Reduced dislocation density in Ge/Si epilayers

Mismatched epilayers, for which the coherency strain has been relieved by misfit dislocation introduction, typically exhibit high epithreading dislocation densities. When the misfit is substantial, as for Ge or GaAs grown by molecular beam epitaxy onto (001) Si substrates, the density is regularly over 10 ⁹ cm^-2. We have grown Ge on Si (111) and (001) with epithreading dislocation densities in the 10⁶ cm^-2 range by use of chemical vapor deposition. This is because longer, and thus fewer, misfit dislocations appear for strain relief. Potential explanations for this are postulated. The most likely reason is that thermally activated dislocation glide is much faster at chemical vapor deposition growth temperatures.