Lattice distortion in a strain-compensated Si1−x−yGexCy layer on silicon

Abstract

A number of ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$ ${\mathrm{Ge}}_{\mathit{x}}$ ${\mathrm{C}}_{\mathit{y}}$ layers with different concentrations of Ge and C were grown by molecular-beam epitaxy on a Si(001) substrate to investigate the possibility of strain compensation. The layers were characterized by transmission electron microscopy, x-ray diffraction, and Raman scattering and modeled using a valence-force field model. For a [Ge]/[C] ratio of approximately 10, the lattice constant in the growth direction is equal to that of the substrate, indicating the absence of macroscopic strain. Experimental and theoretical results are compatible with Vegard’s rule. The bond lengths in the alloy exhibit a significant relaxation away from the ideal ‘‘chemical’’ value as given by the sum of the corresponding covalent radii. The measured shifts of the Raman frequencies relative to the constituents cannot be understood in a straightforward description based purely on the softening or hardening of the interatomic bonds as deduced from the Grüneisen parameters.