The depth profile of thermally, anodically and plasma grown oxides on GaAs has been studied by means of X-ray photoelectron spectroscopy (XPS). The ion sputtering effects are taken into account in obtaining quantitative chemical depth profiles. Thermally oxidized film is composed of Ga
2O
3 (98%) with a large pile up of elemental-arsenic at the interface. In contrast, anodically oxidized film shows As
2O
3/Ga
2O
3 ratio of 0.8-0.9 except in the interfacial region, where the concentration ratio of Ga
2O
3: As
2O
3: elemental-As is equal to 1:0.7:0.2 for the as-grown oxide and 1:0.55:0.35 for the annealed oxide. The plasma oxidized film consists of Ga
2O
3 (_??_65%), As
2O
3 (_??_30%) and elemental-As (_??_5%) with evidence of As
5+ at the surface. In the interfacial region, the plasma oxidized film is observed to contain less elemental-As than the anodic oxide/GaAs.
The literature on the chemical composition of oxide/GaAs is also discussed.
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