p-type doping of MoS2 thin films using NbMasihhur R. Laskar, Digbijoy N. Nath, Lu Ma, Edwin W. Lee II, Choong Hee Lee, Thomas Kent, Zihao Yang,
Rohan Mishra, Manuel A. Roldan, Juan-Carlos Idrobo, Sokrates T. Pantelides, Stephen J. Pennycook, Roberto
C. Myers, Yiying Wu, and Siddharth Rajan
Citation: Applied Physics Letters 104, 092104 (2014); doi: 10.1063/1.4867197 View online: http://dx.doi.org/10.1063/1.4867197 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhancing the photocurrent and photoluminescence of single crystal monolayer MoS2 with resonant plasmonicnanoshells Appl. Phys. Lett. 104, 031112 (2014); 10.1063/1.4862745 Controlled growth of vertically aligned MoO3 nanoflakes by plasma assisted paste sublimation process J. Appl. Phys. 114, 184310 (2013); 10.1063/1.4830278 Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition Appl. Phys. Lett. 102, 193107 (2013); 10.1063/1.4804546 Physical properties of Cu2ZnSnS4 thin films deposited by spray pyrolysis technique J. Renewable Sustainable Energy 5, 023113 (2013); 10.1063/1.4795399 Effect of Li doping in NiO thin films on its transparent and conducting properties and its application inheteroepitaxial p-n junctions J. Appl. Phys. 108, 083715 (2010); 10.1063/1.3499276
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p-type doping of MoS2 thin films using Nb
Masihhur R. Laskar,1,a) Digbijoy N. Nath,1,a) Lu Ma,2,a) Edwin W. Lee II,1,a)
Choong Hee Lee,1 Thomas Kent,3 Zihao Yang,1 Rohan Mishra,4,5 Manuel A. Roldan,5,6
Juan-Carlos Idrobo,7 Sokrates T. Pantelides,4,5,8 Stephen J. Pennycook,9
Roberto C. Myers,1,3 Yiying Wu,2 and Siddharth Rajan1,3,b)
1Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210, USA2Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA3Department of Material Science and Engineering, The Ohio State University, Columbus, Ohio 43210, USA4Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA5Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,USA6Department Fisica Aplicada III, Universidad Complutense de Madrid, Madrid 28040, Spain7Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,USA8Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville,Tennessee 37235, USA9Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA
(Received 21 December 2013; accepted 9 February 2014; published online 4 March 2014)
We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented
chemical vapor deposited MoS2. Niobium was found to act as an efficient acceptor up to relatively
high density in MoS2 films. For a hole density of 3.1� 1020 cm�3, Hall mobility of 8.5 cm2 V�1 s�1
was determined, which matches well with the theoretically expected values. X-ray diffraction scans
and Raman characterization indicated that the film had good out-of-plane crystalline quality.
Absorption measurements showed that the doped sample had similar characteristics to high-quality
undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope
imaging showed ordered crystalline nature of the Nb-doped MoS2 layers stacked in the [0001]
direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in
realizing a wide variety of electrical and opto-electronic devices based on layered metal
dichalcogenides. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4867197]
Transition metal dichalcogenides (TMDs), such as
MoS2, WS2, WSe2, etc., have recently attracted widespread
attention for a variety of next-generation electrical1,2 and
optoelectronic3,4 device applications including low cost,
flexible,5 and transparent electronics.6,7 These layered mate-
rials provide ultra-high confinement and are intrinsically
two-dimensional (2D) in nature, which is therefore promis-
ing for highly scaled vertical transistor topologies.8 Besides,
through van der Waals epitaxy, they circumvent limitations
such as lattice mismatch in heterostructure growths of con-
ventional semiconductors. Devices including field effect
transistors (FETs) with excellent on/off ratio and high cur-
rent densities,1,9 as well as integrated circuits10 have been
reported using flakes of MoS2 mechanically exfoliated from
bulk geological samples. More recently, large area (0001)
oriented MoS2 with excellent crystalline and structural qual-
ities grown by chemical vapor deposited (CVD) on (0001)
sapphire was reported.11 Such CVD-grown MoS2 eliminates
the limitations associated with the commonly used exfoliated
approach, such as control on thickness and area, and are
therefore viable for large scale device fabrication.
