Comparison of the structural, electrical, and optical properties of amorphous silicon-germanium alloys produced from hydrides and fluorides

Amorphous silicon-germanium alloys have been prepared in the same rf glow-discharge reactor from (${\mathrm{SiH}}_4$+${\mathrm{GeH}}_4$) and (${\mathrm{SiF}}_4$+${\mathrm{GeF}}_4$+${\mathrm{H}}_2$) mixtures at substrate temperatures between 200 and 400 °C. The principal aim of the investigation has been to discover whether preparation of these alloys from fluorides rather than from hydrides will result in better photoelectronic properties, and if so, whether the underlying cause is the substitution of fluorine for hydrogen in the alloy, or some other structure-related alteration of the material. Thus the apparatus, preparation procedures, characterizational techniques, and property measurements follow those of an earlier publication on alloys produced from hydrides alone. The only significant difference in photoelectronic properties found has been an order-of-magnitude improvement of the photoconductivity of alloys with band gaps near 1.5 eV. Evidence is assembled to assert that fluorine substitution for hydrogen in the alloy is not the cause of the changes in photoelectronic properties, but that these are more probably related to changes in a two-phase heterostructure, which are revealed most directly by transmission electron microscopy. A two-phase, two-transport-path model is proposed to explain the improved photoconductivity obtained with the fluoride-derived alloys.