Clustering/anticlustering effects on the GeSi Raman spectra at moderate (Ge,Si) contents: Percolation scheme vs. ab initio calculations

We test a presumed ability behind the phenomenological percolation scheme used for the basic description of the multi-mode Raman spectra of mixed crystals at one dimension along the linear chain approximation, to determine, via the Raman intensities, the nature of the atom substitution, as to whether this is random or due to local clustering/anticlustering. For doing so, we focus on the model percolation-type GeySi1-y system characterized by six oscillators {1 x (Ge-Ge), 3 x (Ge-Si), 2 x (Si-Si)} and place the study around the critical compositions y similar to (0.16, 0.71, and 0.84) corresponding to nearly matching of intensities between the like Raman modes from a given multiplet (Ge-Si triplet or Si-Si doublet). The interplay between the GeySi1-y Raman intensities predicted by the percolation scheme depending on a suitable order parameter k of local clustering/ anticlustering is found to be consistent with ab initio calculations of the GeySi1-y Raman spectra done with the Ab Initio Modeling PROgram code using large (64-, 216-, and 512-atoms) disordered cubic supercells matching the required (y, k) values. The actual percolation vs. ab initio comparative insight at moderate/dilute-(Ge,Si) limits, with an emphasis on the k-induced intra-bond transfer of oscillator strength, extends a pioneering one earlier achieved at an intermediate composition (y similar to 0.50) by using small (32-atom) supercells [O. Pages et al., J. Appl. Phys. 114, 033513 (2013)], mainly concerned with the inter-bond transfer of oscillator strength, providing altogether a complete picture.