The use of Niobium (Nb) as a substitutional impurity on
the metal site to get p-type conductivity was suggested sev-
eral decades ago12–14 for bulk MoS2 and WSe2. Hole
mobility of 10 cm2/Vs in Nb-doped MoS2 flakes obtained by
transport reaction had been reported15 earlier, but such MoS2
flakes were few microns to few hundred microns thick.
Besides, no detailed investigations into the material quality
of such thick Nb-doped MoS2 were reported. P-type conduc-
tivity has also been electrostatically achieved16,17 on MoS2
mechanically exfoliated from geological samples using
back-gating and liquid-gating approaches. However, no
report exists for in situ p-doping using an acceptor dopant in
epitaxial (and even mechanically exfoliated) MoS2 thin
films, which is necessary for demonstration of MoS2-based
bipolar devices such as heterojunction bipolar transistors
(HBT), light emitting diodes (LEDs), and photodetectors.
In this work, we show that as predicted by density func-
tional theory (DFT) based calculations,18 Nb behaves as an
acceptor in MoS2. Raman spectroscopy, X-ray diffraction
(XRD) scans, and aberration-corrected scanning transmis-
sion electron microscopy (STEM) imaging indicated that the
crystalline nature of MoS2 was preserved after Nb-doping,
and optical absorbance measurements showed an absorption
edge at 1.8 eV indicative of a direct band-to-band transition
in the semiconductor. Finally, electrical measurements dem-
onstrated p-type conductivity in these films with a mobility
of 8.5 cm2/Vs at room temperature and a low contact resist-
ance of 0.6 X-mm.
We followed a vapor deposition method for the growth
of MoS219 that was demonstrated to lead to high crystalline
quality MoS2 in previous work 11. A series of three (0001)
a)M. Laskar, D. Nath, L. Ma, and E. Lee contributed equally to this work.b)Author to whom correspondence should be addressed. Electronic mail:
0003-6951/2014/104(9)/092104/4/$30.00 VC 2014 AIP Publishing LLC104, 092104-1
APPLIED PHYSICS LETTERS 104, 092104 (2014)
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sapphire samples were prepared for MoS2 growth by e-beam
evaporation of Mo (2.5 nm)/Nb/Mo (2.5 nm) (schematic in
Fig. 1(a)) layers with thickness of Nb varied as 0.3 nm (sam-
ple A), 0.2 nm (sample B), and 0 nm (control sample C,
undoped). The typical dimensions of the samples were 1–
2 cm� 0.7 cm, with the breadth of the samples being limited
by the size of the quartz tube used for the deposition. The
sample was then subjected to sulfurization in the CVD cham-
ber at 900 �C for 10 min. Further details of the CVD growth
of undoped MoS2 on sapphire are reported elsewhere.11 The
conditions optimum for growing large area undoped MoS2
with excellent structural and surface qualities as reported in
Ref. 11 were used in this study to grow all the samples.
Using high-resolution STEM techniques, the thickness of
MoS2 samples grown under these conditions was found to be
10 nm and 13 nm for the undoped and Nb-doped samples,
respectively. The difference in thickness is attributed to ex-
perimental variation, rather than a systematic effect of Nb
doping.
A Ni (30 nm)/Au (50 nm)/Ni (30 nm) metal stack was
deposited by e-beam evaporation to form Ohmic contacts to
Nb-doped MoS2 (samples A and B). Devices were isolated
by etching MoS2 using BCl3/Ar-based inductively coupled
plasma/reactive ion (ICP-RIE) etch chemistry. Hall measure-
ments were performed using standard van der Pauw pads,
and both samples A and B had positive Hall coefficients indi-
cating hole transport. Temperature dependent Hall measure-
ments for sample A showed no carrier freeze-out even at
20 K (Fig. 1(b)), indicating that degenerate p-type doping
had been achieved. Negligible dependence of hole mobility
on temperature was observed for sample A. The room tem-
perature hole mobility measured for sample A was 0.5
cm2/Vs with a hole density of NA¼ 1.5� 1021 cm�3.
Sample B with reduced Nb thickness (0.2 nm) was found to
exhibit a room temperature hole mobility of 8.5 cm2/Vs with
a corresponding p-type charge density of 3.1� 1020 cm�3.
