Energy band structure and Frenkel excitons in PbGa2S4

Optical reflection spectra are measured and calculated in PbGa₂S₄ crystals in the region of resonances related to excitons with large oscillator strength and binding energy (Frenkel excitons). The splitting of the upper valence band in the center of the Brillouin zone due to crystal field (Δ_cf) and spin orbit (Δ_so) interaction are determined. Optical reflection spectra are measured and calculated according to Kramers-Kronig relations in the region of 3-6 eV in Е⊥с and Е∥с polarizations, and the optical constants n, k, ε₁ and ε₂ are determined. The observed electronic transitions in PbGa₂S₄ crystals are discussed in the frame of theoretical energy band structure calculation for thiogallate crystals.     

Birefringence of CuGa2S4 crystals

The interference of optical transmission spectra of thin CuGaS₂ single crystals is measured in E||c and E⊥c polarizations. The spectral dependencies of the refractive indexes n_o, n_e and Δn = n_o − n_e near the absorption edge have been determined from interference spectra. The intersection of refractive indexes at two wavelengths has been revealed at 300 K and 10 K. The characteristics of Band-Pass-Mode Filter and Band-Elimination-Mode Filter have been measured, which possess 7 narrow absorption (transmission) bands and represent a comb filter. The characteristics of these filters have been studied.     

Interference of ordinary and extraordinary waves in AgAsS2 crystals

The anisotropy of the near-bandgap absorption is investigated in AgAsS₂ crystals. The refraction indices, n^‖ and n^⊥ respectively for the E‖c and E⊥c polarizations as well as the spectral dependence of the refraction indices difference, Δn=n‖−n^⊥ are determined from the interference spectra of AgAsS₂ crystals. A transmission band with four maxima is observed in the transmission spectra of crystals placed between crossed polarizers. The optical parameters n, k, ε₁, and ε₂ for the E‖c and E⊥c polarizations are calculated from the reflection spectra by using the Kramers–Kronig relations.     

Exciton mechanism of energy transfer to F-centers in dosimetric corundum crystals

The optical absorption (OA) and thermoluminescence of oriented -Al₂O₃ crystals colored in aluminum vapor have been studied. Optical absorption measurements of an X-irradiated sample at the orientation EC₃ during isochronal annealing revealed mutual transformations of F- and F⁺-centers within the dosimetric peak from 380 to 500 K. The transformations occurred simultaneously with emptying of the electron trap whose OA is at 2.8 eV. The predominant F→F⁺ conversion, which could be explained by delocalization of holes, was observed in the low-temperature part of the peak near 380–440 K. From the available experimental data it was inferred that the simultaneous release of electrons and holes in the low-temperature part of the peak facilitated formation of excitons bound on F-type centers. The excitons transferred part of the energy stored in the irradiated crystal to F⁺- and F⁻-centers during thermal stimulation, leading to luminescence of these centers.     

Ab initio calculations of band structure and thermophysical properties for SnS2 and SnSe2

The electronic band structure and elastic constants of SnS₂ and SnSe₂ have been calculated by using density-functional theory (DFT). The calculated band structures show that SnS₂ and SnSe₂ are both indirect band gap semiconductors. The upper valence bands originate mainly from S_p and Sn_d electrons, while the lowest conduction bands are mainly from (S, Se) _p and Sn_s states. The calculated elastic constants indicate that the bonding strength along the [100] and [010] direction is stronger than that along the [001] direction and the shear elastic properties of the (010) plane are anisotropic for SnS₂ and SnSe₂. Both compounds exhibit brittle behavior due to their low B/G ratio. Relationships among volumes, the heat capacity, thermal expansion coefficients, entropy, vibrational energy, internal energy, Gibbs energy and temperature at various pressures are also calculated by using the Debye mode in this work.     

Optical properties of Tl3SbS3 acousto-optic crystals

Optical properties of acousto-optical material Tl₃SbS₃ were investigated at the fundamental absorption edge. The basic (n = 1) and excited (n = 2, 3) exciton states were obtained from the λ-modulated reflection spectra for polarizations E⊥c at 10 K. Taking into account the spatial dispersion we determined the parameters of excitons by calculating the spectra shapes of λ-modulated reflection of line n = 1 and estimated values of the zone-translation masse and the reduced effective masse of excitons, the effective masse of electrons , heavy and light holes.     

Exciton spectra, valence band splitting, and energy band structure of CuGaXIn1−XS2 and CuGaXIn1−XSe2 crystals

Exciton spectra are studied in CuGa_XIn_1−XS₂ solid solutions by means of photoreflectivity and wavelength modulation spectroscopy at liquid nitrogen temperature. The exciton parameters, dielectric constants, and free carrier effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The crystal field and spin orbit valence band splitting is calculated as a function of X taking into account the energy position of excitonic lines. The energy band structure of CuGa_XIn_1−XS₂ and CuGa_XIn_1−XSe₂ compounds is derived from optical spectra at photon energies higher than the fundamental band gap. The energies of optical transitions are tabulated for X values from 0 to 1.     

Band structure and optical transitions in the Hg3Se2Cl2 crystals

First-principles calculations of the band structure for Hg₃Se₂Cl₂ crystal were performed by means of density functional theory (DFT). The exchange and correlation potential was described within a framework of the local density approximation based on exchange-correlation energy optimization to calculate the total energy. DOS/PDOS and valence charge distribution were studied in details. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions. For device applications based on Hg₃Se₂Cl₂ crystal, understanding these fundamental issues is highly important and essential. It should be noted that optoelectronic applications of Hg₃Se₂Cl₂ are caused by coexistence of the large polarized Hg cations and a huge contribution of an harmonic phonon subsystem caused by anions.     

