  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.
