Oxidation of Ge(100) and Ge(111) surfaces: an UPS and XPS study

In situ and ex situ oxidation studies are carried out on Ge(100) and Ge(111) employing techniques of ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). In situ oxidation produces mainly GeO on both the surfaces. Ge 3d and 2p levels show a chemical shift of about 1.4 and 1.8 eV respectively corresponding to this oxide. GeO desorbs from the surface on annealing to 400°C without undergoing any transformation. On exposing to air, a mixture of oxides consisting mainly of GeO and GeO2 are formed on both the surfaces. Ge 3d and 2p levels show a chemical shift of 3.2 eV for GeO2. The amount of GeO2 increases with increasing time of exposure to air. The spectral features of both the species have been identified by UPS and XPS. GeO2 can be selectively removed from the surface by rinsing the sample in warm water. Atomically clean surfaces of Ge have been achieved by thermally decomposing a thin oxide layer prepared ex situ by chemical means, similar to Ishizaka and Shiraki's method for Si. O 1s and O 2p spectra of SiO and GeO species exhibit distinctly different binding energy values and this helps unambiguous assignment of the bonding partner.