojanadara, J. Osiecki, A. Zakharov, R. Yakimova, R. Uhrberg, and L.I. Johansson, Hydrogen intercalation of graphene grown on 6H-SiC (0001). Surface Science, 2011. 605(17): p. 1662-1668.
33. J. Sforzini, L. Nemec, T. Denig, B. Stadtmüller, T.L. Lee, C. Kumpf, S. Soubatch, U. Starke, P. Rinke, V. Blum, F.C. Bocquet, and F.S. Tautz, Approaching truly freestanding graphene: the structure of hydrogen-intercalated graphene on 6H-SiC(0001). Physical Review Letters, 2015. 114(10): p. 106804.
34. C. Yu, Q. Liu, J. Li, W. Lu, Z. He, S. Cai, and Z. Feng, Preparation and electrical transport properties of quasi free standing bilayer graphene on SiC (0001) substrate by H intercalation. Applied Physics Letters, 2014. 105(18): p. 183105.
35. J.A. Robinson, M. Hollander, M. LaBella, K.A. Trumbull, R. Cavalero, and D.W. Snyder, Epitaxial graphene transistors: enhancing performance via hydrogen intercalation. Nano Letters, 2011. 11(9): p. 3875-3880.
36. M. Winters, O. Habibpour, I.G. Ivanov, J. Hassan, E. Janzén, H. Zirath, and N. Rorsman, Assessment of H-intercalated graphene for microwave FETs through material characterization and electron transport studies. Carbon, 2015. 81: p. 96-104.
37. C. Yu, Z. He, J. Li, X. Song, Q. Liu, S. Cai, and Z. Feng, Quasi-free-standing bilayer epitaxial graphene field-effect transistors on 4H-SiC (0001) substrates. Applied Physics Letters, 2016. 108(1): p. 013102.
96
38. Y. Murata, T. Cavallucci, V. Tozzini, N. Pavliček, L. Gross, G. Meyer, M. Takamura, H. Hibino, F. Beltram, and S. Heun, Atomic and electronic structure of Si dangling bonds in quasi-free-standing monolayer graphene. Nano Research, 2017. 11(2): p. 864-873.
39. T. Ciuk, P. Caban, and W. Strupinski, Charge carrier concentration and offset voltage in quasi-free-standing monolayer chemical vapor deposition graphene on SiC. Carbon, 2016. 101: p. 431-438.
40. J. Hassan, M. Winters, I.G. Ivanov, O. Habibpour, H. Zirath, N. Rorsman, and E. Janzén, Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0001) layers. Carbon, 2015. 82: p. 12-23.
41. M. Ostler, F. Fromm, R.J. Koch, P. Wehrfritz, F. Speck, H. Vita, S. Böttcher, K. Horn, and T. Seyller, Buffer layer free graphene on SiC(0001) via interface oxidation in water vapor. Carbon, 2014. 70: p. 258-265.
42. C. Xia, L.I. Johansson, Y. Niu, A.A. Zakharov, E. Janzén, and C. Virojanadara, High thermal stability quasi-free-standing bilayer graphene formed on 4H–SiC(0001) via platinum intercalation. Carbon, 2014. 79: p. 631-635.
43. T. Hu, H. Bao, S. Liu, X. Liu, D. Ma, F. Ma, and K. Xu, Near-free-standing epitaxial graphene on rough SiC substrate by flash annealing at high temperature. Carbon, 2017. 120: p. 219-225.
44. B. Gupta, I. Di Bernardo, P. Mondelli, A. Della Pia, M.G. Betti, F. Iacopi, C. Mariani, and N. Motta, Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
45. A. Sandin, J.J. Rowe, and D.B. Dougherty, Improved graphene growth in UHV: pit-free surfaces by selective Si etching of SiC (0001)–Si with atomic hydrogen. Surface Science, 2013. 611: p. 25-31.
46. P. Mondelli, B. Gupta, M.G. Betti, C. Mariani, J.L. Duffin, and N. Motta, High quality epitaxial graphene by hydrogen-etching of 3C-SiC (111) thin-film on Si (111). Nanotechnology, 2017. 28(11): p. 115601.
47. B. Gupta, Notarianni, M., Mishra, N., Shafiei, M., Iacopi, F., & Motta, N., Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
48. E. Sokolowski, C. Nordling, and K. Siegbahn, Magnetic analysis of X-ray produced photo and auger electrons. Arkiv Fysik, 1957.
49. J.M. Hollander and W.L. Jolly, X-ray photoelectron spectroscopy. Accounts of chemical research, 1970. 3(6): p. 193-200.
50. C. Riedl, C. Coletti, and U. Starke, Structural and electronic properties of epitaxial graphene on SiC (0 0 0 1): a review of growth, characterization, transfer doping and hydrogen intercalation. Journal of Physics D: Applied Physics, 2010. 43(37): p. 374009.
51. L.H. de Lima, D. Handschak, F. Schonbohm, R. Landers, C. Westphal, and A. de Siervo, The atomic structure of a bare buffer layer on SiC(0001) chemically resolved. Chemical Communications, 2014. 50(88): p. 13571-13574.
97
52. N. Sieber, T. Seyller, L. Ley, D. James, J. Riley, R.C. Leckey, and M. Polcik, Synchrotron x-ray photoelectron spectroscopy study of hydrogen-terminated 6 H− SiC 0001 surfaces. Physical Review B, 2003. 67(20): p. 205304.
53. E. Rollings, G.H. Gweon, S.Y. Zhou, B.S. Mun, J.L. McChesney, B.S. Hussain, A.V. Fedorov, P.N. First, W.A. De Heer, and A. Lanzara, Synthesis and characterization of atomically thin graphite films on a silicon carbide substrate. Journal of Physics and Chemistry of Solids, 2006. 67(9): p. 2172-2177.
54. N. Sieber, B.F. Mantel, T. Seyller, J. Ristein, L. Ley, T. Heller, D.R. Batchelor, and D. Schmeißer, Electronic and chemical passivation of hexagonal 6H–SiC surfaces by hydrogen termination. Applied Physics Letters, 2001. 78(9): p. 1216-1218.
55. M.A. Vanhove, W.H. Weinberg, and C.-M. Chan, Low-energy electron diffraction: experiment, theory and surface structure determination. Vol. 6. 2012: Springer Science & Business Media.
56. C. Ehlert, W.E. Unger, and P. Saalfrank, C K-edge NEXAFS spectra of graphene with physical and chemical defects: a study based on density functional theory. Physical Chemistry Chemical Physics, 2014. 16(27): p. 14083-14095.
57. B.J. Schultz, C.J. Patridge, V. Lee, C. Jaye, P.S. Lysaght, C. Smith, J. Barnett, D.A. Fischer, D. Prendergast, and S. Banerjee, Imaging local electronic corrugations and doped regions in graphene. Nature Communications, 2011. 2: p. 372.
