Density of states of the tetragonal-phase germanium nanocrystals using x-ray photoelectron spectroscopy

Abstract

The valence-band x-ray photoelectron spectroscopy (XPS) spectra of the Ge nanocrystals deposited by the cluster-beam evaporation technique were studied, and suggest the presence of the tetragonal crystal structure (ST-12) of Ge in the Ge nanocrystals. Although the ST-12 structure of Ge is known to be metastable and is transformed to the diamond structure, which is the crystal structure of bulk Ge, at relatively low temperatures, the XPS and absorption spectra of the Ge nanocrystals annealed at various temperatures show that the obtained ST-12 structure is thermally stable. The critical temperature for the phase transformation of the Ge nanocrystals is found to be higher than 700 °C. These results are consistent with our earlier study using Raman spectroscopy.