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Étude Raman de CdxZn1-xSe cubique (x ≤ 0.3) dans le cadre du schéma de percolation : Phonons, phonon-polaritons, haute pression

Abstract : CdxZn1-xSe is special in the classification of the Raman spectra of the A1-xBxC semiconductor mixed crystals based on the historical MREI (1-bond→2-mode) and cluster (1-bond→4-mode) models that view a mixed crystal in terms of a uniform continuum at the macroscopic scale along the virtual crystal approximation (VCA) : it is representative of the rare 2-bond→1-mode behavior, explained as an ultimately compact MREI sub-case. However, recent infrared absorption measurements reveal a phonon fine structure, motivating a deeper study. Additional information is gained in this work by performing a polarization-dependent pure transverse-optic (TO) Raman study of CdxZn1-xSe single crystals with zincblende (cubic) structure (x ≤ 0.3) covering both the phonon and phonon-polariton regimes of the TO modes, using suitable backward and near-forward scattering geometries, respectively. Ab initio insight into the native phonon regime of the phonon-polaritons is obtained at intermediate composition (x=0.5, 0.3) and at the (Cd,Zn)-dilute limits (x~0,1; using prototype impurity motifs) using large disordered supercells (64 – 216 atoms), with special attention to the Raman intensities and the phonon frequencies. The experimental (Raman) and ab initio results converge onto a three-phonon [1 × (Cd − Se), 2 × (Zn − Se)] pattern for CdxZn1-xSe falling out of the scope of the MREI and cluster schemes. This is naturally explained within our generic 1-bond→2-mode percolation model based on a description of a A1-xBxC mixed crystal at the mesoscopic scale in terms of a AC/BC- like composite. Generally, seen from the angle of the percolation model, Raman scattering breaks new ground for studying mixed crystals and gives grounds for hope to solve certain critical issues behind alloying. One refers to the nature of the A⇿B atom substitution, as to whether this is random or not. Another one, is to elucidate how lattice-supported complex media engage their pressure-induced structural transition at the local scale. The two issues are presently addressed, using CdxZn1-xSe as a case study. A pronounced trend towards local clustering in our CdxZn1-xSe crystals is diagnosed in view of the disconcerting interplay between the oscillator strengths of the two Zn-Se Raman modes, 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 k. 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 k (adjusted by simulated annealing). Besides, the generic 1-bond⇾2-mode percolation-type Raman signal 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 an intrinsic capacity behind alloying to reduce the thermal conductivity.
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Submitted on : Wednesday, May 19, 2021 - 10:48:39 AM
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Mohamad Baker Shoker. Étude Raman de CdxZn1-xSe cubique (x ≤ 0.3) dans le cadre du schéma de percolation : Phonons, phonon-polaritons, haute pression. Matière Condensée [cond-mat]. Université de Lorraine, 2020. Français. ⟨NNT : 2020LORR0207⟩. ⟨tel-03229703⟩

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