Effects of oxygen on the properties of amorphous Ge films

Abstract

Structural, electrical, and optical properties are investigated of a-Ge films deposited at normal and oblique incidence in the presence of different gas ambients. The presence of oxygen at pressures ~10^-5 Torr during deposition of a-Ge films, both at normal and oblique incidence, does not affect significantly the commonly observed physical properties of a-Ge. In sharp contrast, an increase in the low temperature activation energy and thermoelectric power and a decrease in the void and columnar structure, electrical conductivity, and absorption at low energies with an associated shift of the absorption edge towards higher energies are observed in films deposited in the presence of oxygen at pressures gE; 10^-4 Torr, particularly at elevated temperatures (>400 K). It seems that the presence of oxygen during deposition helps in removal of voids and associated dangling bonds by reacting chemically with Ge atoms. A reduction in the tailing and associated sharpening of the band edges and decrease in the density of localized states at the Fermi level is also indicated.