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We report on room temperature, polarization-resolved Raman scattering measurements on layered crystals of the series MoSxSe(2-x) (0⩽x⩽2) grown by chemical vapor transport technique. The results reveal two distinct sets of features related to the E12g and A1g modes of pure members of series. As composition x changes, the in-plane E12gmode shows two-mode behavior, whereas the out-of-plane A1g mode presents more complex evolution. The MoSe2-like branch reveals the splitting associated with the altering arrangement of S and Se atoms around Mo and the resulting changes in the dipole moment of the molecule. The X-ray diffraction measurements confirm that the samples are single-phase materials of 2H-type structure over the entire range of the sulfide composition x, while the scanning transmission electron microscopy imaging reveals a random arrangement of the S and Se atoms. Modified random-element-isodisplacement model is adopted to predict the behavior of the individual modes in the alloys. The model successfully confirms the two-mode behavior exhibited by the MoSxSe(2-x) series. [ABSTRACT FROM AUTHOR]

A detailed optical characterization of a freestanding wurtzite semi-insulating Fe-doped GaN (SI-GaN:Fe) grown by hydride-vapor-phase-epitaxy was carried out by photoluminescence (PL) and contactless electroreflectance (CER) at 10 and 300 K, respectively, and photoreflectance at 300 K. Low-temperature PL spectrum of the Ga-face consisted of the Fe3+ impurity characteristic series of IR peaks with a sharp zero-phonon line (ZPL) at 1.299 eV, yellow and blue broad emission bands, and near-band-edge (NBE) emission in the ultraviolet region. The narrow linewidth of 135 μeV of the ZPL and the measured energy difference of ∼70 meV between the ZPL and E2 (high) phonon replica, which is sensitive to the lattice strain, shows good crystal quality and a strain-free incorporation of iron. The obtained transition energies of A, B, and C excitonic features in the CER spectra and the n = 2 excited states (2s) of the A and B excitons enable the estimation of the exciton binding energies. In addition to the free excitonic recombination, the PL spectrum of the Ga-face exhibited clear donor and acceptor related features in the NBE region, while the N-face exhibited a broad emission band related to the free-to-bound recombination only. The differences were explained by the presence of impurity-induced band-tail states in the N-face SI-GaN:Fe due to an increased impurity density and the incorporation of large volume vacancy-type defects. [ABSTRACT FROM AUTHOR]

In this study, anisotropic near band edge transitions of Cu2ZnSiS4 single crystals grown by chemical vapor transport were characterized by using polarization-dependent absorption, piezoreflectance (PzR) and surface photovoltage (SPV) spectroscopy techniques at room temperature. The measurements were carried out on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. Analysis of absorption and SPV spectra reveal indirect allowed transitions for the absorption edge of Cu2ZnSiS4. The estimated values of indirect band gap are 2.97 eV and 3.07 eV, respectively, for E⊥c and E∥c polarization configurations. The polarization-dependent PzR and SPV spectra in the vicinity of the direct band gap of Cu2ZnSiS4 reveal features E⊥ex and E∥ex at around 3.32 eV and 3.41 eV for E⊥c and E∥c polarizations, respectively. Both features E⊥ex and E∥ex are associated with the interband excitonic transitions at point Γ and can be explained by crystal-field splitting of valence band. Based on the experimental observations, a plausible band structure near band edge of Cu2ZnSiS4 is proposed. [ABSTRACT FROM AUTHOR]

In this study, anisotropic near band edge transitions of Cu2ZnSiS4 single crystals grown by chemical vapor transport were characterized by using polarization-dependent absorption, piezoreflectance (PzR) and surface photovoltage (SPV) spectroscopy techniques at room temperature. The measurements were carried out on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. Analysis of absorption and SPV spectra reveal indirect allowed transitions for the absorption edge of Cu2ZnSiS4. The estimated values of indirect band gap are 2.97 eV and 3.07 eV, respectively, for E⊥c and E∥c polarization configurations. The polarization-dependent PzR and SPV spectra in the vicinity of the direct band gap of Cu2ZnSiS4 reveal features E⊥ex and E∥ex at around 3.32 eV and 3.41 eV for E⊥c and E∥c polarizations, respectively. Both features E⊥ex and E∥ex are associated with the interband excitonic transitions at point Γ and can be explained by crystal-field splitting of valence band. Based on the experimental observations, a plausible band structure near band edge of Cu2ZnSiS4 is proposed. [ABSTRACT FROM AUTHOR]

An optical characterization of two wurtzite ZnCdMgSe crystalline alloys grown by the modified high-pressure Bridgman method was 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. Low temperature PL spectra of the investigated samples consisted of the 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 were shifted slightly toward lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase in the CER-PL shift with the increasing of Mg content in the investigated crystals was explained by the rising of compositional disorder causing the smearing of the band edge energies. In addition, the parameters that describe the temperature dependence of the transition energies and broadening parameters of the band edge excitonic transitions were evaluated and discussed. [ABSTRACT FROM AUTHOR]

We report a detailed investigation of the temperature dependence of the band-edge excitonic transitions of three Zn1-x-yBexMgySe mixed crystals using contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15–400 K. The samples were grown by the modified high pressure Bridgman methods. The fundamental transition energy E0 and broadening parameters are determined via a lineshape function fit to the CER and PR spectra. The parameters that describe the temperature dependence of the band-edge excitonic transition energy and broadening parameters are evaluated and discussed. [ABSTRACT FROM AUTHOR]

