L.V.Yashina, S.P.Kobeleva, T.B.Shatalova, V.P.Zlomanov, V.I.Shtanov, XPS study of fresh and oxidized GeTe and (Ge,Sn)Te surface. Solid State Ionics, 2001, v.141-142, p.513-522.
The energies of Ge 2p3/2, Ge 3d, Te 3d5/2, Te 4d5/2, Sn 3d5/2 photopeaks were measured for vacuum cleaved GeTe and Ge0.9Sn0.1Te crystals obtained by vapour-solid liquid technique (VLS). The oxidation of cleaved surface of GeTe in air at ambient temperature and humidity was studied by XPS. The oxidation is seems to take place to significant degree even after few minutes of air exposure. First stage of process results in entire Ge oxidation to the oxidation state 4+ and ~25% Te oxidation to elemental. The intermediate oxidation product is supposed to be GeO2-xTex. Second stage is slower than the first one and results in further Te oxidation to +4 and surface enrichment in Ge.