58. V. Lee, C. Park, C. Jaye, D.A. Fischer, Q. Yu, W. Wu, Z. Liu, J. Bao, S.-S. Pei, C. Smith, P. Lysaght, and S. Banerjee, Substrate hybridization and rippling of graphene evidenced by near-edge X-ray absorption fine structure spectroscopy. The Journal of Physical Chemistry Letters, 2010. 1(8): p. 1247-1253.
59. A. Preobrajenski, M.L. Ng, A. Vinogradov, and N. Mårtensson, Controlling graphene corrugation on lattice-mismatched substrates. Physical Review B, 2008. 78(7): p. 073401.
60. L.G.M. Pettersson, H. Ågren, Y. Luo, and L. Triguero, Benzene adsorbed on Cu(110): theoretical X-ray absorption, emission and shake calculations. Surface Science, 1998. 408(1): p. 1-20.
98
Nanotechnology, 2018. 29(14): p. 145601. DOI: https://doi.org/10.1088/1361-6528/aaab1a
5.8 SUPPORTING INFORMATION
Quasi Free-Standing Epitaxial Graphene Fabrication on 3C-SiC/Si(111)
Mojtaba Amjadipour1, Anton Tadich2, John J Boeckl3, Josh Lipton-Duffin4, Jennifer MacLeod1,
Francesca Iacopi5, and Nunzio Motta1*
1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, QLD, Australia.
2 Australian Synchrotron, 800 Blackburn Road, Clayton, 3168 VIC, Australia.
3 Materials and Manufacturing Directorate, Air Force Research Laboratories, Wright-Patterson
AFB, 45433 OH, United States of America.
4 Central Analytical Research Facility, Institute for Future Environments, Science and Engineering
Faculty, Queensland University of Technology, QLD, Australia.
5 School of Computing and Communications, Faculty of Engineering and Information Technology,
University of Technology Sydney, NSW, Australia.
* Corresponding author. Tel: + 61731385104. E-mail: [email protected]
99
Figure 5-8. C 1s core-level photoemission spectrum at 330 eV photon energy before and after H-intercalation (a) monolayer graphene (b) Bilayer graphene.
Figure 5-9. C 1s core-level photoemission spectrum at 330 eV photon energy for bilayer graphene after H-intercalation and being exposed to ambient conditions for 5 days.
100
Table 5-2. Fitting results for the core-level photoemission spectrum of binding energy (BE) position (±0.2 eV), full-width at half-maximum (FWHM), and relative intensity (peak areas).
Condition Component Position FWHM Line shape Intensity
As-Grown
Graphene
SiC 283.64 1.14 0.2 1.4E+05
Graphene 284.76 0.8 0.95 1.3E+06
Buffer 285 1 0.2 1.4E+05
Buffer2 285.6 0.92 0.3 2.7E+05
H-Intercalated
SiC 283.35 1 0.2 1.5E+05
Graphene 284.63 0.7 0.95 1.8E+07
Buffer - - - 0.0E+00
700 °C
SiC 283.4 1.1 0.2 1.4E+05
Graphene 284.61 0.65 0.9 1.5E+06
Buffer 285 0.7 0.2 7.2E+04
Buffer2 285.57 0.9 0.3 1.4E+05
850 °C
SiC 283.64 1.13 0.2 1.6E+05
Graphene 284.75 0.8 0.95 1.4E+06
Buffer 285 0.9 0.2 1.3E+05
Buffer 2 285.63 0.84 0.3 2.5E+05
Figure 5-10. LEED pattern (a) after H-intercalation (b) after being exposed to ambient conditions for 5 days.
101
Figure 5-11. C 1s NEXAFS spectrum after annealing to 1000 °C (a) full spectrum (b) σ* peak region at normal incidence angle (c) π* peak region at grazing incidence angle.
Figure 5-12. C 1s NEXAFS spectrum before and after H-intercalation (a) bilayer graphene (b) three-layer graphene
sample.
102
Chapter 6: Electron Effective Attenuation Length in Epitaxial Graphene on SiC
6.1 ABSTRACT
The inelastic mean free path (IMFP) for carbon-based materials is notoriously challenging to
model, and moving from bulk materials to 2D materials may exacerbate this problem, making the
accurate measurements of IMFP in 2D carbon materials critical. The overlayer-film method is a
common experimental method to estimate IMFP by measuring electron effective attenuation
length (EAL). This estimation relies on an assumption that elastic scattering effects are negligible.
We report here an experimental measurement of electron EAL in epitaxial graphene on SiC using
photoelectron spectroscopy (PES) over an electron kinetic energy range of 50-1150 eV. We find a
significant effect of the interface between the 2D carbon material and the substrate, indicating that
the attenuation length in the so-called ‘buffer layer’ is smaller than for free-standing graphene. Our
results also suggest that the existing models for estimating IMFPs may not adequately capture the
physics of electron interactions in 2D materials.
103
Keywords: epitaxial graphene; effective attenuation length (EAL); inelastic mean free path
(IMFP); photoelectron spectroscopy.
104
Nanotechnology, 2012019, 30 (2), 025704. DOI: https://dx.doi.org/10.1088/1361-6528/aae7ec
Effective Attenuation Length in Epitaxial Graphene on SiC
Mojtaba Amjadipour1, Jennifer MacLeod1, Josh Lipton-Duffin2, Anton Tadich3, John J Boeckl4,
Francesca Iacopi5, and Nunzio Motta1*
1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, QLD, Australia
2 Central Analytical Research Facility, Institute for Future Environments, Science and
Engineering Faculty, Queensland University of Technology, QLD, Australia.
3 Australian Synchrotron, 800 Blackburn Road, Clayton, 3168 VIC, Australia.
4 Materials and Manufacturing Directorate, Air Force Research Laboratories, Wright-Patterson
AFB, 45433 OH, United States of America.
5 School of Electrical and Data Engineering, Faculty of Engineering and Information
Technology, University of Technology Sydney, NSW, Australia.
* Corresponding author. Tel: + 61731385104. E-mail: [email protected]
105
6.2 INTRODUCTION
The average distance that electron moves through a material before undergoing an energy-
loss scattering event is known as the inelastic mean free path (IMFP) [1, 2]. Energy-loss events
are caused primarily by electron-electron or electron-phonon scattering. Electronic scattering
significantly depends on the electron kinetic energy, and therefore in photoemission these losses
are proportional to the photoemission excitation energy. Conversely, electron-phonon scattering
does not show any significant energy dependence and the losses are very low (~ 50 meV). As a
result, for electrons with kinetic energy greater than ~ 6 eV, energy loss due to electron-phonon
scattering is negligible and electron-electron scattering dominates [3]. A precise understanding of
the IMFP for different materials is vital for a number of measurement techniques such as
photoelectron spectroscopy (PES) [4], photoelectron diffraction [5], Auger electron spectroscopy
(AES) [6], and electron microscopy [7].
