Electronic properties of hydrogenated amorphous silicon-germanium alloys

Abstract

The electronic properties of some binary hydrogenated amorphous silicon-germanium alloys a-SixGe1-x: H in the silicon rich region (x > 0.6) are investigated. Experimental evidence is presented of photo-induced effects similar to those described in a-Si:H (Staebler-Wronski effect). The electronic properties are then studied from the dual point of view of the germanium content dependence and of the photo and thermal histories of the films. The dark conductivity changes between the annealed state and the light-soaked state are interpreted in terms of the variation of the temperature coefficient of the Fermi level. The photoconductivity efficiency is shown to remain close to that of a-Si:H for 1 > x ≽ 0.9 and to strongly decrease when the germanium content is further increased : the photoresponse of the Si0.62Ge 0.38 alloy is 104 times smaller than that of a-Si:H. This deterioration of the photoconductive properties is explained in terms of the increase of the density of gap states following Ge substitution. This conclusion is based on the study of the width of the exponential absorption edge and on the results of photoconductivity time response studies. The latter data are interpreted by means of the model of Rose of trapping and recombination kinetics and it is found that for x ≃ 0.6 the density of states at 0.4-0.5 eV below the mobility edge is 7 x 10 17 eV-1 cm-3 as compared to 2.4 x 1016 eV-1 cm-3 for x = 0.97.