In this paper, the optical properties of Au-doped rhenium diselenide (ReSe2:Au) single crystals have been reported. The doping effects of the material were characterized by polarization-dependent transmittance, piezoreflectance (PzR), and photoreflectance (PR) measurements in the temperature range of 15–300 K. The indirect energy gap of ReSe2:Au shows a slight redshift with respect to the undoped sample. The low temperature PzR and PR spectra reveal an E1ex feature at E∥b polarization as well as two features, E2ex and E3ex, at E⊥b polarization. The angular dependence of the excitonic feature amplitudes agrees well with Malus’ law. In comparison with the undoped ReSe2, the excitonic transition energies remain practically unchanged, while the broadening parameter of the excitonic transition features slightly increases due to impurity scattering. In addition, the parameters that describe the temperature variations of the band-edge excitonic transitions were evaluated and discussed. [ABSTRACT FROM AUTHOR]

In this study, the optical properties of niobium-doped rhenium disulphide (ReS2:Nb) single crystals have been reported. The doping effects of the material were characterized by polarization-dependent transmittance, photoluminescence (PL), and piezoreflectance (PzR) measurements in the temperature range of 10–300 K. The indirect energy gap of ReS2:Nb shows a slight redshift with respect to the undoped sample. The low temperature PL spectra reveal two near direct band-edge excitonic peaks as well as two additional prominent features at higher energy side. The results agree well with that of the PzR investigation of the sample. In comparison with the undoped ReS2, the excitonic transition energies remain practically unchanged, while the broadening parameter of the excitonic transition features slightly increases due to impurity scattering. In addition, the parameters that describe the temperature variations of the band-edge excitonic transitions were evaluated and discussed. [ABSTRACT FROM AUTHOR]

Anisotropic near band edge transitions of [Cu.sub.2]ZnSi[S.sub.4] single crystals grown by chemical vapor transport are analyzed by using polarization-dependent absorption, piezoreflectance (PzR) and surface photovoltage (SPV) spectroscopy techniques at room temperature. The analysis of absorption and SPV spectra has shown indirect allowed transitions for the absorption edge of [Cu.sub.2]ZnSi[S.sub.4].

Abstract: Single crystals of Cu2ZnSn(S x Se1− x )4 (CZTSSe) solid solutions were grown by chemical vapor transport technique using iodine trichloride as a transport agent. As confirmed by X-ray investigations, the as-grown CZTSSe solid solutions are single phase and crystallized in kesterite structure. The lattice parameters of CZTSSe were determined and the S contents of the obtained crystals were estimated by Vegard’s law. The composition dependent band gaps of CZTSSe solid solutions were studied by electrolyte electroreflectance (EER) measurements at room temperature. From a detailed lineshape fit of the EER spectra, the band gaps of CZTSSe were determined accurately and were found to decrease almost linearly with the increase of Se content, which agreed well with the recent theoretical first-principle calculations by S. Chen, A. Walsh, J.H. Yang, X.G. Gong, L. Sun, P. X. Yang, J.H. Chu, S.H. Wei, Phys. Rev. B 83 (2011) 125201 (5pp). [Copyright &y& Elsevier]

Abstract: In this study, the near band edge anisotropic optical properties of wurtz-stannite (WS) Cu2ZnGeS4 single crystals were characterized using polarization-dependent transmittance and electrolyte electroreflectance (EER) techniques. Single crystals of Cu2ZnGeS4 were grown by chemical vapor transport method using iodine as a transport agent. Analysis of absorption spectra revealed indirect allowed transitions for Cu2ZnGeS4 with the band gaps of 2.02 (2.07) and 2.08 (2.14) eV for E‖b and Е‖a polarization configurations at 300 (10) K. The room-temperature EER spectra in the vicinity of the direct band edge showed anisotropic transitions at around 2.38, 2.44 and 2.45eV for E‖b, Е‖a and Е‖c polarizations, respectively. Based on the experimental observations and recent band-structure calculations a plausible band diagram near band edge of WS-Cu2ZnGeS4 was constructed. [Copyright &y& Elsevier]

Abstract: Polarization-dependent absorption characterization of Cu2ZnSiQ4 (Q=S, Se) quaternary single crystal compound semiconductors were carried out in the temperature range of 10–300K. The absorption measurements were performed on the as grown basal plane with the normal along [210] and the axis c parallel to the long edge of the crystal platelet. A significant shift towards lower energy was observed in the absorption spectra of E⊥c polarization with respect to those corresponding to E∥c polarization. A comprehensive analysis of the absorption spectra revealed that the absorption edges of the studied crystals are indirect allowed transitions. A schematic representation of the plausible assignments for the observed near band edge optical transitions for Cu2ZnSiQ4 was proposed. The variation of the indirect transition energies with temperature were analyzed by Varshni and Bose–Einstein expressions. The parameters that describe the temperature dependence of the indirect transition energies with different polarizations were evaluated and discussed. [Copyright &y& Elsevier]

Abstract: The temperature dependence of the band-edge excitonic transitions of Cu2ZnSiS4 single crystals were characterized by using polarization-dependent piezoreflectance (PzR) in the temperature range of 10–300K. The PzR measurements were carried out on the as-grown basal plane with the normal along [210] and the c axis parallel to the long edge of the crystal platelet. The PzR spectra revealed polarization-dependent and features for E⊥c and Е||c polarization, respectively. Both and features are associated with the interband excitonic transitions at Γ point and can be explained by crystal-field splitting of valence band. From a detailed lineshape fit to the PzR spectra, the temperature dependence of the transition energies and broadening parameters of the band-edge excitons were determined accurately. The temperature dependence of near band-edge excitonic transition energies were analyzed using Varshni and Bose–Einstein expressions. The temperature dependence of the broadening parameter of excitonic features also has been studied in terms of a Bose–Einstein equation that contains the electron (exciton)–longitudinal optical phonon-coupling constant. The parameters that describe the temperature variation of the excitonic transition energies and broadening parameters were evaluated and discussed. [ABSTRACT FROM AUTHOR]