Comprehensive theoretical models have been developed to predict the IMFP in a number of
solids over a range of kinetic energies from 50 eV to 200 keV [8-14]. A series of works by Tanuma
et al report calculated IMFPs for a number of elements [8-11, 15-18]. These calculations rely on
an algorithm developed by Penn [19] based on experimental optical data from synchrotron
radiation studies [20], which relates the inelastic scattering probability to energy loss, and the
Lindhard dielectric function, which describes the scattering probability as a function of momentum
transfer. They introduced an equation called Tanuma, Powell, and Penn (TPP-2M) to predict IMFP
of electrons in different compounds and elemental solids at 50-2000 eV energy range [21]. The
parameters in TPP-2M model can be derived from material properties such as density, atomic
weight, and number of valence electrons per atom [15, 21].
106
IMFP can be experimentally estimated by photoelectron spectroscopy (PES) [3] and Auger
electron spectroscopy (AES) using the overlayer-film method [22, 23]. This method is based on
growing or depositing a thin layer of one material on another and investigating the peak intensity
change of the substrate as a function of thickness of the deposited layer [24]. The intensity change
is used to calculate the effective attenuation length (EAL), which is a projected distance of the
IMFP along the incident electron direction [25]. EAL and IMFP are frequently used
interchangeably, but it is important to distinguish between them [1, 2]. The overlayer method
estimates IMFP to be equal to the EAL by assuming that the elastic scattering effects are negligible
and that electrons travel through a straight-line trajectories [2]. Electron elastic scattering is
dependent on the atomic number of the particular substrate, the electron emission angle, and the
acceptance solid angle of the electron energy analyser, making its influence experiment-
dependent; for example, it is different for PES and AES experiments [26-28]. Accurate estimation
of the film thickness and any possible contamination will affect the extracted EAL values. Here
we follow the principle of the overlayer method to measure EAL in epitaxial graphene on SiC as
an estimation for IMFP.
In recent years graphene has emerged at the vanguard of materials science research due to
its extraordinary properties [29-36]. It has been the subject of many electron-collection-based
studies such as PES, LEED, AES and electron microscopy [22, 32, 37-40]. Having a clear
understanding of the electron IMFP is crucial for accurate data interpretation and analysis in these
techniques especially in calculating the surface sensitivity factor, signal intensity and estimating
the graphene layer thickness [27]. However, the IMFP of carbon polymorphs has consistently been
difficult to model; this challenge has been recognized for decades and remains an outstanding
problem [8-11, 15]. Tanuma et al measured the IMFP of graphite using elastic-peak electron
107
spectroscopy (EPES), and found the experimental data to be about 50% higher than their
theoretical predictions [40]. Characterizing the IMFP is even more challenging for graphene as
interface effects dominate and anisotropies are magnified [10]. Very little is known about how
IMFPs (EALs) vary in moving from bulk to 2D materials [22], and this represents an emerging
challenge for the burgeoning field of 2D materials, since accurate materials characterization
requires a good understanding of IMFP (EAL).
Here we present a measurement of the EAL in epitaxial graphene grown on SiC using
synchrotron PES, supported by thickness measurements using transmission electron microscopy
(TEM). Our experiments are performed in ultra-high vacuum (UHV) chamber without being
exposed to air which eliminates any chance of contamination significantly. Graphene growth via
high temperature annealing of SiC is an attractive approach for device fabrication because it does
not require a transfer step, which can adversely affect the properties of the graphene [41-43].
Epitaxial graphene growth on SiC comprises a buffer layer at the interface between the graphene
and the SiC; this layer is a graphene-like but is partially bound to the substrate, disrupting the in-
plane bonding [44, 45]. The buffer layer can be suppressed using a hydrogen intercalation process
that cleaves the backbonds to the surface and converts it to graphene [39, 46]. We examine the
effect of the buffer layer on the EAL values and compare it to that of free-standing graphene on
SiC produced by hydrogen intercalation.
6.3 EXPERIMENTAL DETAILS
A 1 µm thick 3C-SiC(111) layer grown on Si(111) was purchased from NOVASIC (France).
The SiC/Si(111) wafer was chemically and mechanically polished to the surface roughness of ~ 1
nm (StepSiC® by NOVASIC (France)) [47]. Each sample was cleaned by 10 minutes of successive
sonication steps in acetone, isopropanol and deionised water.
108
PES measurements were conducted at the soft X-ray beamline of the Australian Synchrotron.
A SPECS Phoibos 150 hemispherical analyser operating at a pass energy of 10 eV was used for
PES measurements. The system has a base pressure of 1 × 10-10 mbar. Peak intensity calculation
was performed by integrating the area under the peak after subtracting a Shirley background.
Samples were annealed at 400 °C for several hours to remove contaminants. Temperature
measurements were made using an optical pyrometer (IRCON Ultimax UX-20P with emissivity =
0.9). Atomic hydrogen exposure was achieved using EFM-H atomic hydrogen source (FOCUS
GmbH) operating at an apparent chamber pressure of ~ 5×10-6 mbar and 40 W power. Atomic
hydrogen etching assists with elimination of contamination and improving the flatness of the SiC
surface, and has been routinely employed as a preparation step prior to graphene fabrication [48-
50].
Epitaxial graphene fabrication was performed by annealing at 1200 - 1250 °C for 5 - 10
minutes. The thickness of the resultant graphene layer depends on annealing time and temperature,
with longer and hotter anneal cycles producing larger numbers of graphene layers [37, 51]. A FEI
Tecnai F30 TEM system operating at 200 keV was used for graphene layer thickness
measurements. TEM sample preparation was conducted by lift out procedure using a FEI Strata
DB235 FIB/SEM system.
6.4 RESULTS AND DISCUSSIONS
Core-level PES of Si 2p using a photon energy of 1253 eV are presented in Figure 6-1a. The
measurement was conducted first on a reference SiC/Si(111) sample and then on the same sample
covered with graphene. The intensity is decreased for the graphene/SiC sample (I) compared to
the reference sample (I0), because electrons originating in the SiC must transit through the
graphene layer resulting in additional scattering and energy loss events, and these electrons are no
109
longer detected at the kinetic energy of the Si 2p core level. We use this intensity loss to calculate
the EAL values in epitaxial graphene. Figure 6-1b represents the electron transmission (I/ I0) with
respect to photon energy, the experimental data points are fitted by an exponential equation to
illustrate the trend. The transmission is later used to calculate the EAL.
Figure 6-1. (a) Si 2p spectra for the SiC reference sample and the graphene/SiC sample indicating the intensity
decrease. (b) Transmission with respect to the photon energies, points represent the experimental values fitted by an exponential equation.
