Orientation dependence of oxygen adsorption on a cylindrical GaAs sample : II. Photoemission measurements

Abstract The orientation dependence of oxygen adsorption has been investigated by XPS and UPS on the surface of a cylindrically shaped GaAs single crystal with [110] as its axis. This sample exposes the main low-index orientations (001), (111)Ga, (110), (111)As, etc. and all intermediate orientations. It was prepared by ion bombardment and annealing [see part I, Surface Sci. 120 (1982) 67). XPS was used to separate the known two forms of adsorbed oxygen quantitatively. The less tightly chemisorbed a-oxygen has an O 1s binding energy 1.50 ± 0.05 eV larger than the β-oxygen for all surface orientations. Comparison with UPS shows that for h v =40.8 eV the excitation probability of the O 2p orbitals for α-oxygen is four times larger than for β-oxygen. The Ga 3d peak shows the known chemical shift of 1.1 ± 0.1 eV induced by β-oxygen whereas α-oxygen causes no Ga 3d shift. Most of the α-oxygen desorbs thermally starting near room temperature, only a portion smaller than 20% is converted to β-oxygen; β-oxygen desorbs starting at 300–350°C slightly depending on orientation. These results confirm the interpretation of β-oxygen as dissociated oxygen. However, the nature of α-oxygen still is not clear. The strong orientation dependence of the total amount of oxygen is consistent with the Auger results published recently (see part I). The nature of α- and β-oxygen is found to be the same for all orientations. Also the ratio of adsorbed α- to β-oxygen depends surprisingly weakly on the orientation and on the exposure varying only between 0.35 and 0.7 as the extreme values. This suggests that the adsorption of α- and β-oxygen is connected. A possible model is that adsorption, probably of β-oxygen, starts from edges or other energetically less favourable sites and that this disturbance creates stresses or defects which serve as adsorption site for the α-oxygen.