Raman-backscattering studies on amorphous hydrogenated silicon-germanium alloys

Abstract

Structural properties of amorphous hydrogenated silicon-germanium alloys (a-SiGe: H) have been studied over the whole range of compositions by means of standard transmission/reflection measurements, photothermal deflection spectroscopy (PDS), Raman backscattering and X-ray photoelectron spectroscopy (XPS). The average bond-angle distortion in the silicon sub-matrix, as evaluated from a detailed analysis of a-Si_1-xGe_x: H Raman spectra, does not change in films deposited in a triode glow-discharge system, whereas the transverse-optic-like Raman halfwidth (full width at half-maximum), TO-f.w.h.m., increases significantly in films deposited in a diode system once x = 0·35 is exceeded. The Ge-Ge TO-f.w.h.m. on the other hand remains constant in germanium-rich material (0·7 ≤ x ≤ 1). In both cases this corresponds to the transition from a fully or overcoordinated network (coordination number ≥, √6) to a mixed structure that is determined by Si-Ge bonds. Bond statistics for Si-Si, Si-Ge, and Ge-Ge bonds provide an estimate of the ratio of the Raman coupling constants for the Si and Ge sub-lattices. In general the first is considerably more rigid than the latter which results in large Ge-Ge distortions as well as in additional dangling-bond creation with increasing Ge content.