Figure 6-2 shows the intensity and lineshape variation with photon energy of the C 1s core
level spectra of the graphene/SiC sample, acquired over photon energies from 330-1253 eV. It is
clear that higher photon energies produce more substrate-related (SiC substrate) signal whereas
lower energies enhance the surface-related (graphene/buffer layer) signals (Figure 6-2). To
calculate the EAL for electrons inside the epitaxial graphene layer we studied the Si 2p signal as
this is generated almost exclusively from the SiC substrate. The intensity change (a.k.a.
Transmission, I/I0) with photon energy of the Si 2p peak for graphene/SiC compared to the
reference sample (SiC/Si) is shown in Figure 6-1b (data is reported in the supporting information
Table 6-1).
110
Figure 6-2. PES C 1s spectrum of graphene/SiC measured with different photon energies.
By studying the intensity changes as a function of kinetic energy one can calculate the IMFP
of electrons in the graphene layer. The attenuation of a low-energy electron signal can be expressed
as:
II0
= 𝑒𝑒−𝑡𝑡𝜆𝜆𝑒𝑒 (12)
where I is the intensity of the SiC component with graphene on top, I0 is the intensity of the
bare SiC component as the reference sample, t is the thickness of the graphene layer, and 𝜆𝜆𝑒𝑒 is the
EAL. The thickness of the graphene layer was measured directly by TEM of the sectioned sample,
whereby we find on average two graphene-like layers on the SiC substrate (Figure 6-3). The first
layer on the substrate is commonly called buffer layer (see above), with its characteristic
111
backbonding shown schematically in the inset of Figure 6-3; the presence of the buffer layer is
confirmed by PES data (Figure 6-2 and supporting information Figure 6-5). The thickness of the
graphene ‘film’ is thus 0.6 nm, corresponding to one graphene layer plus the buffer layer. Here,
we point out that TEM image shows a local representation of the graphene layer thickness, but
considering the similarity between our PES C 1s spectrum and previously published works for
monolayer graphene on SiC we believe that we have predominantly one monolayer plus the buffer
layer coverage in the measured PES spot size (~100 µm × 80 µm) [52, 53]. For further discussion
about the graphene layer thickness refer to the supporting information. Based on this measured
thickness the EAL was calculated at six different excitation energies from the intensity attenuation
equation above.
Figure 6-3. TEM image showing that the sample has the coverage of two graphene like layers (first layer
corresponds to the buffer layer and the second one is graphene supported by the PES data).
The EAL values are plotted in Figure 6-4. In order to fit this data a modified Bethe equation
[16] introduced by Tanuma et al [18] was employed, whereby:
λe =
E
Ep2[β ln(γE)− C
E + DE2]
(13)
112
where 𝜆𝜆𝑒𝑒 is the electron EAL (nm), E is the electron energy (eV), Ep is the free-electron plasmon
energy (eV) (for carbon Ep = 22.3 eV). β, γ, C and D are constant parameters of the equation [18].
Our fit parameters can be found in the supporting information Table 6-2. For comparison to the
measured EAL values for the epitaxial graphene in the present work, EAL values for exfoliated
graphene transferred to a SiO2 substrate and IMFPs for graphite are shown in Figure 6-4. EALs
for mechanical exfoliated graphene flakes transferred to a SiO2 substrate are measured using AES
(80-500 eV) by Xu et al [22]. Depicted IMFP values for graphite are experimentally measured data
using EPES [40] and predicted values by TPP-2M model [11].
Figure 6-4. EAL values measured in the present work for monolayer graphene/buffer layer and bilayer free-standing graphene compared with EAL values measured using AES for exfoliated graphene transferred to SiO2 substrate by Xu et al [22] for monolayer graphene (ML) and averaged data over bilayer and three layer graphene. IMFP values for graphite estimated by TPP-2M model [11] and experimentally measured using EPES by Tanuma et al [40] are
reported. The solid lines represent the fitting of the data using the modified Beth equation [18].
The present EAL data for monolayer epitaxial graphene with buffer layer on SiC are about
45% higher than EALs for monolayer graphene reported by Xu et al [22] and approximately double
their averaged EAL values for bilayer and three layer graphene. Xu et al [22] studied the EAL in
exfoliated graphene transferred to SiO2 substrate using AES in the 80-500 eV energy range, and
113
used Raman spectroscopy to approximate the thickness of the transferred graphene. They indicated
that their data derived for monolayer graphene did not match with their measurements for bilayer
and three-layer graphene, a discrepancy that they attributed to interface diffraction and the two-
dimensional nature of graphene. The variation observed between the present data and Xu et al’s
values could stem from the presence of some degree of folding or residue from the transfer process
employed by Xu et al, resulting in extra attenuation of electrons and an underestimation of the
EAL for graphene [54, 55]. Using different methods to measure EALs (here PES compared to AES
used by Xu et al [22]) can affect the measured values due to the different elastic scattering effects,
which are ignored in both experiments [28]. Different techniques used for thickness measurement
may affect the results as well. Lastly, the variation in graphene synthesis method and
graphene/substrate interaction effects may play some role in observing such difference.
We find that our experimental EALs for monolayer epitaxial graphene with the buffer layer
on SiC are about 50% higher than experimental IMFP values for graphite measured using EPES
[40] and the values predicted by the TPP-2M model [11]. IMFP and EAL values are assumed to
be comparable based on ignoring the elastic scattering effects (as discussed above) [28]. The TPP-
2M model can describe the experimental IMFP values for graphite, but still under-predicts the
EAL values found here for graphene [11]. Interface effects and the strong anisotropy of graphene
may account for the variation between experimental IMFP values for graphite and present EALs
for graphene.
To discern the effect of the buffer layer on the attenuation length, we repeated the
measurements after hydrogen intercalation of the graphene layer. The intercalation process breaks
the bonds between the buffer layer and the SiC substrate and converts it to a complete graphene
layer [39]. Hence our original sample, monolayer graphene with a buffer layer, is converted to a
114
bilayer free-standing graphene on SiC as a result of the hydrogen intercalation (supporting
information Figure 6-7). Comparing the extracted EALs in the free-standing bilayer graphene (red
crosses) to the one with buffer layer (black dots) in Figure 6-4, it can be noted that the EAL for
the free-standing graphene increases by about 40% indicating that the buffer layer scatters
electrons substantially more than a free-standing graphene layer. This effect is likely due to the
presence of sp3 bonds between the buffer layer and the SiC substrate, which increases the scattering
probability of electrons crossing the buffer layer compared to the free-standing graphene layer.
This is also observable in the C 1s PES spectra where the substrate-related peak intensity increases
after hydrogen intercalation indicating that more electrons can travel through the graphene layer
in the free-standing graphene compared to the buffer layer (supporting information Figure 6-7).
