Quantum Electroluminescence Yield of EuGa2S4 Single Crystals

The quantum yield of the electroluminescence of EuGa₂S₄:Nd single crystals for the dependence of the mean free path of the electron on its kinetic energy and the kinetic energy square are calculated, as well as the impact excitation cross section of Nd³⁺ in the single crystals indicated. The mechanism of electron scattering is considered.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Cathodoluminescence of Eu2+ in EuGa2S4 and Ga2S3:Eu2O3

We present the results of an investigation of the cathodoluminescence of the Eu²⁺ ion in gallium chalcogenides (Ga₂S₃) and europium thiogallate (EuGa₂S₄). We show that the intense wide-band radiation of these compounds is due to the 4f-5d transitions of the Eu²⁺ ions. With increase in the Eu²⁺ percent composition from 0.1 to 5.0 mol.% the luminescence brightness increases and then decreases. Translated from Zhurnal Prikladnoi Spektrskopii, Vol. 67, No. 2, pp. 268–270, March–April, 2000.     

Optical properties of Tl2In2Se3S layered single crystals

The optical properties of Tl₂In₂Se₃S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. Dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters – oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index – were found to be 4.67 eV, 45.35 eV, 1.38 × 10¹⁴ m ^{? 2} and 3.27, respectively. Transmission measurements were also performed in the temperature range 10–300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. γ = ?5.6 × 10^?4 eV/K, and the absolute zero value of the band gap energy, E _gi(0) = 2.09 eV, were obtained.     

Optical energy gaps of undoped and Co-doped ZnIn2Se4 single crystals

Single crystals of undoped and Co-doped ZnIn₂Se₄ were grown by the vertical Bridgman technique. The optical energy gaps of the single crystals were investigated in the temperature range of 10–300 K from the optical absorption measurements. The indirect optical energy gaps of the single crystals were found to be 1.624 eV for undoped ZnIn₂Se₄ and 1.277 eV for Co-doped one at 300 K. Also, the direct optical energy gaps were given by 1.774 and 1.413 eV for undoped ZnIn₂Se₄ and co-doped one, respectively. The temperature dependence of the optical energy gaps was well fitted by the Varshni equation.     

Decay Time of 2.09 eV Luminescence of S2--Vacancy Centers and Their Optical and Thermal Stability in NaCl:S2 Crystals

An investigation was carried out on decay time of the 2.09 eV emission of S²–vacancy pair centers in NaCl crystals. When the crystal was excited by a pulsed 337 nm light from an N₂ laser, the 2.09 eV yellow luminescence principally decayed with the lifetime of 14.2 μs at low temperatures, and exhibited a weak thermal quenching with an activation energy of 51.3 meV above about 150 K. Such S^2--vacancy pair centers responsible for the 2.09 eV luminescence were thermally stable at room temperature, and not bleached even when exposed to ultraviolet lights below about 5 eV. Thus, the 2.09 eV emission center possibly can be used as a solid-state laser active center working at room temperature.     

Neodymium luminescence in chalcogenides of europium-gallium: EuGaS4 and EuGa2Se4

Photoluminescence of chalcogenides of europium-gallium, EuGa₂S₄ and EuGa₂Se₄, doped with neodymium is investigated. The positions of Stark levels are determined from the spectra. The symmetry of luminescence centres is shown to be lower than cubic and the existence of nonequivalent centers is established. At 77 K the decay time of luminescence from the excited levels of Nd³⁺ depends on the spin of the states. That indicates a slow relaxation rate in the crystals under investigation. It is probable that these crystals can be used as effective luminophores.     

СВОЙСТВА СУРЬМЯНИСТОГО КАДМИЯ, ЛЕГИРОВАННОГО СЕРЕБРОМ, ПРИ НИЗКИХ ТЕМПЕРАТУРАХ

The temperature dependence of the resistivity and the Hall coefficient of Ag doped CdSb single crystals was investigated in the temperature range of 1.5–300°K. It was shown that Ag in CdSb gives shallow acceptor levels with an activation energyE _a =3.5×10^–3 eV. Presence of the admixture band determines the electrical properties of crystals at low temperatures. The energy width of this band is about 10^–3 eV for slightly doped crystals.     

Polarized transmission intensity studies at and below energy gap of wide-gap single crystalline CaGa2S4

Polarized transmission intensity studies have been performed on wide-gap single crystalline CaGa₂S₄ at room temperature. The effective value of the partial birefringence in (100) plane is determined to be comparatively large varying from 0.14 to 0.50 with increasing photon energy from 1.9 to 3.5 eV. Four structures, which were preliminary ascribed to the critical points of interband optical transitions are displayed by ellipsometric examination with the aid of a spectroscopic phase-modulated ellipsometer in the spectral range 1.5–6.3 eV. These points, together with band gap singularity, are believed to largely contribute into the optical properties below the energy gap of biaxial CaGa₂S₄. The pseudo values of two major components of dielectric function tensor of CaGa₂S₄ are given for the first time.     

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb₂S₃ single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K _α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb₂S₃ crystal. The chemical shifts in Sb₂S₃ crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T _c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.     

