In-situ X-ray photoelectron spectroscopy study of the oxidation of CuGaSe2

The thermal and native oxidation of CuGaSe2 thin films was studied by in-situ X-ray photoelectron spectroscopy (XPS). The special design of the XPS chamber allowed to measure XP-spectra under oxidizing gas atmospheres at pressures of up to 5mbar (in-situ) or in ultra high vacuum (UHV). During thermal oxidation, the formation of predominantly Ga2O3 and some amount of SeO2 were observed, but no copper oxides could be detected in the near surface region of the thin films. The same oxides were found after native oxidation in air under ambient conditions. Only after long term native oxidation for longer than four months Cu(OH)2 was detected. An additional sodium oxide compound formed at the thin film surface, NaxO and Na2CO3 after thermal and native oxidation, respectively. The amount of these sodium oxide compounds depends on the Na content on the as prepared surface. The formation of SeO2 under humid conditions at 100°C was found to depend on the surface composition of the thin film.