XPS study of fresh and oxidized GeTe and (Ge,Sn)Te surface

The energies of Ge 2p3/2, Ge 3d, Te 3d5/2, Te 4d5/2 and Sn 3d5/2 photopeaks were measured for vacuum-cleaved GeTe and Ge0.9Sn0.1Te crystals, obtained by the ‘vapour–solid liquid’ technique (VLS). The oxidation of cleaved surfaces in air at ambient temperature and humidity was studied by X-ray photoelectron spectroscopy (XPS). The oxidation seems to take place even after few minutes of air exposure. The first stage of the oxidation results in entire Ge oxidation to the oxidation state 4+ and partial Te oxidation to the elemental state. The intermediate oxidation product is supposed to be GeO2−xTex. The second stage is slower than the first one, and it leads to further Te oxidation to 4+ oxidation state and also the significant surface enrichment in Ge.