Morphology dependence of interfacial oxidation states of gallium arsenide under near
ambient conditions
Xueqiang Zhang1,2, Edward Lamere1,3, Xinyu Liu3, Jacek K. Furdyna3 and Sylwia Ptasinska1,3*
1
Radiation Laboratory, 2Department of Chemistry and Biochemistry, and 3Department of
Physics, University of Notre Dame, Notre Dame, IN 46556, USA
* Author to whom correspondence should be addressed. Electronic mail:
Sylwia.Ptasinska.1@nd.edu. Telephone: (574) 631-4506. Postal Address: Radiation Laboratory,
University of Notre Dame, Notre Dame, IN 46556 USA.
Supporting Information
1. Near Ambient Pressure XPS (NAP XPS) system
The in-situ interfacial chemistry of oxygen adsorption on the GaAs surface with two different
morphologies was monitored by the NAP XPS system in the Notre Dame Radiation Laboratory.
The analysis chamber of NAP XPS is equipped with a differentially pumped SPECS PHOIBOS
150 hemispherical analyzer, a Micro-FOCUS 600 X-ray Monochromator and a reaction cell. The
reaction cell can operate up to pressures of 20 mbar in a continues–flow mode with a sample
stage heated up to 800 K. A more detailed description of this instrument is reported in our
previous work.1
All photoemission spectra were recorded using an X-ray beam generated by an aluminum
(Al) Kα (1486.6 eV) X-ray source at a power of 100 W with an anode voltage and a current
of 15 kV and 6.7 mA, respectively. The angle between the incident X-ray beam was kept at
54.7o with respect to the normal of sample surface. The electron energy hemispherical
analyzer was operated at high resolution mode with the pass energy of 20 eV and the energy
step of 0.05 eV to resolve small chemical shifts due to changes in GaAs oxidation states.
1
Survey photoemission spectra were measured with the pass energy of 70 eV and the energy
step of 0.5 eV. Under these conditions, the measured full width at half maximum (FWHM) of
Au 4f7/2 peaks is 0.5 eV and 1.2 eV for high resolution and survey spectra, respectively. The
energy calibration of the spectra was referenced to the method reported by Surdu-Bob et al.2
in which the As 3d5/2 peak was fixed at 41.1 eV. Peak fitting of all photoemission spectra was
performed using SpecsLab2 software and Computer Aided Surface Analysis for X-ray
Photoelectron Spectroscopy (CasaXPS) with a combination of Gaussian/Lorentzian functions
in the ratio of 70:30 after subtracting a Shirley background. The BE of each fitted component
and its FWHM during peak deconvolution were constrained throughout all data processing
with the variability kept below ± 0.1 eV.
2. Sample pretreatment
The undoped simple planar GaAs crystalline surface with (100) orientation was
purchased from AXT. Inc, USA. The GaAs nanowires (NW) used in this study were grown by
Molecular Beam Epitaxy (MBE) using a Riber 32 system at the University of Notre Dame. Prior
to the nanowire growth a 1 nm to 2 nm Au layer was deposited on a GaAs(111)B substrate by an
electron-beam evaporator installed in the MBE chamber. The substrate was then heated to 873 K
to form Au-Ga eutectic nano-droplets under As2 flux. After the substrate temperature was cooled
to 803 K, the Ga and As shutters were opened to grow Au-catalyzed GaAs nanowires via the
vapor–liquid–solid (VLS) growth mechanism.3-6 GaAs nanowire templates were grown for 4
hours, with a nominal film growth rate of ~0.4 Å/sec. A Scanning Electron Microscopy (SEM)
image of the NW ensemble used in the present measurements is shown in Fig. S1.
2
Fig. S1 SEM images of GaAs NWs before (a) and after Ar+ bombardment (b).
Both samples were cleaned by cycles of argon ion (Ar+) bombardment (1 keV, 10 min)
followed by annealing to 825 K in order to remove native contaminants (oxides, hydrocarbons,
etc.) and to restore the surface. The Ar+ beam was kept at 90o with respect to the sample surface.