Finally, we note that surface/volume considerations may mean that the measured EAL values
for graphene will depend critically on the thickness of the graphene layer, so that measurements
from a bilayer graphene (present work) may not linearly extrapolate to thicker graphene layers
(seen in Xu et al’s [22] work as well). In this context, it may be reasonable to assume quite different
behavior between EALs (IMFPs) in monolayer/bilayer graphene and bulk graphite. Such a
disparity would mean that the theoretical constructs established for bulk materials are unlikely to
capture the physics associated with the IMFP in thin films, where interface/anisotropy effects are
expected to be significant.
6.5 CONCLUSIONS
Using synchrotron PES in the energy range 50-1150 eV, we have measured the EAL of
electrons in epitaxial graphene on SiC. Conducting graphene synthesis and EAL measurements in
UHV without any exposure to air provided an ideal experimental condition. Comparing EAL
values for the sample with the buffer layer to the free-standing one fabricated by hydrogen
115
intercalation has enabled us to demonstrate that the interface effects are significant and show that
buffer layer scatters electrons more than a graphene layer.
We have also found a significant variation between previous theoretical and experimental
IMFP values for graphite and present EAL values for epitaxial graphene, while the energy
dependence remains very similar. 2D materials such as graphene obey different physics than bulk
materials, including a more pronounced influence from surfaces, interfaces and anisotropy; these
effects are not fully captured in the current models. This work highlights the need for more research
into measuring the EAL (IMFP) in 2D materials, requiring systematic measurements over a range
of different thicknesses and types of graphene (epitaxial, CVD, etc.). Epitaxial graphene is
subjected to a number of electron-collection-based measurements, therefore, having a better
understanding of EAL in epitaxial graphene on SiC is expected to improve the accuracy of data
analysis particularly in calculating the surface sensitivity factor, signal intensity and estimating the
graphene layer thickness.
6.6 ACKNOWLEDGMENTS
The Queensland Government support through the Q-CAS Collaborative Science Fund 2016
is kindly acknowledged. M.A. acknowledges scholarship funding provided by Science and
Engineering Faculty (QUT). PES measurements were conducted at the Australian Synchrotron
(Soft X-Ray Beamline), and the authors acknowledge the support of the Australian Synchrotron.
The authors kindly acknowledge Prof Patrick Soukiassian and Dr Neeraj Mishra for their help in
this research.
6.7 REFERENCES
1. C.J. Powell and A. Jablonski, Surface sensitivity of X-ray photoelectron spectroscopy. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009. 601(1): p. 54-65.
116
2. ISO 18115-1. Surface Chemical Analysis–Vocabulary. 2013. 3. I. Lindau and W.E. Spicer, The probing depth in photoemission and auger-electron
spectroscopy. Journal of Electron Spectroscopy and Related Phenomena, 1974. 3(5): p. 409-413.
4. P. Cumpson, Elastic scattering corrections in AES and XPS. III. Behaviour of electron transport mean free path in solids for kinetic energies in the range 100 eV< E< 400 eV. Surface and Interface Analysis, 1997. 25(6): p. 447-453.
5. F. Hillebrecht, H. Rose, T. Kinoshita, Y. Idzerda, G. van der Laan, R. Denecke, and L. Ley, Photoelectron diffraction in magnetic linear dichroism. Physical Review Letters, 1995. 75(15): p. 2883.
6. W.S.M. Werner, W. Smekal, H. Störi, H. Winter, G. Stefani, A. Ruocco, F. Offi, R. Gotter, A. Morgante, and F. Tommasini, Emission-depth-selective Auger photoelectron coincidence spectroscopy. Physical Review Letters, 2005. 94(3): p. 038302.
7. K. Kimoto, T. Asaka, T. Nagai, M. Saito, Y. Matsui, and K. Ishizuka, Element-selective imaging of atomic columns in a crystal using STEM and EELS. Nature, 2007. 450: p. 702.
8. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths for 31 materials. Surface and Interface Analysis, 1988. 11(11): p. 577-589.
9. H. Shinotsuka, S. Tanuma, C. Powell, and D. Penn, Calculations of electron inelastic mean free paths. X. Data for 41 elemental solids over the 50 eV to 200 keV range with the relativistic full Penn algorithm. Surface and Interface Analysis, 2015. 47(9): p. 871-888.
10. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. IX. Data for 41 elemental solids over the 50 eV to 30 keV range. Surface and Interface Analysis, 2011. 43(3): p. 689-713.
11. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. Surface and Interface Analysis, 2005. 37(1): p. 1-14.
12. H.T. Nguyen-Truong, Low-energy electron inelastic mean free path in materials. Applied Physics Letters, 2016. 108(17): p. 172901.
13. H.T. Nguyen-Truong, Analytical formula for high-energy electron inelastic mean free path. The Journal of Physical Chemistry C, 2015. 119(41): p. 23627-23631.
14. W.S.M. Werner, K. Glantschnig, and C. Ambrosch-Draxl, Optical constants and inelastic electron-scattering data for 17 elemental metals. Journal of Physical and Chemical Reference Data, 2009. 38(4): p. 1013-1092.
15. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. Surface and Interface Analysis, 1994. 21(3): p. 165-176.
16. H. Bethe, Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie. Annalen der Physik, 1930. 397(3): p. 325-400.
17. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths. III. Data for 15 inorganic compounds over the 50–2000 eV range. Surface and Interface Analysis, 1991. 17(13): p. 927-939.
117
18. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electorn inelastic mean free paths. II. Data for 27 elements over the 50–2000 eV range. Surface and Interface Analysis, 1991. 17(13): p. 911-926.
19. D.R. Penn, Electron mean-free-path calculations using a model dielectric function. Physical Review B, 1987. 35(2): p. 482-486.
20. H.J. Hagemann, W. Gudat, and C. Kunz, Optical constants from the far infrared to the x-ray region: Mg, Al, Cu, Ag, Au, Bi, C, and Al2O3. Journal of the Optical Society of America, 1975. 65(6): p. 742-744.
21. S. Tanuma, C.J. Powell, and D.R. Penn, Calculations of electron inelastic mean free paths (IMFPs) VI. analysis of the gries inelastic scattering model and predictive IMFP equation. Surface and Interface Analysis, 1997. 25(1): p. 25-35.
22. M. Xu, D. Fujita, J. Gao, and N. Hanagata, Auger electron spectroscopy: a rational method for determining thickness of graphene films. ACS Nano, 2010. 4(5): p. 2937-2945.
23. C.J. Powell, R.J. Stein, P.B. Needham, and T.J. Driscoll, Attenuation lengths of low-energy electrons in solids derived from the yield of proton-excited Auger electrons: beryllium and aluminum. Physical Review B, 1977. 16(4): p. 1370-1379.
