Measurement of gap states in undoped a-SiGe: H alloys by modulated photocurrent spectroscopy

Abstract

Modulated photocurrent spectroscopy has been used to investigate the energy location and nature of dangling bonds in undoped a-SiGe: H alloys, with particular emphasis on the doubly occupied dangling-bond (D⁻) centre. It is found that the Ge D⁻ centre lies about 0·1 eV below the Si D⁻ centre, while a sharp switch of the predominant deep centres occurs from Si dangling bonds to Ge dangling bonds as the Ge concentration approaches about 35 at.%, which corresponds to an optical bandgap energy of around 1·5 eV. The analysis for the pre-exponential factor of the thermal emission rate of electrons from the D⁻ centre suggests that different electron transition processes are operative for Si and Ge dangling bonds.