XPEEM with energy filter
S. Heun, Laboratorio TASC-INFM, Trieste, Italy,
A. Locatelli and M. Kiskinova,Sincrotrone Trieste, Basovizza, Trieste, Italy
MotivationWhy XPS?
chemical state informationsurface sensitiveease of quantificationnondestructive
Why spectromicroscopy ?(semicond.) nanostructures: self-organization, lithographydevicesdiffusion, segregationalloying (silicide formation)chemical reactionssurface magnetism (XMCD)
Scanning vs. direct imaging type
Photon optics is demagnifying the beam:Scanning Instrument
Whole power of XPS in a small spot mode.Flexibility for adding different detectors.Rough surfaces can be measured.Limited use for fast dynamic processes.Lower lateral resolution than imaging instruments
Electron optics to magnify irradiated area:Imaging Instrument
High lateral resolution (20 nm).Multi-method instrument (XPEEM/PED).Excellent for monitoring dynamic processes.Poorer spectroscopic ability.Sensitive to rough surfaces.
Monochromaticlight
Sample(scanned)
Photo-emissiondetector
Detector hv
Object plane
Object lensBackfocal planeStigmator
Intermediate imageIntermediatelens
Projectivelens
Image planeMCPScreen CCD camera
Sample
Intermediate image
e-
PEEM: instrumentation
PEEM without energy filterPEEM with imaging retarding field filterPEEM with area selective spectroscopyPEEM with full spectromicroscopic capabilities
PEEM with sector field (LEEM)PEEM with aberration correction
PEEM with retarding field filter 1Sample
Imaging DeviceImaging Device
Objective lensObjective lens
Contrast aperture
Stigmator/Deflector
First Projective lens
Multi Channel PlateFluorescent Screen
Iris ApertureIris Aperture
Second Projective lens
GmbH
Double Grid System
Retard LensImaging
Energy Filter
Photons (UV..VUV..X rays)
retard lensdouble grid
MultiMulti--channelplatechannelplate
Fluorescent Screen
divergent rays
parallel rays
E0E1< < E2
PEEM with retarding field filter 2
Counting mode:Screen as collector for integral electron flux Numerical derivation delivers N(E) MicrospectroscopyFOV > 0.5µm and dE > 0.5eVAlternative: Modulation technique
Gm
bH
Imaging mode:High pass filtered image differences:Spectromicroscopy
PEEM with retarding field filter 3
M. Merkel et al.: Surf. Sci. 480 (2001) 196
Raw data 1st derivative
Micro-XPS using soft X-ray synchrotron radiation,Bessy I, hv = 100 eV, W(110),
spectra taken with retarding field filterin counting mode,
field of view: 100 µm, 109 ph/s
PEEM with band pass filter
R. Wichtendahl et al.: Surf. Rev. Lett. 5 (1998) 1249Y. Sakai et al.: Surf. Rev. Lett. 5 (1998) 1199G. Schönhense et al.: Surf. Sci. 480 (2001) 180
Time Of FlightWien FilterHemispherical AnalyzerDouble Hemispherical AnalyzerOmega Filter
The SPELEEM
Spectroscopic Photoemission and Low Energy Electron MicoscopePEEM with full spectromicroscopic capabilitiesPEEM with magnetic sector field in combination with MEM and LEEMNo aberration correction yetModes of operation:
Imaging XPEEM (spectromicroscopy)Dispersive plane (microspectroscopy)Diffraction mode (PED)
XPEEM: Spectroscopic Microscopy
Images from a Field Effect Transistor (FET) at different binding energies.Photon energy 131.3 eV.
Imaging of Dispersive Plane
W{110} clean surfaceW 4f core level hv = 98 eVResolution 210 meV
(1) Riffe et al., PRL 63 (1989) 1976.(2) Webelements
SCLS (eV)
W7/2-W5/2 BE diff. (eV)
Gauss. Broad. (eV)
Alpha S
Gamma S (eV)
Alpha B
Gamma B (eV)
parameter
0.3040.321
2.142.2 (2)
0.210.04
Fixed0.063
Fixed0.084
Fixed0.035
Fixed0.06
Our fitRef. (1)
Lateral resolution
C 1s image (hv= 350 eV, KE = 62 eV)
Lateral resolution: 32 nm
inte
nsity
(a.u
.)100806040200
distance (pixel)
lateral resolution:0.7 * (4 pixel /435 pixel ) * 5000 (nm)= 32 nm acquisition time: 1min
275
270
265
260
255
250
Inte
nsity
(arb
. uni
ts)
10008006004002000Length (nm)
C 1s光電子像
∼ 43 nm
光電子強度プロファイル
放射光
62.5 eV
ラインパタンパタンの影
SWNT(束)
Satoru Suzuki, Yoshio Watanabe and Yoshikazu HommaStefan Heun and Andrea Locatelli
AFM NanolithographyStudied by Spectromicroscopy
S. Heun, M. Lazzarino, G. Mori, D. Ercolani, B. Ressel, and L. Sorba,Laboratorio TASC-INFM, S.S. 14, km 163.5, I-34012 Trieste, Italy,
A. Locatelli, S. Cherifi, A. Ballestrazzi, and K. C. Prince,Sincrotrone Trieste, S.S. 14, km 163.5, I-34012 Trieste, Italy,