24. P.W. Palmberg and T.N. Rhodin, Auger electron spectroscopy of fcc metal surfaces. Journal of Applied Physics, 1968. 39(5): p. 2425-2432.
25. Y.F. Chen, C.M. Kwei, and C.J. Tung, Electron inelastic mean free paths versus attenuation lengths in solids. Journal of Physics D: Applied Physics, 1992. 25(2): p. 262.
26. C.J. Powell and A. Jablonski, Evaluation of calculated and measured electron inelastic mean free paths near solid sSurfaces. Journal of Physical and Chemical Reference Data, 1999. 28(1): p. 19-62.
27. C.J. Powell and A. Jablonski, Progress in quantitative surface analysis by X-ray photoelectron spectroscopy: Current status and perspectives. Journal of Electron Spectroscopy and Related Phenomena, 2010. 178-179: p. 331-346.
28. A. Jablonski and H. Ebel, Comparison of electron attenuation lengths and escape depths with inelastic mean free paths. Surface and Interface Analysis, 1988. 11(12): p. 627-632.
29. A.K. Geim and K.S. Novoselov, The rise of graphene. Nature Materials, 2007. 6(3): p. 183-191.
30. A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, and A.K. Geim, The electronic properties of graphene. Reviews of Modern Physics, 2009. 81(1): p. 109-162.
31. V.Y. Aristov, G. Urbanik, K. Kummer, D.V. Vyalikh, O.V. Molodtsova, A.B. Preobrajenski, A.A. Zakharov, C. Hess, T. Hänke, and B. Buchner, Graphene synthesis on cubic SiC/Si wafers. Perspectives for mass production of graphene-based electronic devices. Nano Letters, 2010. 10(3): p. 992-995.
32. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, and W.A. De Heer, Electronic confinement and coherence in patterned epitaxial graphene. Science, 2006. 312(5777): p. 1191-1196.
118
33. Q. Wang, R. Kitaura, S. Suzuki, Y. Miyauchi, K. Matsuda, Y. Yamamoto, S. Arai, and H. Shinohara, Fabrication and in situ transmission electron microscope characterization of freestanding graphene nanoribbon devices. ACS Nano, 2016. 10(1): p. 1475-1480.
34. G. Fisichella, S. Lo Verso, S. Di Marco, V. Vinciguerra, E. Schilirò, S. Di Franco, R. Lo Nigro, F. Roccaforte, A. Zurutuza, A. Centeno, S. Ravesi, and F. Giannazzo, Advances in the fabrication of graphene transistors on flexible substrates. Beilstein Journal of Nanotechnology, 2017. 8: p. 467-474.
35. B.V. Cunning, M. Ahmed, N. Mishra, A.R. Kermany, B. Wood, and F. Iacopi, Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers. Nanotechnology, 2014. 25(32): p. 325301.
36. A. Mohsin, K. Mohamad, N. Marco, W. Bei, G. Dayle, G. Bharati, J.B. John, T. Arash, M. Nunzio, E.S. Stephen, and I. Francesca, A thin film approach for SiC-derived graphene as an on-chip electrode for supercapacitors. Nanotechnology, 2015. 26(43): p. 434005.
37. B. Gupta, M. Notarianni, N. Mishra, M. Shafiei, F. Iacopi, and N. Motta, Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
38. E. Rollings, G.H. Gweon, S.Y. Zhou, B.S. Mun, J.L. McChesney, B.S. Hussain, A.V. Fedorov, P.N. First, W.A. De Heer, and A. Lanzara, Synthesis and characterization of atomically thin graphite films on a silicon carbide substrate. Journal of Physics and Chemistry of Solids, 2006. 67(9): p. 2172-2177.
39. M. Amjadipour, A. Tadich, J.J. Boeckl, J. Lipton-Duffin, J. MacLeod, F. Iacopi, and N. Motta, Quasi free-standing epitaxial graphene fabrication on 3C–SiC/Si(111). Nanotechnology, 2018. 29(14): p. 145601.
40. S. Tanuma, T. Shiratori, T. Kimura, K. Goto, S. Ichimura, and C.J. Powell, Experimental determination of electron inelastic mean free paths in 13 elemental solids in the 50 to 5000 eV energy range by elastic‐peak electron spectroscopy. Surface and Interface Analysis, 2005. 37(11): p. 833-845.
41. A. Van Bommel, J. Crombeen, and A. Van Tooren, LEED and Auger electron observations of the SiC (0001) surface. Surface Science, 1975. 48(2): p. 463-472.
42. M. Suemitsu, Y. Miyamoto, H. Handa, and A. Konno, Graphene formation on a 3C-SiC (111) thin film grown on Si (110) substrate. e-Journal of Surface Science and Nanotechnology, 2009. 7: p. 311-313.
43. A. Ouerghi, A. Kahouli, D. Lucot, M. Portail, L. Travers, J. Gierak, J. Penuelas, P. Jegou, A. Shukla, and T. Chassagne, Epitaxial graphene on cubic SiC (111)/Si (111) substrate. Applied Physics Letters, 2010. 96(19): p. 191910.
44. S. Goler, C. Coletti, V. Piazza, P. Pingue, F. Colangelo, V. Pellegrini, K.V. Emtsev, S. Forti, U. Starke, F. Beltram, and S. Heun, Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene. Carbon, 2013. 51: p. 249-254.
45. B. Gupta, E. Placidi, C. Hogan, N. Mishra, F. Iacopi, and N. Motta, The transition from 3C SiC (111) to graphene captured by ultra high vacuum scanning tunneling microscopy. Carbon, 2015. 91: p. 378-385.
119
46. C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, and U. Starke, Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation. Physical Review Letters, 2009. 103(24): p. 246804.
47. B. Gupta, I. Di Bernardo, P. Mondelli, A. Della Pia, M.G. Betti, F. Iacopi, C. Mariani, and N. Motta, Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
48. A. Sandin, J.J. Rowe, and D.B. Dougherty, Improved graphene growth in UHV: pit-free surfaces by selective Si etching of SiC (0001)–Si with atomic hydrogen. Surface Science, 2013. 611: p. 25-31.
49. M. Amjadipour, M. MacLeod, J. Lipton-Duffin, F. Iacopi, and N. Motta, Epitaxial graphene growth on FIB patterned 3C-SiC nanostructures on Si (111): reducing milling damage. Nanotechnology, 2017. 28(34): p. 345602.
50. P. Mondelli, B. Gupta, M.G. Betti, C. Mariani, J.L. Duffin, and N. Motta, High quality epitaxial graphene by hydrogen-etching of 3C-SiC (111) thin-film on Si (111). Nanotechnology, 2017. 28(11): p. 115601.
51. F. Zarotti, B. Gupta, F. Iacopi, A. Sgarlata, M. Tomellini, and N. Motta, Time evolution of graphene growth on SiC as a function of annealing temperature. Carbon, 2016. 98: p. 307-312.