Photoluminescence characterization of rare-earth stoichiometric compounds of EuGa2S4

Photoluminescence properties of the stoichiometric compound of EuGa₂S₄ has been investigated for the first time. The emission appearing in the region of 510–620 nm has a high quantum efficiency value reaching 100% at low temperatures. The characteristics of the emission are similar to those of CaGa₂S₄:Eu, and can be considered to be due to phonon-terminated transition of the Eu ion. Based on the measured luminescence data, construction feasibility of unique high gain tunable lasers is discussed.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Optical absorption in layered MnGaInS<Subscript>4</Subscript> single crystals

Optical absorption in MnGaInS₄ single crystals has been studied. Direct and indirect optical transitions are found to occur in the range of photon energies of 2.37–2.74 eV and in the temperature range of 83–270 K. The temperature dependence of the band gap has been determined; its temperature coefficients E _gd and E _gi are −5.06 × 10⁻⁴ and −5.35 × 10⁻⁴ eV/K, respectively. MnGaInS4 single crystals exhibit anisotropy in polarized light at the absorption edge; the nature of this anisotropy is explained.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Low-temperature absorption edge and photoluminescence in layered structured Tl2Ga2S3Se single crystals

The absorption edge of undoped Tl₂Ga₂S₃Se crystals have been studied through transmission and reflection measurements in the wavelength range 440–1100 nm and in the temperature range 10–300 K. The absorption edge was observed to shift toward lower energy values with increasing temperature. As a result, the rate of the indirect band gap variation with temperature γ=−2.6×10⁻⁴ eV/K and the absolute zero value of the band gap energy E_gi(0)=2.42 eV were obtained.     

Birefringence of PbGa2S4 crystals

Birefringence of PbGa₂S₄ single crystals is studied in the spectral range of 0.45–0.8 μm at T = 300 K. The obtained results are analyzed, and conclusions on the bond character in lead thiogallate are drawn.     

Positron annihilation and Hall effect in electron irradiatedn-InP crystals

Positron annihilation and Hall effect inn-InP crystals as a function of electron irradiation up to 1 · 10¹⁹ cm^–2 and post-irradiated isochronal annealing up to 550 °C have been studied. It is concluded that in irradiatedn-InP samples positrons interact with negatively charged acceptor-type defect with level atE _c –0.33 eV, probablyV _In (primary defect). In post-irradiated isochronal annealed (up to 330 °C) samples ofn-InP positron trapping occurs preferably in secondary defects-vacancy clusters, which are formed in the temperature range (150–300 °C). Inn-InP crystals containing radiation induced defects the trapping rate was found to decrease with temperature in the range (300–77) K.     

Synthesis and optical properties for crystals of a novel organic semiconductor [ Ni(Cl) 2{(Ph 2P) 2CHC(R 1R 2)NHNH 2}]

In this investigation, the synthesis process of the apyrazole derivative for diphenylphosphino-methane hydrazine complexes [ Ni(Cl)₂{(Ph₂P)₂CHC(R₁R₂)NHNH₂}] was reported, and the obtained crystals were analyzed by X-ray diffraction. As a result of the growth process, a set of complexes were formed. The structures of these complexes are discussed on the basis of Elemental analysis (EA), IR, ¹H-NMR, ³¹P-NMR spectroscopic data and FAB mass spectra. The compound under investigation shows typical semiconductor behavior as a result of delocalization of the π-electrons in the structure. The reflectance and transmittance spectra for the crystals were measured and analyzed in the incident photon energy range 1.29 to 3.93 eV and in the temperature range 77 to 300 K. The optical study revealed that the optical transition is direct allowed. Below the absorption edge, the refractive index as a function of wavelength was determined in the low energy region of the used incident photon energy range. From the refractive index-wavelength variations, the oscillator and dispersion energies of the refractive index for the obtained crystals were determined. The static refractive index and static dielectric constant were evaluated.     

Fine structure in the absorption edge spectrum of NbS2Y2 (Y = Cl,Br, I)

High resolution absorption spectra of single crystals of NbS₂Y₂ (Y = Cl, Br, I), obtained at temperatures between 4.2 and 300 K revealed extensive fine structure in the absorption edge. This structure has been analysed and interpreted in terms of allowed indirect interband transitions, involving phonons corresponding to SS and NbS vibrations, followed by forbidden and by allowed direct transitions. From the shape of the absorption curve associated with the phonon branches of the indirect transitions a binding energy of 23 cm^? for indirect excitons is obtained. A binding energy of 28 cm^? for direct excitons is deduced from the exciton lines observed at the long-wavelength side of the direct transitions. A detailed interpretation of the optical transitions is given in terms of a molecular orbital diagram for the Nb₂S₄ clusters, present in these crystals.     

Identification of new absorption bands in Zn1-xMnxTe

We report optical absorption measurements on Zn_1-xMn_xTe at 300K, 77K, and LHe temperatures. In an x ～ 0.6 sample, new absorption bands are found at 2.605 eV and 2.745 eV, together with two lower energy bands seen also in an x ～ 0.1 sample. All bands are attributed to intra-ion transitions of Mn²⁺ in the 3d⁵ configuration.