Structural, electrical, and optical properties of a-Si1−xGex:H and an inferred electronic band structure

Abstract

Amorphous hydrogenated silicon-germanium alloys have been prepared over the full composition range by rf glow discharge of silane-germane mixtures at substrate temperatures between 200 °C and 400 °C. A detailed description of the deposition apparatus, preparation procedures, and techniques of property measurement is given. The dependences on substrate temperature and germanium content of the optical-absorption, photoconductivity, and photoluminescence spectra and of the conductivity-temperature relation are reported and analyzed. Electronic band structures (density of states versus energy) are proposed which incorporate the changes in the optical energy gap with composition and the energies of the dangling-bond states on germanium and silicon atoms. The observed properties are examined in detail in relation to the proposed new density-of-states structure. The possibility is explored of explaining the poor photoelectronic properties with and without invoking a higher density of defects in the alloys. The occurrence of a two-phase heterostructure and the possibility of changes in its constitution, especially the location in it of the hydrogen, with changing substrate temperature and film composition are also discussed.