Surface purity and NW morphology were monitored by XPS and SEM, respectively. Neither
contamination nor NW destruction (except removing the gold caps from the top of nanowires)
were observed after such treatments as shown in Figs. S1 and S2. However, in the present
experiment the Ar+ bombardment results in preferential removal of As due to weaker surface
binding energy of As compared to that for Ga atoms, leaving a Ga-rich surface.7,8 It is worth
noting that the majority of Ga atoms are still believed to be bound to tetrahedral sites with a
minor Ga-Ga bond formation, despite the As deficiency. Since Ga 2p spectra are very surface
sensitive, the formation of considerable amounts of Ga-Ga bonds can certainly cause a shift to
lower binding energy in the Ga 2p spectra. However, no such a shift is observed in the Ga 2p
spectra.
3
Fig. S2 Survey photoemission spectra of “as received” and argon ion bombardment treated simple
planar GaAs crystalline (100) surface.
3. Surface chemistry evolution
After cleaning, the contamination-free samples are transferred to the reactor chamber.
Two types of experiments were performed on samples with the planar (100) GaAs surface and
on the nanowire samples: exposure to different O2 pressures (i) and temperatures (ii). i) The
pressure-dependent investigation was performed in the O2 pressure in the range from UHV
(5×10-10 mbar) to 5 mbar at room temperature (RT, ~300 K), and ii) the temperature-dependent
study was carried out in the temperature range from RT to 773 K under a constant O2 pressure
environment (0.1 mbar).
4
Figures S3-S6 display the evolution of photoemission spectra for both morphologies
under different pressures and temperatures. To assist the analysis, the photoemission spectra are
normalized in order to better reveal the shift and broadening of the different peaks. This
normalization is particularly helpful for comparing the spectra obtained in the pressuredependent study, in which elevated pressures significantly decrease the mean free path of the
photoelectrons, thus resulting in weakened XPS peak intensities.
i) Pressure-dependent studies
Fig. S3 Evolution of high resolution photoemission spectra observed on planar GaAs surface at
RT and different pressures and after O2 is pumped away (“evacuated”). The Ga 2p3/2 and As 2p3/2
5
spectra were normalized for better comparison due to their shorter mean free paths at higher
pressures.
Fig. S4 Evolution of high resolution photoemission spectra observed on GaAs nanowires at RT
and different pressures and after O2 is pumped away (“evacuated”). The Ga 2p3/2 and As 2p3/2
spectra were normalized for better comparison due to their shorter mean free paths at higher
pressures.
ii) Temperature-dependent studies
6
Fig. S5 Evolution of high resolution photoemission spectra for a planar GaAs surface at 0.1 mbar
of O2 and different temperatures.
7
Fig. S6 Evolution of high resolution photoemission spectra for GaAs nanowire ensemble at 0.1
mbar of O2 and different temperatures.
(1) X. Zhang, S. Ptasinska, Journal of Physical Chemistry C 2014, 118, 4259.
(2) C.C. Surdu-Bob, S.O. Saied, J.L. Sullivan, Applied Surface Science 2001, 183, 126.
(3) R.S. Wagner, W.C. Ellis, Applied Physics Letters 1964, 4, 89.
(4) V.G. Dubrovskii, G.E. Cirlin, I.P. Soshnikov, A.A. Tonkikh, N.V. Sibirev, Y.B. Samsonenko, V.M. Ustinov,
Physical Review B 2005, 71, 205325.
(5) J.C. Harmand, M. Tchernycheva, G. Patriarche, L. Travers, F. Glas, G. Cirlin, Journal of Crystal Growth
2007, 301, 853.
(6) K. Tivakornsasithorn, R.E. Pimpinella, V. Nguyen, X. Liu, M. Dobrowolska, J.K. Furdyna, Journal of
Vacuum Science & Technology B 2012, 30, 02B115.
(7) H.J. Kang, Y.M. Moon, T.W. Kang, J.Y. Leem, J.J. Lee, D.S. Ma, Journal of Vacuum Science &
Technology A-Vacuum Surfaces and Films 1989, 7, 3251.
(8) K.A. Gschneidner Jr, In Solid State Physics; S. Frederick, T. David, Eds.; Academic Press: 1964; Vol. 16,
p 275.
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