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

Abstract

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

“…Now, Zn 1−x Cd x Se is prone to clustering on approach to its composition-induced zincblende → wurtzite structural transition 19 ( x ~ 0.3), which might significantly impact its mesostructure, especially at compositions near the Cd-Se bond percolation threshold as in the present case. A Raman insight into the clustering rate κ of Zn 1−x Cd x Se with moderate-to-large Cd content-on a scale of 0 (random substitution) to 1 (full clustering, i.e., phase separation)-using the terminology of the κ-based formalism developed within the cluster model 8 -has earlier been obtained within the percolation scheme based on a description of Zn 1−x Cd x Se in terms of a CdSe/ ZnSe-like "composite" (see above).…”

“…The pressure dependencies of the bond force constants ( k i ) and vibration dampings ( γ i )-governing the closure/opening and collapse processes of each system, respectively (see text)-are schematically indicated (using single or double arrows), together with the critical zincblende → rock-salt pressure transition ( P T ). 19 (details are given in the Supplementary Section I) are shown in Fig. 2.…”

“…Seen from the angle of the percolation model, Raman scattering breaks new ground for studying mixed crystals. For instance, the first issue raised above has been solved recently: when exploited within the percolation scheme, the Raman spectra of zincblende 19 as well as diamond 20 mixed crystals can be used to shed light on the nature of the atom substitution (random vs. clustering/anticlustering) on a quantitative basis (using an ad hoc order parameter—see below). In this work, we address the second issue and test the Raman doublet of the short bond as a sensitive chemical probe to “see” how mixed crystals engage their pressure-induced structural transition at the mesoscopic scale.…”

“…The zincblende ↔ rock-salt transition pressures are correspondingly larger, smaller and comparable to P ZnSe 23 . (vi) The Zn-Se bond stands out among the II-VI's in that its capacity to stiffen under pressure hits the lowest level, judged by the minimal volume derivative of its bond ionicity ( df * i /dlnV=0.127), that makes roughly half the value for Zn-S and Cd-Se, and one sixth as much as the Be-Se one 24 19 ) AC-singlet and BC-doublet-to a point, in the latter case, that the spacing is larger for the doublet ( δ ~ 20 cm −1 ) than between the doublet and the singlet ( ~ 10 cm −1 ) -(see, e.g., Supplementary Fig. S6b).…”

“…(viii) Zn-Se is either the "passive" (AC-singlet, Zn 1−x Be x Se and ZnSe 1−x S x ) or the "active" (BC-doublet, Zn x Cd 1−x Se) bond; the active bond is sensitive to its local environment either up to first-(Zn 1−x Be x Se) or second-neighbors (ZnSe 1−x S x , Zn 1−x Cd x Se), and the active bond is either the minor (Be-Se, Zn-S) or the dominant (Zn-Se of Zn 1−x Cd x Se) species19,21,22 .…”