From transfer length method (TLM) measurements, a low
contact resistance of 0.6 X mm was extracted for sample B
(Fig. 1(c)). The sheet resistance extracted from TLM was
1.8 kX/� which was found to match that obtained from Hall
measurement. The significant improvement in hole mobility
(8.5 cm2/Vs from 0.5 cm2/Vs) with a reduction in p-doping
density indicates that the mobility is limited mainly by ion-
ized impurity scattering at such high degenerate doping
densities.
A simple estimate was made for impurity scattering lim-
ited hole mobility in bulk MoS2 using three-dimensional
(3D) formalism, given by lh¼ q seff/m*, where lh is the
hole mobility in MoS2 while seff and m* are effective hole
scattering time and effective hole mass, respectively. An
in-plane effective hole mass of 0.43 is used for the mobility
estimate based on DFT calculations for unstrained bulk
MoS2.20 The momentum scattering time sm(NA,EF) at Fermi
level EF was calculated using21
1
smðNA;EFÞ¼ p NA
�h
q2LTF2
e0es
� �2
gcðEFÞ: (1)
Here, �h is reduced Planck constant, es is the dielectric con-
stant9 of MoS2 (¼7.6), and T is temperature. gc(E) in Eq. (1)
is the 3D density of states for holes and LTF is the
Thomas-Fermi screening length for a degenerate gas given
by LTF ¼ffiffiffiffiffiffiffiffiffiffiffiffiffi
es
q2gcðEFÞ
q. Fig. 1(d) shows hole mobility as a func-
tion of acceptor density (NA) in p-MoS2, as calculated by
Eq. (1). Given the large uncertainty in various parameters
used in the model, the agreement with the experimental data
is satisfactory.
The material and structural quality of the MoS2 samples
were assessed using Raman spectroscopy, high resolution
XRD scans, aberration-corrected STEM imaging, and optical
absorbance measurements. The undoped MoS2 (sample C),
as well as Nb-doped p-type samples A and B were found to
FIG. 1. (a) Schematic of Nb-doping in
CVD grown MoS2. (b) Temperature-
dependent Hall measurement on
degenerately doped sample A (p¼ 1.5
� 1021 cm�3) showing no carrier
freeze-out at 20 K. (c) TLM fitting to
extract sheet and contact resistance on
sample B using Ni/Au/Ni metal con-
tacts. (d) Ionized impurity scattering
limited hole mobility in p-MoS2, theo-
retically estimated using 3D formalism,
compared with measured data.
092104-2 Laskar et al. Appl. Phys. Lett. 104, 092104 (2014)
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be crystalline in nature exhibiting good structural and mate-
rial qualities as evident from the measurements using these
various characterization techniques. Fig. 2 shows the on-axis
XRD scans for the samples A, B, and C. The (002) peak of
MoS2 was observed in all the three scans although the inten-
sity for sample A was lower, indicating degradation of mate-
rial quality under such high doping levels (1.5� 1021 cm�3).
Sample B exhibited high intensity (002), (004), and (006)
peaks, indicative of good crystalline quality of MoS2. Fig.
3(a) shows the Raman spectra of the samples taken with a
Renishaw, 514 nm laser (with 60mW power). Characteristic
in-plane (E12g) and out-of-plane (A1g) vibrational modes
were observed at 381 and 407 cm�1, respectively,22,23 for all
the three samples. This indicates that the phonon spectrum of
MoS2 remains unchanged despite the addition of Nb.
Optical absorbance measurements were performed on
all the samples using a broad UV-VIS-NIR deuterium-tung-
sten-halogen white light source. The absorbance spectra was
taken for a reference piece of sapphire in order to determine
a reference intensity I0(k). The MoS2 sample was measured
and the absorbance was determined using A ¼ IðkÞI0ðkÞ, where
I(k) is the intensity collected by the monochromator after the
light is transmitted through the MoS2 grown on sapphire.
The normalized absorbance spectra (Fig. 3(b)) show that all
three samples A, B, and C exhibit an absorption edge at
1.8 eV indicative of a direct band-to-band transition in these
semiconductors. DFT calculations in Ref. 24 show that
although the lowest energy transition in bulk MoS2 is
approximately 1.2 eV, the direct transition of approximately
1.8 eV still exists at the K point. This is verified25 by the fact
that both PL as well as absorbance measurements on bulk
MoS2 have been found to exhibit a prominent transition or
peak around 1.8–1.9 eV which has been attributed to
direct-gap transitions between split valence band maxima
(v1 and v2) and conduction band minima all located at Kpoint of Brillouin zone. For the Nb-doped samples A and B,
the 1.8 eV direct gap is found to exhibit clear band tails,
which is a good indicator of the shallow Nb states. The mul-
tiple blue/UV peaks are possibly due to transitions involving
higher bands.
