Growth and optical characterization of Cd1-xBexSe and Cd1-xMgxSe crystals

Exceptional phonon point versus free phonon coupling in Zn1-xBexTe under pressure: an experimental and ab initio Raman study

Raman scattering and ab initio Raman/phonon calculations, supported by X-ray diffraction, are combined to study the vibrational properties of Zn1-xBexTe under pressure. The dependence of the Be-Te (distinct) and Zn-Te (compact) Raman doublets that distinguish between Be- and Zn-like environments is examined within the percolation model with special attention to x ~ (0,1). The Be-like environment hardens faster than the Zn-like one under pressure, resulting in the two sub-modes per doublet getting closer and mechanically coupled. When a bond is so dominant that it forms a matrix-like continuum, its two submodes freely couple on crossing at the resonance, with an effective transfer of oscillator strength. Post resonance the two submodes stabilize into an inverted doublet shifted in block under pressure. When a bond achieves lower content and merely self-connects via (finite/infinite) treelike chains, the coupling is undermined by overdamping of the in-chain stretching until a «phonon exceptional point» is reached at the resonance. Only the out-of-chain vibrations «survive» the resonance, the in-chain ones are «killed». This picture is not bond-related, and hence presumably generic to mixed crystals of the closing-type under pressure (dominant over the opening-type), indicating a key role of the mesostructure in the pressure dependence of phonons in mixed crystals.

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

The generic 1-bond → 2-mode "percolation-type" Raman signal inherent to the short bond of common A_1-xB_xC 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 Zn_1-xCd_xSe. The Raman doublet, that distinguishes between the AC- and BC-like environments of the short bond, is reactive to pressure: either it closes (Zn_1-xBe_xSe, ZnSe_1-xS_x) or it opens (Zn_1-xCd_xSe), 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.

Pressure-induced phonon freezing in the ZnSeS II-VI mixed crystal: phonon-polaritons and ab initio calculations

Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the 'phonon-polariton' transverse optical modes (with mixed electrical-mechanical character) of the II-VI ZnSe1-x S x mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly-resolved percolation-type Zn-S Raman doublet of the three oscillator [1  ×  (Zn-Se), 2  ×  (Zn-S)] ZnSe0.68S0.32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende  →  rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe0.68S0.32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn-S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar 'phonon freezing' was earlier evidenced with the well-resolved percolation-type Be-Se doublet of Zn1-x Be x Se (Pradhan et al 2010 Phys. Rev. B 81 115207), that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse/convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition.

Optical characterization of a Cd(0.85)Mg(0.15)Se mixed crystal

We present an optical characterization of a Bridgman-grown wurtzite-type Cd(0.85)Mg(0.15)Se mixed crystal in the near-band-edge interband transitions using temperature-dependent contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range 15-400 K. The interband excitonic transitions A and C originating from the band edge and spin-orbital splitting critical points of the sample, respectively, have been observed in the CER/PR spectra. The transition energies and broadening function of the excitonic features are determined via a lineshape fit to the CER/PR spectra. The parameters that describe the temperature dependence of the transition energies of excitons A and C, and the broadening function of exciton A, are evaluated and discussed.