Synthesis of Silicon-Germanium Film Alloys Based on Chemically Formed Porous Silicon Layers

Formation of silicon-germanium alloy films by electrochemically filling a porous silicon matrix with germanium and subjecting it to rapid thermal processing at 950 °C in argon flow is investigated. Low-porosity porous silicon layers are obtained using metal-assisted chemical etching of lightly-doped silicon wafers. It is shown that the alloy film formed in the employed temperature regime is always located on a residual porous underlayer. The difference in the thickness of the initial porous silicon layer determines not only the thickness of this underlayer, but also that of the alloy film itself, as well as its elemental composition. This behavior is attributed to the difference in the distribution of the temperature gradient, as heat transfer from the subsurface region is greatly complicated due to reduced thermal conductivity of thicker porous layers, causing it to be subjected to higher temperatures and leading to the growth of a thicker alloy layer with increased silicon contents. Assumingly, the presence of a porous underlayer can thermally and electrically insulate the alloy film from the monocrystalline wafer, eliminating the need to transfer the film to a dielectric substrate for subsequent use in thermoelectric converters and other electronic devices.