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

TL;DR

This study uses Raman spectroscopy to investigate how pressure induces structural transitions in ZnSe-based mixed crystals, revealing bond behavior and potential for thermal conductivity reduction.

Contribution

It introduces a phonon-based partitioning approach to understand pressure-induced phase transitions at the mesoscopic scale in mixed semiconductor crystals.

Findings

Raman doublet behavior varies with pressure, indicating different bond environment responses.

Pressure can cause either closure or opening of the Raman doublet, depending on the system.

Bond decoupling at the transition point leads to a rigid backbone, affecting thermal properties.

Abstract

The generic 1-bond:2-mode percolation type Raman signal inherent to the short bond of common (A,B)C 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 ZnCdSe. The Raman doublet, that distinguishes between the AC- and BC-like environments of the short bond, is reactive to pressure: either it closes (ZnBeSe, ZnSeS) or it opens (ZnCdSe), 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.