Structural, optical and transport properties of sputtered hydrogenated amorphous silicon films in relation to Si-H bonding configurations

Abstract

The hypothesis of a relationship between the structural effects of hydrogen on the Si matrix and the electronic structure of hydrogenated a-Si has been tested on d.c, sputtered 1 μm thick a-Si_t-x H _x films, with x ranging from 0 to 9 at.%. Comparative optical, electrical and X-ray diffractometry measurements have been performed on samples in which different kinds of H distribution and Si-H bonds were achieved by varying the total pressure (P _tot) of Ar and H₂ and the substrate temperature (T _s).

For H concentrations in the range 0–5 at.%, materials with either a predominant 2000cm⁻¹ or 2100cm⁻¹ infrared (i.r.) absorption stretching band could be prepared (without the 890cm⁻¹ bending mode). It has been observed that, for a given H content, the optical gaps of these two materials are the same, despite the fact that their atomic distribution functions (ADF) exhibit significant differences in the region 4–6 Å. The behaviour of the dark conductivity α with temperature is sensitive to the type of H distribution through the pre-exponential factor and the range over which it is thermally activated. When the H content is increased from 5 to 9 at%, the optical gaps increase as well as the activation energies of α, but there is no correlative trend of the interference function towards a more relaxed structure as occurs just before the crystallisation of non-hydrogenated materials.

It is concluded that if H atoms play a role in relaxing the Si network at low concentrations this indirect structural effect cannot explain by itself the enlargement of the energy gap with the H content. A tentative explanation is given of the inferior photovoltaic performances of sputtered materials compared to glow-discharge ones.