P. Pingue,NEST-INFM and Scuola Normale Superiore, I-56012 Pisa, Italy.
Local Anodic Oxidation (LAO)
Water electrolysis H2O → H+ + OH-.OH- groups migrate towards the sample.Oxide penetration induced by the intense local electric field.
Versatile tool at relatively low costHigh lateral resolution but small area
Si Oxide: Image Contrast at Si 2p
Field of view 12 µm, hv = 132.5 eV, energy resolution: 1 eV
M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.
Binding energy 104.7 eV
a-14V
-12V
-10V
-8V
Binding energy 102.3 eV
b
Si Oxide: Spectroscopy at Si 2p
Spectra show high unformity of each stripeBulk silicon peak: position and width constantNative oxide: SiO2 peak (3.9 eV shifted)AFM oxide shows higher binding energyThere is no evidence of broadeningSpectra explain contrast inversion
M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.
104.7eV
102.3eV
ba
110 109 108 107 106 105 104 103 102 101 100 99 98 97 96 950.0
0.2
0.4
0.6
0.8
1.0
native oxideAFM oxide
Inte
nsity
(a.u
.)
binding energy (eV)
Si Oxide: Writing Voltage Effect
14V14V
nativenative
12V12V
10V10V
8V8V
1.0
0.8
0.6
0.4
0.2Nor
mal
ized
Pho
toem
issi
on In
tens
ity
110 105 100 95Binding Energy (eV)
: native oxide: 14 V: 12 V: 10 V: 8 V
Si 2p
siliconoxide
bulksilicon
M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.
ΔE = 3.97 eV (native)ΔE = 4.62 eV (U = 14V)ΔE = 4.46 eV (U = 12V)ΔE = 4. 41 eV (U = 10V)ΔE = 4.39 eV (U = 8V)
Shift (Sibulk - SiOx) increases with increasing writing voltage (oxide thickness).
Si Oxide: Charging Effects
H. Kobayashi, T. Kubota, H. Kawa, Y. Nakato, and M. Nishiyama: Appl. Phys. Lett. 73 (1998) 933.
+ +
+ +
+ +
Intensity decreases due to escape depth
effectCharged (yellow)
layer contribution is more important
Squared points: our data
Line: Kobayashi model
GaAs Oxide: Photon Exposure
AFM after: height 13 nm
AFM before: height 18 nm
Images taken with secondary electronsPhoton energy: 125 eVKinetic energy: 4 eVField of view: 10 µmOne image every 2 sec
Spectra From AFM GaAs Oxide
Sample S03BHole (3,2)Writing voltage 15 VStructure height 3 nmImage taken with secondary electrons:
Photon energy: 130 eVKinetic energy: 0.3 eVField of view: 10 µm
Time resolved spectroscopy with SPELEEM using Dispersive Plane (hv = 130 eV)
Spectra From AFM GaAs Oxide
Time resolved spectroscopy with SPELEEM using Dispersive Plane (hv = 130 eV)
2500
2000
1500
1000
Inte
nsity
(a.
u.)
12 10 8 6 4 2Relative Binding Energy (eV)
Exposure Time 3 min 5 min 7 min 13 min 20 min 28 min 43 min 63 min
As 3dhv = 130 eV
1000
800
600
400
200
Inte
nsity
(a.
u.)
12 10 8 6 4 2Relative Binding Energy (eV)
Exposure Time 4 min 6 min 10 min 17 min 25 min 54 min 74 min
Ga 3dhv = 130 eV
As oxide GaAsO 2s
Ga oxides+ GaAs
The As-oxide signal decreases and disappears.The Ga-oxide signal remains unchanged (early stage of exposure).
