Long-wave phonons inZnSe−BeSemixed crystals: Raman scattering and percolation model

Abstract

We extend to longitudinal-optical (LO) phonons the percolation model set for the basic understanding of the atypical transverse-optical (TO) $\text{one}\text{\ensuremath{-}}\text{bond}\ensuremath{\rightarrow}\text{two}\text{\ensuremath{-}}\text{mode}$ behavior observed by Raman scattering in the $\mathrm{Be}\text{\ensuremath{-}}\mathrm{Se}$ spectral range of the random ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Be}}_{x}\mathrm{Se}$ alloy $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$, which opens the class of mixed crystals with contrast in the bond stiffness. The study is supported by contour modeling of the TO and LO Raman line shapes. This is achieved via application of the Hon and Faust treatment to a version of the modified-random-element-isodisplacement model generalized to multioscillators. While the TO signal clearly discriminates between $\mathrm{Be}\text{\ensuremath{-}}\mathrm{Se}$ vibrations within the hard $\mathrm{Be}$-rich region and the soft $\mathrm{Zn}$-rich one, complexity arises in the LO symmetry due to vibration coupling via the 1ong-range longitudinal polarization field. In particular this generates a massive transfer of oscillator strength from the low-frequency $({\mathrm{LO}}^{\ensuremath{-}})$ (hard, soft)-mixed mode to the high-frequency $({\mathrm{LO}}^{+})$ one, which results in an apparent ${\mathrm{LO}}^{+}$ single-mode behavior. Moreover the contrasts between the $\mathrm{Zn}\text{\ensuremath{-}}\mathrm{Se}$ and $\mathrm{Be}\text{\ensuremath{-}}\mathrm{Se}$ bond lengths and bond stiffness are proposed to force a Verleur and Barker-like (VB) discrete multimode Raman response from each region. Accordingly ${\mathrm{LO}}^{\ensuremath{-}}$ and ${\mathrm{LO}}^{+}$ intramode transfers of oscillator strength superimpose to the ${\mathrm{LO}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{LO}}^{+}$ intermode one. This accounts for the spectacular distortions of the ${\mathrm{LO}}^{+}$ line shape. On the whole, the puzzling LO behavior can be regarded as the result of a cooperative phenomenon between two discrete assemblies of polar LO phonons, driven by the long-range longitudinal polarization field. Also, the Verleur and Barker description accounts for subtle unexplained behaviors in the TO symmetry. More generally it appears to provide a much attractive area for the discussion of the asymmetries of the TO and LO Raman line shapes in random alloys, as a possible alternative to the much debated spatial correlation model or to internal/external strain effects.