We overview the study of vibration modes in the impurity limits of pseudobinary (substitutional) semiconductor alloys of the III-V or II-VI composition. On the side of experiments, such modes carry information about internal strains (the force constants being extremely sensitive to tiny variation/stretching of the bond lengths. On the side of ab initio calculations, the modes in question help to identify impurity-related peaks in experimental spectra (amidst a possibly confusing complex pattern), and relate the observed trends in their positions to the underlying structure models which can be thus tested. In the context of alloys study, the single impurity, as well as a couple of nearest (on the same, cation or anion, sublattice) impurities have a virtue of being clear-cut model systems, which allow nevertheless certain "extrapolation" towards further alloying effects.

A systematic study of single AND coupled impurities helps, moreover, to establish quantitative context for the so-called "percolation model" in the vibrational behaviour of mixed semiconductors. This model implies, throughout the whole concentration range, a manifestation of TWO separated vibration modes due to EACH type of cation-anion bond in a given pseudobinary alloy. The onset of such bimodal behaviour occurs already on coming from the single-impurity to impurity-pair regimes. We look into the microscopics of this transitions and discuss why in certain systems the corresponding splitting parameter is almost zero and in others quite large (up to tens of inverse cm).

The systems under study are (Zn,Be)Se, Zn(Se,Te), (Ga,In)As, (Ga,In)P etc., as well as some wurtzite systems. First-principles calculations of phonon spectra are done within the density functional theory, using either planewave/pseudopotential method with linear response (the Pwscf code), or finite-displacements approach with local-orbital method (Siesta).

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1. Unification of the phonon mode behavior in semiconductor alloys via a simple percolation concept