Chemical Bonding on GaAs (001) Surfaces Passivated Using SeS 2
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formation of a thermally and chemically stable passivating layer [4]. As in other cases of chemical-based surface passivation, the underlying passivating mechanism of the SeS 2-treated GaAs (001) surface still is not clearly understood. Knowledge of the chemical bonding at the treated surfaces and the corresponding electronic properties has been incomplete and is presently addressed in this study. The GaAs (001) surfaces passivated by SeS2 , using wet chemical techniques, have been studied with synchrotron radiation photoemission spectroscopy. Core-level spectra exhibiting predominantly either surface or bulk features have been obtained using the tunability of synchrotron radiation source. These results reveal a rich surface-derived substructure that is due to changes in the near surface chemical bonding environment. The core-level shifts induced by surface Fermi level movements have been identified. A structural layer model is proposed based on the analysis of the spectroscopic results. EXPERIMENT Si-doped n-type GaAs (001) wafers with a carrier density of lxl018 cm"3 were used this study. The native oxide on the surface was initially removed with HCl:H 20 (1:1) solution and blown dry with N 2 gas. The samples were subsequently dipped into a saturated solution of SeS2 in CS 2 for 40 seconds. Any excess SeS 2 on the surface was removed by repeated CS 2 rinsing. The above treatment was performed at room ambient. Samples prepared using above described 589 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
SeS 2-treatment will be referred to as the 'as-treated' samples. The photoemission experiments were carried out at the Synchrotron Radiation Center (SRC) of the University of WisconsinMadison. Monochromatic light of different photon energies was used to control the emitted electron kinetic energy to enable the depth profiling. The photoelectrons were analyzed with a double pass cylindrical mirror electron kinetic energy analyzer (CMA). Kinetic energies (KE) of 40 eV and 300 eV, corresponding to photoelectron escape depths of 0.5nm and 1.5 nm, were utilized to measure the surface-sensitive and bulk-sensitive spectra, respectively. A clean GaAs (001) surface, as a reference for subsequent measurements, was obtained by thermal annealing up to 6000 C in the photoemission chamber under ultra-high vacuum (UHV). The Fermi level position on the sample holder was determined relative to a reference sample of Ta foil that was cleaned by Argon sputtering and thermal annealing. In order to investigate the nature of chemical bonding on the surface, the 'as-treated' samples were annealed in the photoemission chamber under UHV at increasing temperatures. The sample temperature during thermal annealing was measured by an optical pyrometer. RESULTS The changes of photoemission peak intensity ratios between the different elements were used to deduce the in-depth elemental composition resulting from the SeS 2 -treatment. The atomic concentration ratios can be approximately determined from the photoemission peak inte
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