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ε(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH₄SO₄, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5m_e. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO₄) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.     

Optical constants of Tl4Ga3InSe8 layered single crystals

The optical properties of Tl₄Ga₃InSe₈ layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 500–1100 nm. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 1.94 and 2.20 eV, respectively. Transmission measurements carried out in the temperature range of 10–300 K revealed that the rate of change of the indirect band gap with temperature is γ=−4.1×10⁻⁴ eV/K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.03 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 4.10 eV, 23.17 eV, 6.21×10¹³ m⁻² and 2.58, respectively. From X-ray powder diffraction study, the parameters of monoclinic unit cell were determined.     

Life-time resolved emission spectra in CdI2 crystals

The emission spectrum of CdI₂ is composed of ultraviolet (UV), green (G) and yellow (Y and Y′) bands peaking at 3.38, 2.50, 2.16 and 2.25 eV, respectively. In order to determine the initial states of the Y- and G-luminescence, decay curves have been measured at 6 and 80 K by varying emission energy. The observed decay curves are composed of two or three exponential components. These decay components were named τ₁, τ₂, τ₃, τ_3′ and τ₄. The emission spectrum for each decay component, i.e., the life-time resolved emission spectrum, was constructed from the observed decay curves. At 6 K, three bands at 2.12, 2.49 and 2.64 eV are obtained for τ₁, τ₂ and τ₃ components, respectively. At 80 K, a dominant band for the τ₄ component and a weak band for the τ_3′ component appear on the same energy position at 2.25 eV. The origin of each emission band in the life-time resolved emission spectra will be briefly discussed.     

Optical properties of new photovoltaic materials: AgCuO2 and Ag2Cu2O3

AgCuO₂ and Ag₂Cu₂O₃ are new types of semiconductor materials. A theoretical study is presented for both the electronic and optical properties of these new photovoltaic materials in the framework of density functional theory (DFT). The calculated cohesive energy is −3.606 eV/atom and −3.723 eV/atom for Ag₂Cu₂O₃ and AgCuO₂, respectively. Electronic calculations indicate that AgCuO₂ is a small band gap semiconductor and Ag₂Cu₂O₃ is metallic in nature. The valency state of Cu is divalent in Ag₂Cu₂O₃ and trivalent in AgCuO₂. The largest absorption coefficient of CuO₂ is 332 244, which is significantly greater than that of CuInSe₂, CdTe, GaAs, etc.     

Optical spectra and energy levels of the Cr ions in MWO4 (M=Mg, Zn, Cd) and MgMoO4 crystals

Single crystals of MWO₄ (M=Mg, Zn, Cd) and MgMoO₄ doped with Cr³⁺ have been grown by the flux growth method. Their optical spectra have been systematically measured and assigned on the basis of the classical Ligand Field Theory. The exchange charge model of the crystal field has then been applied to calculate the crystal field parameters (CFPs) and the energy levels of the Cr³⁺ ion in all studied crystals. These are in reasonable agreement with the experimental data. Systematic trends in the CFPs values, crystal field splittings and Racah parameters have been evidenced and their relation with sizes and symmetry properties of the host cavities occupied by Cr³⁺ has been pointed out.     

Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

The optical and electronic properties of (GaAs)_n/(InAs)_n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.     

Spectroscopic studies of Nd and Er in KGd(WO4)2 single crystals

Single crystals of KGd(WO₄)₂ doped with Nd³⁺ and Er³⁺ ions were grown by the top seeded solution growth method. Detailed analysis of the room-temperature absorption spectra was performed using the conventional Judd-Ofelt theory and actual dependence of the refractive index on the wavelength. In addition to the intensity parameters Ω_2,Ω_4,Ω₆, the branching ratios and radiative lifetimes were estimated for all possible transitions in the studied spectral region. Those transitions which can be potentially used for laser applications have been identified for both ions.     

Optical band gap and optical constants in a-Se80Te20−xPbx thin films

The optical constants (absorption coefficient, refractive index, extention coefficient, real and imaginary part of dielectric constant) have been studied for a-Se₈₀Te_20−xPb_x (where x = 0, 2, 6, 10) thin films as a function of photon energy in the wave length range (500–1000 nm). It has been found that the optical band gap increases while the refractive index and the extinction coefficient (k) decreases on incorporation of lead in Se–Te system. The value of absorption coefficient (α) and the extinction coefficient (k) increases, while the value of refractive index (n) decreases with incident photon energy. The results are interpreted in terms of the change in concentration of localized states due to the shift in fermi level.     

太阳能材料CuInS2的晶体结构、电子和光学特性

用全电势线性缀加平面波法加局域轨道方法调查了黄铜矿半导体CuInS2的结构、电子和光学特性。我们计算的带隙0.17 eV是直接的,其它实验和理论也表明这种材料有一个直接带隙。在 In 4d和S 3p轨道之间有相当强的杂化,构成了(InS2)4-阴离子。我们计算的反射率光谱,介电函数的实部和虚部,消光系数和折射率和实验结果取得了很好的一致。     

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.     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

Luminescence properties of spark-processed Si in air, O2, and N2 with low pressure

This paper reports the luminescence properties of spark-processed Si (sp-Si) prepared with different atmospheres such as air, O₂, and N₂ in low vacuum range (50-760 Torr). Three main luminescence bands are observed from spark-processed Si (sp-Si). In addition to the well-known two luminescence bands in the blue/violet peaking at 410 nm and green peaking at 500 nm, a novel UV luminescence band is detected for the sp-Si prepared in N₂. The temperature dependence of photoluminescence (PL) characteristics of the newly detected UV luminescence band is examined. Further studies of photoluminescence excitation (PLE) have been performed and origins of luminescence are discussed based on the experimental results.