Phonon-based partition of (ZnSe-like) semiconductor mixed crystals on approach to their pressure-induced structural transition

Abstract

The generic 1-bond → 2-mode “percolation-type” Raman signal inherent to the short bond of common A1−xBxC semiconductor mixed crystals with zincblende (cubic) structure is exploited as a sensitive “mesoscope” to explore how various ZnSe-based systems engage their pressure-induced structural transition (to rock-salt) at the sub-macroscopic scale—with a focus on Zn1−xCdxSe. The Raman doublet, that distinguishes between the AC- and BC-like environments of the short bond, is reactive to pressure: either it closes (Zn1−xBexSe, ZnSe1−xSx) or it opens (Zn1−xCdxSe), depending on the hardening rates of the two environments under pressure. A partition of II–VI and III–V mixed crystals is accordingly outlined. Of special interest is the “closure” case, in which the system resonantly stabilizes ante transition at its “exceptional point” corresponding to a virtual decoupling, by overdamping, of the two oscillators forming the Raman doublet. At this limit, the chain-connected bonds of the short species (taken as the minor one) freeze along the chain into a rigid backbone. This reveals a capacity behind alloying to reduce the thermal conductivity as well as the thermalization rate of photo-generated electrons.