Non-random Be-to-Zn substitution in ZnBeSe alloys: Raman scattering and ab initio calculations

Abstract

We show how the 1-bond ↦ 2-mode percolation behaviour, as observed in the transverse optical (TO) Raman spectra of several A1-xBxC semiconductor (SC) alloys, can be used to detect and to estimate a possible deviation from randomness in the A↔B substitution. As a case study we focus on the Zn1-xBexSe system, which shows an uniquely well-resolved percolation-type TO fine structure in the spectral range of Be–Se vibrations, and discuss differences in the Be–Se Raman signals from bulk crystals and epitaxial layers. In the epilayers, the longer (shorter) Be–Se bonds from the BeSe-like (ZnSe-like) region appear to be slightly but systematically over- (sub-) represented with respect to the random case. This indicates a trend towards local ordering in analogy with the known case of InGaP2. The discussion is supported by ab initio insight into the phonon/bond length properties of a prototype ZnBeSe supercell at x ~ 0.5 in their dependence on the amount of CuPt ordering (the most frequent type of spontaneous ordering). Besides, the ab initio calculations reveal some singularity in the lattice dynamics/relaxation around the intermediate value of the order parameter η ~ 0.5, coinciding with the so far experimentally achievable limit of CuPt spontaneous ordering.