Percolation Picture for Long Wave Phonons in Zinc-Blende Mixed Crystals: From (Zn, Be) Chalcogenides to (Ga, In)As

We propose a simplified version of the one-bond two-mode percolation model originally developed for the long wave phonons related to the stiff Be-VI bond in (Zn, Be) chalcogenides, which open the class of random mixed crystals with contrast in the bond stiffness. This is deduced from the comparison between the Raman responses from the stiff Be-VI bond and the soft Zn-VI one in these systems. The simplified version is tested on (Ga, In)As, made of soft-like bonds only and taken here as a representative challenging system. This results in a successful reinterpretation of the puzzling multi-phonon behaviour in the Raman/infrared spectra of this alloy, that has been a subject of debate. The discussion is supported by contour modelling of the TO and LO Raman lineshapes by applying the Hon and Faust treatment to a version of the modified-random- element-isodisplacement model generalized to multi-oscillators. Also, the assignment of the long wave phonons in (Ga, In)As is supported by atomistic calculations of the bond length distributions of the minority bond species in large (Ga, In)As supercells corresponding to alloy compositions close to the In-As (In ∼0.19) and Ga-As (In ∼0.81) bond percolation thresholds. The configurations are analysed to distinguish between isolated and connected bonds, not in the usual terms of next nearest neighbours.