Adsorption of CO, O2, and H2O on GaAs(100): Photoreflectance studies

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Abstract
Adsorption of CO, O2, and H2O on semi-insulating GaAs(100) has been examined with photoreflectance (PR). This work represents the first use of PR for quantitative adsorption measurements on semiconductors. In PR, the laser-induced change in surface reflectance is monitored as a function of wavelength. The resulting spectra are sensitive to changes in both the surface potential and the nature of surface states. Results at the E1 (3.0 eV) and E0 (1.4 eV) transitions are complementary to each other. Sticking coefficients S were obtained from E1 data for these gases, and S at low coverage was found to increase in the order CO≪O2<H2O. S decreases by at least four orders of magnitude for all the gases as saturation is approached. The results suggest that oxygen has two binding states that fill sequentially. Gas adsorption generally improves the communication between isolated surface states and the bulk.

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