52. S. Watcharinyanon, C. Virojanadara, J. Osiecki, A. Zakharov, R. Yakimova, R. Uhrberg, and L.I. Johansson, Hydrogen intercalation of graphene grown on 6H-SiC (0001). Surface Science, 2011. 605(17): p. 1662-1668.
53. K.V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G.L. Kellogg, L. Ley, J.L. McChesney, T. Ohta, S.A. Reshanov, and J. Röhrl, Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials, 2009. 8(3): p. 203-207.
54. X. Liang, B.A. Sperling, I. Calizo, G. Cheng, C.A. Hacker, Q. Zhang, Y. Obeng, K. Yan, H. Peng, Q. Li, X. Zhu, H. Yuan, A.R. Hight Walker, Z. Liu, L.-m. Peng, and C.A. Richter, Toward clean and crackless transfer of graphene. ACS Nano, 2011. 5(11): p. 9144-9153.
55. J. Kang, D. Shin, S. Bae, and B.H. Hong, Graphene transfer: key for applications. Nanoscale, 2012. 4(18): p. 5527-5537.
120
Nanotechnology 2019, 30 (2), 025704. DOI: https://dx.doi.org/10.1088/1361-6528/aae7ec
6.8 SUPPORTING INFORMATION
Electron Inelastic Mean Free Path in Epitaxial Graphene on SiC
Mojtaba Amjadipour1, Jennifer MacLeod1, Josh Lipton-Duffin2, Anton Tadich3, John J Boeckl4,
Francesca Iacopi5, and Nunzio Motta1*
1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, QLD, Australia.
2 Central Analytical Research Facility, Institute for Future Environments, Science and Engineering
Faculty, Queensland University of Technology, QLD, Australia.
3 Australian Synchrotron, 800 Blackburn Road, Clayton, 3168 VIC, Australia.
4 Materials and Manufacturing Directorate, Air Force Research Laboratories, Wright-Patterson
AFB, 45433 OH, United States of America.
5 School of Electrical and Data Engineering, Faculty of Engineering and Information Technology,
University of Technology Sydney, NSW, Australia.
* Corresponding author. Tel: + 61731385104. E-mail: [email protected]
121
Figure 6-5. C 1s spectra of graphene/SiC sample with buffer layer.
Based on our PES data and comparison with previously published works, we believe we have
predominantly a single layer of graphene plus the buffer layer over our SiC substrate [1, 2]; TEM
data also confirms it. Under growth conditions similar to ours, it has been reported that some islands
of bilayer graphene may be present on the surface as well [1, 3]. Here, we used high temperature
annealing and we employed H-etching procedure to improve the graphene quality and homogeneity.
Our group has previously reported that this procedure results in a very good quality graphene using
scanning tunneling microscopy (STM) and Raman spectroscopy [4, 5]. It has been previously
reported that graphene grows over the SiC step edges forming a fairly uniform coverage on the SiC
substrate [6]. Finally, we would like to point out that having more than one layer of graphene results
in even higher EAL (IMFP) which further confirms our claim that current theoretical and
experimental values for graphite under-predicts IMFP for graphene.
We have additionally considered how the EAL varies due to the different layer spacing
distances reported in the literature for monolayer graphene plus buffer layer, and this consideration
is presented in Figure 6-6, which shows that the sensitivity of EAL (IMFP) to these changes is
minimal, with less than 5% variation in values.
122
Figure 6-6. EALvariation for monolayer graphene plus buffer layer due to different layer spacing values [7, 8]. The solid lines represent the fitting of the data using the modified Beth equation.
Table 6-1. Si 2p peak intensity change for the sample covered with graphene plus buffer layer (I) and the reference sample (I0).
Electron energy I0 (a.u.) Error (%)
I0 I (a.u.) Error (%) I
Transmission I/I0
1151 70699.1 0.18 60970.3 0.20 0.86
698 278147 0.17 226175 0.20 0.81
498 647245 0.13 513497 0.16 0.79
298 1436070 0.11 1040180 0.13 0.72
228 1184090 0.17 801872 0.25 0.68
48 4506550 0.07 1986820 0.13 0.44
Table 6-2. Constant parameters of the modified version of the Bethe equation calculated for the fittings in Figure 6-4.
Condition β (eV-1nm-1) γ (eV-1) C (nm-1) D (eVnm-1)
Present work - Graphene 0.08 0.50 7.66 70.00
Present work – Free-Standing Graphene 0.05 0.80 7.00 150.00
123
As-grown epitaxial graphene on SiC is characterized by the presence of a so-called “buffer
layer” at the interface between the SiC substrate and the sp2 graphene layers; this buffer layer is back-
bonded to the substrate, creating sp3 hybridization (Figure 6-7) [9]. The presence of the buffer layer
is indicated by a unique spectral signature in the C 1s core level (Figure 6-5 and 6-7) [9, 10]. During
the intercalation process, atomic hydrogen penetrates between the buffer layer and the substrate, and
replaces the buffer layer-substrate bonds with H-capping at the substrate, allowing the buffer layer to
convert to a fully sp2-hybridized graphene layer [2, 9].
A literature review of H-intercalation of graphene on SiC shows that it is possible to intercalate
until no trace of the buffer layer is observed [2, 9, 11]. This is also the case in our data: following H-
intercalation, we find no spectral signature of the buffer layer in our C 1s data (Figure 6-7).
The thickness of a monolayer graphene plus buffer layer is quite similar to a free-standing
bilayer graphene. The main difference is that the sp3 backbonds between the buffer layer and the
substrate result in the thickness of monolayer graphene plus buffer layer being somewhat lower, at
about 0.6 nm whereas bilayer graphene is about 0.67 nm [12].
124
Figure 6-7. (a) A model representing monolayer graphene plus the buffer layer on SiC, (b) a model showing the effect of H-intercalation converting the sample to a bilayer free-standing graphene, (c) C 1s spectra indicating the elimination
of the buffer layer components after H-intercalation.
REFERENCES
1. K.V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G.L. Kellogg, L. Ley, J.L. McChesney, T. Ohta, S.A. Reshanov, and J. Röhrl, Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials, 2009. 8(3): p. 203-207.
2. S. Watcharinyanon, C. Virojanadara, J. Osiecki, A. Zakharov, R. Yakimova, R. Uhrberg, and L.I. Johansson, Hydrogen intercalation of graphene grown on 6H-SiC (0001). Surface Science, 2011. 605(17): p. 1662-1668.
3. I.G. Ivanov, J.U. Hassan, T. Iakimov, A.A. Zakharov, R. Yakimova, and E. Janzén, Layer-number determination in graphene on SiC by reflectance mapping. Carbon, 2014. 77: p. 492-500.