Spectra From AFM GaAs Oxide
4000
3500
3000
2500
2000
Inte
nsity
(a.
u.)
888684828078Kinetic energy (eV)
As 3dhv = 130 eV
GaAs
AsO
20x103
15
10
5
0
Inte
nsity
(a.
u.)
110108106104102100Kinetic Energy (eV)
Ga 3dhv = 130 eV
GaAs
Ga2O
Ga2O3
O 2s
After about 1 hour of exposure, only traces of As-oxides are observed in the As 3d core level. The AFM-oxide is mainly composed of Ga2O.
The Knotek-Feibelman mechanism
The valence electrons are mainly localized at O atoms.This Auger decay leads to a final state with two vacancies in valence band weakening the bond between Ga and O.
Valence electrons
E
3d
GaInitial state
Valence electrons
E
3d
Ga
Valence electrons
E
3d
Ga
Valence electrons
E
3d
GaFinal state
Summary
Si oxide:The AFM induced oxidation produces chemically uniform, stoichiometric SiO2 with dielectric properties comparable to those of thermal SiO2.
GaAs oxide:Photon assisted partial desorption of the AFM-grown oxide was observed.All As oxides are desorbed.The AFM-oxide is mainly composed of Ga2O.We proposed a simple model for the dynamics of the desorption.
Ge/Si concentration in Ge/Si(111)
Fulvio Ratto, Federico RoseiNFL, INRS–EMT, Université du Québec, 1650 Boul. Lionel Boulet, J3X 1S2 Varennes (QC) Canada
Andrea Locatelli, Salia Cherifi, Stefano Fontana, Stefan HeunSincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, 34012 Trieste (Italy)
Pierre-David Szkutnik, Anna Sgarlata, Maurizio De CrescenziDip. di Fisica and Unità INFM, Univ. di Roma II, Via della Ricerca Scientifica n.1, 00133 Roma (Italy)
Nunzio MottaDip. di Fisica and Unità INFM, Univ. di Roma TRE, Via della Vasca Navale 84, 00100 Roma (Italy)
Motivation
Goal: integrate optoelectronics and microelectronics on the same Si wafer3D islands could act as quantum dots (QD)Critical Issues in the self–assembly of QD:
Size and shape uniformity of the islandsControlling the nucleation sitesStability of the islandsComposition of 3D structures => determines structural and electronic properties
XPEEM
“top view” mapping of island concentration
X-Ray excited photoelectrons are energy filtered and focused on a
screen, creating a real space image with chemical contrast
Previous work on intermixing:
Cross sectional techniques yielding a “side view”concentration mapping
Our approach:X-Ray photoemission electron microscopyPhotoelectron spectra with high lateral resolution (~30 nm)
Chemical contrast
F. Ratto et al., APL 84 (2004) 4526
10 ML Ge on Si(111), T = 560 °C
Ge segregates in the 3D islands
island
partially shadowed region
Intensity contour plots
Concentration profile in 3D islands
Diffusion mechanisms
Island centre is richer in Si than the borders
95% 100%
island
partially shadowed region
T = 530 °CF. Ratto et al., APL 84 (2004) 4526
Concentration vs. Base area *
F. Ratto et al., APL 84 (2004) 4526
Si concentration increases with island’s base area, up to ~40%
Deposition temperature determines the relation between Si surface content and base area
*Boscherini et al., APL 76 (2000) 682
Diffusion mechanisms
Intermixing may be simply due to surface mobility
Borders richer in Si than centre
Thermodynamically favoured configurations need atomic exchange
Centre richer in Si than borders
Conclusions
XPEEM allows a top view mapping of surface Si concentration
Island’s center is richer in Si than the borders
Si surface content in individual islands increases with their base area
The deposition temperature determines the Si concentration-base area relation
AcknowledgementsXPEEM instrumentation
T. Schmidt, S. Günther (Sincrotrone Trieste)C. Koziol (Elmitec)J. Westermann, B. Holmes (Omicron)
AFM NanolithographyM. Lazzarino, G. Mori, D. Ercolani, B. Ressel, L. Sorba (TASC-INFM, Trieste)S. Cherifi, A. Ballestrazzi, K. C. Prince (Sincrotrone Trieste)P. Pingue (NEST-INFM, Pisa)
Ge / Si islandsF. Ratto, F. Rosei (Université du Québec)S. Cherifi, S. Fontana (Sincrotrone Trieste)P.-D. Szkutnik, A. Sgarlata, M. De Crescenzi (Università di Roma II)N. Motta (Università di Roma TRE)