Multi-phonon (percolation) behavior and local clustering of CdxZn1-xSe-cubic mixed crystals (x ≤ 0.3): A Raman-ab initio study

Abstract

We present a polarization-dependent pure transverse-optic (TO) Raman study of high-quality CdxZn1-xSe single crystals with zincblende (cubic) structures (x ≤ 0.3) covering both the phonon and phonon-polariton variants of the TO modes, using suitable backward and near-forward scattering geometries, respectively. Insight into the native phonon regime of the phonon-polaritons is obtained at intermediate composition of the random crystal (x = 0.5, 0.3) and at the (Cd,Zn)-dilute limits (x ∼ 0.1; using prototype impurity motifs) by applying ab initio codes to large supercells (64-216 atoms), with special attention to both the Raman intensities and the phonon frequencies. The experimental (Raman) and theoretical (ab initio) results converge onto a percolation-type three-phonon [1 × (Cd - Se), 2 × (Zn - Se)] pattern for CdxZn1-xSe. On the practical side, the interplay between the oscillator strengths of the two Zn-Se Raman modes is used to diagnose a pronounced trend toward local clustering in the studied crystals, presumably an early sign of the composition-induced zincblende ↔ wurtzite structural transition (x ∼ 0.3). The deviation from the ideal Zn ↔ Cd random substitution is estimated by working out a zincblende-version of the percolation model equipped with a relevant order parameter κ. The model is based on a sensitivity of the Zn-Se vibration to its local environment at the second-neighbor scale, independently supported by ab initio calculation of the Raman spectra in their dependence on κ (adjusted by simulated annealing).