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[en] The poorly known AgCd2GaS4 single-crystal compounds that crystallize in a rhombic structure (space group Pmn21) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd2GaS4 compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2x1020 cm-3. The AgCd2GaS4 single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (Eg0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd2GaS4 single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd2GaS4 crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06-0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd2GaS4 compounds are drawn
[en] The study is concerned with the photoelectric and optical properties of a AgCd2−xMnxGaSe4 alloy with a Mn → Cd isovalent substitution. The positions of the photoconductivity and photoluminescence peaks are determined, and the band gap of the alloy is estimated, based on compositional analysis. The influence of technological defects on specific features of the alloy’s photoelectric and optical properties is analyzed. It is established that the centers controlling the alloy crystals’ photosensitivity are cation vacancies. The photoluminescence centers responsible for emission at awavelengths from 0.77 to 0.88 μm (dependent on the relation between components in the alloy) are defect complexes consisting of cation and anion vacancies. A physically consistent model is proposed to interpret the effects observed in the alloy.
[en] The quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using a cw 532 nm green laser with energy density about 0.4 J cm−2. The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for polarized and unpolarized photoinducing laser light to separate the contribution of the intrinsic defect states from that of the pure states of the valence and conduction bands. To understand the origin of the observed photoinduced absorption near the fundamental edge, the benchmark first-principles calculations of the structural, electronic, optical and elastic properties of Cu2CdGeS4 were performed by the general gradient approximation (GGA) and local density approximation (LDA) methods. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior (shift of the absorption maxima in different polarizations) within the 0.15–0.20 eV energy range not only near the absorption edge; optical anisotropy was also found for the deeper inter-band transition spectral range. Peculiar features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron density distribution. Possible intrinsic defects are shown to affect the optical absorption spectra considerably. Pressure effects on the structural and electronic properties were modeled by optimizing the crystal structure and calculating all relevant properties at elevated hydrostatic pressure. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu2CdGeS4 are also reported. (paper)
[en] This paper reports a comprehensive phenomenological description and experimental infrared (IR) investigations of the soft-mode-driven lattice instabilities into various commensurately and incommensurately modulated phases of Cs2HgCl4 crystals. Our theoretical analysis shows that the lattice instabilities along the a and c crystallographic directions are related to low-frequency transverse optical (TO) phonon branches of Σ2 and Λ3 symmetry, respectively, which merge together in the center of the Brillouin zone at the point of B3g symmetry. As the temperature decreases both branches fall down, leading first to the direct condensation of the soft TO Σ2 mode in the symmetric Σ direction (k ∥ a*). On the other hand, coupling of the TO and transverse acoustic (TA) modes of Λ3 symmetry causes, at somewhat lower temperatures, a series of frozen modulated commensurate and incommensurate states developing along the symmetric Λ direction (k ∥ c*). Polarized far-infrared (FIR) reflectivity spectra (15-600 cm-1) of Cs2HgCl4 crystals were measured in a broad temperature region, 10-297 K. Despite a rich sequence of structurally modulated phases existing above 163 K we observed rather moderate temperature evolution of IR spectra where only a few new modes of different polarizations have been activated. However, the commensurately modulated phases occurring below 163 K made an essential impact on the spectra of all three polarizations. The process of activation of both the Raman- and the IR-active phonons in the structurally modulated phases is subjected to the phenomenological analysis. (paper)
[en] It is shown that narrow band gap semiconductors Tl_1_−_xIn_1_−_xGe_xSe_2 are able effectively to vary the values of the energy gap. DFT simulations of the principal bands during the cationic substitutions is done. Changes of carrier transport features is explored. Relation with the changes of the near the surface states is explored . Comparison on a common energy scale of the x-ray emission Se Kβ _2 bands, representing energy distribution of the Se 4p states, indicates that these states contribute preliminary to the top of the valence band. The temperature dependence of electrical conductivity and spectral dependence photoconductivity for the Tl_1_−_xIn_1_−_xGe_xSe_2 and Tl_1_−_xIn_1_−_xSi_xSe_2 single crystals were explored and compared with previously reported Tl_1_−_xIn_1_−_xSn_xSe_2. Based on our investigations, a model of centre re-charging is proposed. Contrary to other investigated crystals in Tl_1_−_xIn_1_−_xGe_xSe_2 single crystals for x = 0.1 we observe extraordinarily enormous photoresponse, which exceed more than nine times the dark current. X-ray photoelectron core-level and valence-band spectra for pristine and Ar"+-ion irradiated surfaces of Tl_1_−_xIn_1_−_xGe_xSe_2 (x = 0.1 and 0.2) single crystals have been studied. These results indicate that the relatively low hygroscopicity of the studied single crystals is typical for the Tl_1_−_xIn_1_−_xGe_xSe_2 crystals, a property that is very important for handling these quaternary selenides as infrared materials operating at ambient conditions. (paper)