An atomic force micrograph (AFM) of sample B shown
in Fig. 4 (rms roughness �1.3 nm, scans size: 2 lm� 2 lm)
indicated complete coverage of surface by MoS2 and a rela-
tively smooth surface.
STEM imaging was performed on an aberration-
corrected Nion UltraSTEM-100TM electron microscope oper-
ating at an acceleration voltage of 60 kV.26 Fig. 5 shows an
atomic resolution Z-contrast STEM image of the Nb-doped
sample B which exhibited the highest mobility of 8.5 cm2/Vs.
The Z-contrast image reveals that the ordered crystalline na-
ture of the MoS2 layers stacked in the [0001] direction on a
sapphire substrate is preserved even with degenerate p-type
doping. Based on larger HRTEM images, the grain sizes are
estimated to be larger than 100 nm. Recent MoS2 samples
(unrelated to this present experiment) grown under slightly
different conditions have large area epitaxial ordering (as
shown by off-axis XRD spectra with 6-fold symmetric
peaks). However, the AFM morphology was quite similar to
FIG. 2. High-resolution XRD scans for samples A (p¼ 1.5 � 1021 cm�3), B
(p¼ 3.1 � 1020 cm�3), and C (undoped).
FIG. 3. (a) Raman spectra on samples
A (p¼ 1.5 � 1021 cm�3), B (p¼ 3.1
� 1020 cm�3), and C (undoped). (b)
Absorbance measurements for samples
A (p¼ 1.5 � 1021 cm�3), B (p¼ 3.1
� 1020 cm�3), and C (undoped).
092104-3 Laskar et al. Appl. Phys. Lett. 104, 092104 (2014)
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the samples discussed in this paper. Therefore, the AFM mor-
phology is believed to be probably indicative of adatom dif-
fusion dynamics rather than grain size. More investigations
are currently carried out to estimate the grain size.
In conclusion, we show that Nb can act as an efficient
acceptor in MoS2 leading to high hole density and relatively
high mobility. For a hole concentration of 3.1� 1020 cm�3, a
hole mobility of 8.5 cm2/Vs was measured at room tempera-
ture and was found to be limited by ionized impurity limited
scattering. The use of Nb substitutional impurity for p-type
doping demonstrated here for MoS2 could be extended to
other dichalcogenides and could therefore have wider appli-
cations. Furthermore, the simple deposition scheme used
here could be employed to directly pattern areas with p-type
MoS2, thus providing flexibility for device design. This first
demonstration of substitutional p-type doping in large area
thin film CVD-grown MoS2 is expected to enable several
high-performance electrical and opto-electronic devices.
S.R. acknowledges funding from the NSF NSEC
(CANPD) Program (EEC0914790) and NSF Grant ECCS-
0925529. L.M. and Y.W. acknowledge the support from NSF
(CAREER, DMR-0955471). M.L. was supported by the OSU
NSF MRSEC CEM Seed Program. R.C.M. acknowledges
funding from NSF (CAREER, DMR-1055164) and (CBET-
1133589). This research was partially supported by the Office
of Basic Energy Sciences, Materials Sciences and Engineering
Division, U.S. Department of Energy (RM, MAR, STP, SJP),
the Center for Nanophase Materials Sciences User Facility
(J.C.I.), which is sponsored at Oak Ridge National Laboratory
(ORNL) by the Scientific User Facility Division, Office of
Basic Energy Sciences, U.S. Department of Energy, the
European Research Council Starting Investigator Award
STEMOX #239739 (MAR).
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FIG. 4. AFM of sample B (2 lm� 2 lm scan), data scale: 8 nm; RMS
roughness: 1.3 nm.
FIG. 5. Z-contrast STEM image of Nb-MoS2 film (sample B) on Al2O3 sub-
strate showing the stacking of MoS2 layers in the [0001] direction.
092104-4 Laskar et al. Appl. Phys. Lett. 104, 092104 (2014)
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