A Study of the Optical and Electrical Properties of Amorphous Germanium and Amorphous Germanium-Gallium Alloys

Abstract

This project has consisted of measuring the optical properties and the d.c. resistivity (from 77K to 300K) of amorphous Ge and amorphous GeGa alloys. Thin film samples were prepared by co-evaporation of Ge and Ga in an ultrahigh vacuum system. Four samples were prepared at the same time one pure and three alloys of varying Ga concentration. The optical study consisted of measurements in the far infrared, near infrared and visible spectral regions so that both the vibrational and electronic spectra were studied. A far infrared technique is described that was developed especially to measure the very small absorption by the thin film samples. The far infrared results suggested that a significant fraction of Ga atoms dissolved in amorphous Ge with four fold coordination and that the low frequency far infrared absorption (corresponding to the acoustic modes in crystalline Ge) is due to coupling via static charges centred on voids. The near infrared and visible optical data and the resistivity data are consistent with this analysis, however, the possibility of Ga dissolving in amorphous Ge with a coordination of less than four has not been excluded. Because optical measurements played an important part in this project, a study was made of the absorption in weakly absorbing thin films, unsupported and on both transparent and metallic substrates. In addition an approximation was developed between the abosrption and the optical conductivity.