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An optical characterization of wurtzite Zn1-x-yCdxMgySe crystalline alloys grown by the modified high-pressure Bridgman method has been carried out by temperature-dependent photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 10300 K, and photoreflectance (PR) measurements between 300400 K as well as surface photovoltage spectroscopy (SPS) at 300 K. Low temperature PL spectra of the investigated samples consist of an excitonic line, the “edge emission” due to radiative recombination of shallow donor-acceptor pairs and a broad band related to recombination through deep level defects. Three excitonic features, A, B and C, in the vicinity of band edge were observed in the CER and PR spectra. The peak positions of band-edge excitonic features in the PL spectra are shifted slightly towards lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase of the CER-PL shift with the increasing of Mg content in the investigated crystals is explained by the rising of compositional disorder causing the smearing of the band-edge energies. In addition, the coincidence of energy positions of features obtained from SPS measurements with excitonic transition energies E0A determined from CER/PR data at 300 K confirm the proper surface treatment of the samples.

We present a polarization-dependent pure transverse-optic (TO) Raman study of high-quality CdxZn1−xSe single crystals with zincblende (cubic) structures (x ≤ 0.3) covering both the phonon and phonon-polariton variants of the TO modes, using suitable backward and near-forward scattering geometries, respectively. Insight into the native phonon regime of the phonon-polaritons is obtained at intermediate composition of the random crystal (x = 0.5 , 0.3) and at the (Cd,Zn)-dilute limits (x ∼ 0.1 ; using prototype impurity motifs) by applying ab initio codes to large supercells (64–216 atoms), with special attention to both the Raman intensities and the phonon frequencies. The experimental (Raman) and theoretical (ab initio) results converge onto a percolation-type three-phonon [ 1 × (Cd - Se) , 2 × (Zn - Se) ] pattern for CdxZn1−xSe. On the practical side, the interplay between the oscillator strengths of the two Zn-Se Raman modes is used to diagnose a pronounced trend toward local clustering in the studied crystals, 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 κ. 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 κ (adjusted by simulated annealing). [ABSTRACT FROM AUTHOR]

The Raman selection rules of the (Zn-Se, Be-Se)-mixed phonon-polariton (PPPP) created by "alloying" in the three-mode [1×(Zn-Se), 2×(Be-Se)] Zn 1-xBexSe system, whose dramatic S-like dispersion (~200cm-1) covers the large frequency gap between the Zn-Se and Be-Se spectral ranges, are studied in its wave vector (qq) dependence by near-forward Raman scattering. Both the "collapse" regime away from the Γ point (q=0) and the "reinforcement" regime near Γ are addressed, using appropriate laser lines and Be contents. We find that in both regimes the considered PPPP, in fact a transverse mode with the mixed mechanical-electrical character, obeys the same nominal Raman selection rules as its purely mechanical variant observed in the standard backscattering geometry. The discussion is supported by contour modeling of the multi-PPPP Raman lineshapes in their qq-dependence within the linear dielectric approach. [ABSTRACT FROM AUTHOR]

A detailed investigation of the temperature dependence of the edge excitonic transitions of three Bridgman-grown wurtzite-type Cd(sub 1-x-y)Be(sub x)Zn(sub y)Se-mixed crystals is conducted. The parameters that describe the temperature dependence of the interband transition energies and the broadening function of the excitonic features are evaluated and discussed.

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. [ABSTRACT FROM AUTHOR]

We investigate by near-forward Raman scattering a presumed reinforcement of the (A-C,B-C)- mixed phonon-polariton of a A1-xBxC zincblende alloy when entering its longitudinal optical-like (LO-like) regime near the Brillouin zone centre Γ, as predicted within the formalism of the linear dielectric response. A choice system to address such issue is ZnSe0.68S0.32 due to the moderate dispersion of its refractive index in the visible range, a sine qua non condition to bring the phononpolariton insight near Γ. The LO-like reinforcement regime is actually accessed by using the 633.0 nm laser excitation, testified by the strong emergence of the (Zn-Se,Zn-S)-mixed phononpolariton at ultimately small scattering angles. [ABSTRACT FROM AUTHOR]