Surface Detection of Strain-Relaxed Si1–xGex Alloys With High Ge-Content by Optical Second-Harmonic Generation

We investigated the strain and surface structural properties of a strain-relaxed Si_1-xGe_x alloy layer with high Ge-content using optical surface second-harmonic generation. Here, the Si_1-xGe_x alloys are heteroepitaxial, and they are deposited onto Si substrates via ultra-high-vacuum-chemical vapor deposition. The in-plane strain and the composition x of the Si_1-xGe_x alloys were determined using Raman spectroscopy. The SH signals generated from three Si_1-xGe_x alloy surfaces versus the rotational angle of the substrate were measured. The SH intensities for the combination of s-input/p-output polarization show fourfold symmetry; however, for s-input/s-output, the SH signals show eightfold symmetry with the rotational angle. Residual strain would induce an enhancement of the isotropic p-output SH component in the Fourier transform coefficient. Finally, the degree of symmetry of the SH signals from these three Si_1-xGe_x alloy surfaces versus the rotational angle was related with the surface pit defects (densities and sizes) and surface roughness.