Re-examination of the SiGe Raman spectra - Linear chain approximation and ab initio calculations

We propose a (three-dimension) -> (one-dimension) shift of paradigm for basic understanding of Raman spectra of random Si-Ge. Fair contour modeling of Raman spectra is achieved along the linear chain approximation via 1D-cluster version of the phenomenological Percolation scheme, originally developed for zincblende alloys, after ab initio calibration of the intrinsic Si-Si, Ge-Ge and Si-Ge Raman efficiencies. The 1D-cluster scheme introduces a seven-oscillator [1x(Ge-Ge), 4x(Si-Ge), 2x(Si-Si)] Raman behavior for SiGe, which considerably deviates from the currently admitted six-oscillator [1x(Ge-Ge), 1x(Si-Ge), 4x(Si-Si)] one. The 1D-cluster re-assignment of Raman lines is based on remarkable intensity-interplays with composition, known from the literature, but so far not properly understood. It is independently supported by ab initio calculation of the frequencies of bond-stretching modes along prototype impurity motifs taken as quasi-linear. Different numbers of Raman modes per bond indicate different sensitivities to the local environment of the Ge-Ge (insensitive), Si-Si (sensitive to 1st neighbors) and Si-Ge (sensitive to 2nd neighbors) bond-stretchings. Last, we compare the SiGe Percolation scheme with the current version for zincblende alloys, using GaAsP as a natural reference. The SiGe vs. GaAsP comparison is supported by ab initio calculation of local lattice relaxation/dynamics related to prototype impurity motifs that are directly transposable to the two crystal structures.