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

TL;DR

This paper introduces a new 1D-cluster model for understanding SiGe Raman spectra, supported by ab initio calculations, revealing a seven-oscillator scheme that improves upon previous models and clarifies intensity interplays with composition.

Contribution

It proposes a novel 1D-cluster Raman model for SiGe alloys, calibrated with ab initio methods, and compares it to existing models and other alloy systems.

Findings

The 1D-cluster scheme involves seven oscillators, differing from the traditional six.

Ab initio calculations support the new Raman mode assignments.

The model explains intensity variations with composition and local environment.

Abstract

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.