HELIUM SCATTERING STUDY OF CLEAN AND OXYGEN-COVERED GAAS(100)

The Ga-rich GaAs(100) surface and the adsorption of oxygen on this sur face were investigated with He-diffraction, which is completely non-de structive and particularly sensitive to defects. Although the interpre tation of the data is complex due to the large corrugation of the GaAs (100) surface, useful information is obtained. On the Ga-rich GaAs(100 ) surface two reconstructions are observed, the c(8 x 2) and the disor dered (6 X 6) structure. We found a reversible phase transition betwee n these reconstructions at a temperature of about 450 degrees C. The H e-diffraction results support the model of the c(8 X 2) reconstruction proposed by Skala et al. [Phys. Rev. B 48 (1993) 9138]. A new model f or the disordered (6 X 6) reconstruction is proposed. Oxygen is found to adsorb exclusively on Ga-terminated areas of the surface. On both r econstructions we found evidence for two chemisorption states of the o xygen.