Polytropic coefficient ? in the fluid simulation of the plasma-sheath transition

Silicon-germanium-based compositions comprising silicon, germanium, and carbon (i.e., Si-Ge-C), methods for growing Si-Ge-C epitaxial layer(s) on a substrate, etchants especially suitable for Si-Ge-C etch-stops, and novel methods of use for Si-Ge-C compositions are provided. In particular, the invention relates to Si-Ge-C compositions, especially for use as etch-stops and related processes and etchants useful for microelectronic and nanotechnology fabrication.