4. P. Mondelli, B. Gupta, M.G. Betti, C. Mariani, J. Lipton-Duffin, and N. Motta, High quality epitaxial graphene by hydrogen-etching of 3C-SiC(111) thin-film on Si(111). Nanotechnology, 2017. 28(11): p. 115601.
5. B. Gupta, I. Di Bernardo, P. Mondelli, A. Della Pia, M. Betti, F. Iacopi, C. Mariani, and N. Motta, Effect of substrate polishing on the growth of graphene on 3C–SiC (111)/Si (111) by high temperature annealing. Nanotechnology, 2016. 27(18): p. 185601.
6. A. Ouerghi, R. Belkhou, M. Marangolo, M. Silly, S. El Moussaoui, M. Eddrief, L. Largeau, M. Portail, and F. Sirotti, Structural coherency of epitaxial graphene on 3C–SiC (111) epilayers on Si (111). Applied Physics Letters, 2010. 97(16): p. 161905.
125
7. A. Ouerghi, M. Marangolo, R. Belkhou, S. El Moussaoui, M. Silly, M. Eddrief, L. Largeau, M. Portail, B. Fain, and F. Sirotti, Epitaxial graphene on 3C-SiC (111) pseudosubstrate: Structural and electronic properties. Physical Review B, 2010. 82(12): p. 125445.
8. J. Hass, J.E. Millán-Otoya, P.N. First, and E.H. Conrad, Interface structure of epitaxial graphene grown on 4H-SiC(0001). Physical Review B, 2008. 78(20): p. 205424.
9. C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, and U. Starke, Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation. Physical Review Letters, 2009. 103(24): p. 246804.
10. K.V. Emtsev, F. Speck, T. Seyller, L. Ley, and J.D. Riley, Interaction, growth, and ordering of epitaxial graphene on SiC0001 surfaces: A comparative photoelectron spectroscopy study. Physical Review B, 2008. 77(15): p. 155303.
11. M. Amjadipour, A. Tadich, J.J. Boeckl, J. Lipton-Duffin, J. MacLeod, F. Iacopi, and N. Motta, Quasi free-standing epitaxial graphene fabrication on 3C–SiC/Si(111). Nanotechnology, 2018. 29(14): p. 145601.
12. W. Norimatsu and M. Kusunoki, Transitional structures of the interface between graphene and 6H–SiC (0001). Chemical Physics Letters, 2009. 468(1): p. 52-56.
126
Chapter 7: Conclusions and Future Research
7.1 CONCLUSIONS
The goals of this thesis have been achieved by developing a procedure using a Si protective
layer to grow graphene over FIB patterned SiC nanostructures, demonstrating the success of hydrogen
intercalation in elimination of the buffer layer and presenting a direct estimation of electron inelastic
mean free path in epitaxial graphene on SiC.
7.1.1 Epitaxial Graphene Growth on FIB Patterned 3C-SiC Nanostructures on Si(111): Reducing Milling Damage
This thesis studied the effect of Ga+ ion milling on graphene growth on a 3C-SiC substrate in
UHV. The results indicated that Ga+ ion exposure prevents graphene growth on SiC and results in
formation of some islands on the surface. Using a modified procedure based on depositing a Si cap
layer, it was demonstrated that graphene can be grown on FIB-patterned SiC nanostructures. The Si
protective layer traps the milling residuals and helps to remove them during the normal growth
process. The Si layer evaporates by annealing the sample at about 1000 ˚C before the final annealing
step for graphene growth. Employing a combination of HIM, STM and Raman spectroscopy a
pathway to fabricate epitaxial graphene on FIB-patterned 3C-SiC nanostructures using UHV
annealing was presented. This method paves the way for use of FIB patterning for nanoscale
engineering of graphene on SiC.
7.1.2 Quasi Free-Standing Epitaxial Graphene Fabrication on 3C-SiC/Si(111)
The effectiveness of hydrogen intercalation process to eliminate the buffer layer for epitaxial
graphene fabricated on 3C-SiC/Si(111) thin films was systematically investigated. The presence of
the buffer layer, which is a carbon-rich layer at the interface of graphene and SiC substrate, adversely
affects graphene’s properties. Hydrogen intercalation eliminates the buffer layer and can produce
free-standing graphene on 3C-SiC/Si(111) thin films. Hydrogen atoms break the backbonds between
127
the buffer layer and the SiC substrate, converting the buffer layer into a graphene layer. Reduced
substrate/graphene interaction was confirmed by the NEXAFS data. The PES C 1s spectrum indicated
the complete elimination of the buffer layer after H-intercalation. Furthermore, it was demonstrated
that the free-standing graphene fabricated on the 3C-SiC/Si(111) is stable in air. The H-intercalation
process is reversible: annealing the intercalated sample to 850 ˚C or higher results in hydrogen
desorption and recreation of the buffer layer.
7.1.3 Measuring the Electron Inelastic Mean Free Path in Epitaxial Graphene on SiC
Having a precise understanding about the electron IMFP is vital for a number of electron-
collection-based measurements. The electron IMFP in epitaxial graphene fabricated on 3C-
SiC/Si(111) was directly estimated by measuring EAL using synchrotron PES. The results indicated
that previous theoretical and experimental works for graphite under-predict the absolute value of the
IMFP for epitaxial graphene. Using a hydrogen intercalation procedure the IMFP of electrons in the
buffer layer with free-standing graphene was compared indicating that the buffer layer scatters
electrons significantly more due to having sp3 bonding with the substrate. It can be concluded that
2D materials such as graphene behave differently than bulk materials due to dominant effect of
surface and anisotropies in 2D materials. Such effects are not included in the current models for IMFP
highlighting the necessity for more research into measuring the IMFP in 2D materials.
7.2 FUTURE RESEARCH
Device fabrication using free-standing epitaxial graphene nanoribbons is a promising pathway
for the future research. Combing FIB milling of SiC prior to graphene growth and H-intercalation
procedure results in fabrication of high quality graphene nanoribbons. Figure 7-1 shows one possible
configuration for producing a field effect transistors with the fabricated graphene nanoribbons. For
this purpose, two metal contacts can be deposited on the sides of the FIB milled structures as drain
and source. Furthermore, in order to fabricate very narrow graphene ribbon, Helium ions (via Carl
Zeiss Orion NanoFab) can be used to narrow the structures size to few nanometer scales.
128
Figure 7-1. A possible configuration to fabricate a field effect transistor using the graphene grown on the SiC
nanostructures.
Investigating the epitaxial graphene growth in situ using TEM is another interesting future
research direction. The graphene growth via Si sublimation can be conducted in situ by TEM
employing state of the art manipulators. This would provide the opportunity to observe the growth
process live by TEM. This study would be even more appealing for structured SiC samples revealing
different growth mechanisms over different facets of SiC.
