Temperature Dependence of the Absorption Spectra and Optical Parameters in TGS and Cu2+-Doped TGS Crystals

The effect of temperature on the optical absorption spectra and optical parameters is investigated for pure TGS and TGS doped with Cu²⁺ ions. Absorption measurements cover the range from room temperature to about 355 K in the energy range 3-5.5 eV. The temperature dependence of the band gap E_g(T) reveals an anomaly at the phase transition temperature for both pure and Cu²⁺-doped TGS crystals. In the region of the absorption edge the absorption coefficient is found to display Urbach-rule behaviour. The characteristic Urbach parameters are determined and their temperature dependence is investigated.     

Optical Properties of MnIn2S2Se2 Single Crystals

Optical transmittance and reflectance spectra of MnIn₂S₂Se₂ single crystals are measured in the wavelength range from 0.5 to 30 μm. The interference method is used to determine the refractive index of the compound for wavelengths between 0.8 and 12 μm. From an analysis of the absorption spectrum it follows that the fundamental edge is due to forbidden indirect transitions between parabolic bands for the polarization E ⊥ c with a gap energy of 1.50 eV at room temperature.     

Anisotropy of optical absorption in layered single crystals

The optical energy gap of Re doped MoSe₂ single crystal Mo_0.995Re_0.005Se₂ has been measured at room temperature near the fundamental absorption edge. The incident light was kept normal to the basal plane i.e. along the c‐axis of the grown crystals. Results have been given on the basis of two and three dimensional models. Both direct and indirect transitions are involved in the absorption process. The indirect transition was found to be allowed with two phonons involved in the process. The three dimensional model and not the two dimensional could be used to describe the optical properties of Re doped MoSe₂ single crystal. Also, the optical energy gaps depend upon the amount of the intercalating Re material. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diamagnetic and photoabsorption characterisation of gel‐grown cadmium oxalate single crystals

Perfect single crystals of cadmium oxalate trihydrate have been grown using the slow and controlled reaction between Cd⁺² and (C₂O₄)^‐2 ions in agar‐agar gel, resulting in the formation of insoluble product Cd(COO)₂.3H₂O. The optimum growth parameters have been determined. The variation of magnetic moment of the grown crystals under an applied static magnetic field is studied and the material is found to be diamagnetic. The polarizability is found sensitive to optical band gap. An empirical relation between magnetic susceptibility and electronic polarizability has been established. Optical absorption spectra of the sample recorded in the range 200‐2500 nm reveal transitions involving absorption and emission of phonons. The detailed study supports the existence of allowed direct and indirect allowed gaps in the material. The direct allowed transition prevails in the region of relatively higher photon energy. Some feeble disorder in the crystal is conceived to be present. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of temperature on the microcrystalline structure of thermally evaporated Sb2S3 thin films

Thin films of antimony trisulfide (Sb₂S₃) were prepared by thermal evaporation under vacuum (p=5×10^–5 torr) on glass substrates maintained at various temperatures between 293 K and 523 K. Their microstructural properties have obtained by transmission electron microscopy (TEM). The electron diffraction analysis showed the occurrence of amorphous to polycrystalline transition in the films deposited at higher temperature of substrates (523 K). The polycrystalline thin films were found to have an orthorhombic structure. The interplanar distances and unit‐cell parameters were determined by high‐resolution transmission electron microscopy (HRTEM) and compared with the standard values for Sb₂S₃. The surface morphology of Sb₂S₃ thin films was investigated by scanning electron microscopy (SEM). The optical transmission spectra at normal incidence of Sb₂S₃ thin films have been measured in the spectral range of 400–1400 nm. The analysis of the absorption spectra revealed indirect energy gaps, characterizing of amorphous films, while the polycrystalline films exhibited direct energy gap. From the photon energy dependence of absorption coefficient, the optical band gap energy, E_g, were calculated for each thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature‐tuned band gap energy and oscillator parameters of Tl2InGaSe4 semiconducting layered single crystals

The optical properties of Tl₂InGaSe₄ layered single crystals have been studied through the transmission and reflection measurements in the wavelength range of 500‐1100 nm. The analysis of room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 1.86 and 2.05 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 = – 4.4 × 10^‐4 eV/K. The absolute zero value of the band gap energy was obtained as E_gi(0) = 1.95 eV. The dispersion of the refractive index is discussed in terms of the single oscillator model. The refractive index dispersion parameters: oscillator wavelength and strength were found to be 2.53 × 10^–7 m and 9.64 × 10¹³ m^–2, respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical investigations on the existence of phase transition in ZnO:Li thin films prepared by DC sputtering method

We investigated the effect of temperature on the absorption spectra of Zn_0.8Li_0.2O thin films (ZnO:Li), deposited at 573 K, in the wavelength range 190‐800 nm. The films were deposited on sapphire, MgO or quartz substrates by DC sputtering method. The results show a shift of the optical energy gap (E_g), with direct allowed transition type near the fundamental edge, to lower wavelengths as the temperature increases. The temperature rate of E_g changes considerably showing an anomaly around 320 K depending on type of substrate. The founded results indicated that replacement of Zn ions with Li ions induces a ferroelectric phase in the ZnO wurtzite‐type semiconductor. The exponential dependence of the absorption coefficient on the incident photon energy suggests the validity of the Urbach rule. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of pure and metal ions doped ammonium sulfate single crystals

A study of the optical properties of pure‐and some metal ions doped ammonium sulfate crystals (AS) were made. Optical constants of AS crystals were calculated at room temperature. The optical absorption coefficient (α ) was analyzed and interpreted to be in the allowed direct transition. The introduction of Rb⁺ or Cs⁺ ions gives rise to an intense charge transfer band with a maximum at λ= 310 nm in the optical spectrum. In case of Cr³⁺ ‐doping, the absorption shows a shoulder just before the onset band to band transition. The values of the allowed direct energy gap E_g for undoped and doped crystals were calculated. It was found that E_g values were decreased with metal ions doping. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were calculated as a function of photon energy. The validity of Cauchy‐Sellimeier equation was checked in the wavelength range 4.9 ‐ 5.6 eV and its parameters were calculated. Applying the Single‐Effective‐Oscillator model, the moments of ε (E ) could be estimated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of optical parameters of Ga0.75In0.25Se layered crystals

The optical properties of the Ga_0.75In_0.25Se crystals have been investigated by means of transmission and reflection measurements in the wavelength range of 380–1100 nm. The analysis of the results performed at room temperature revealed the presence of optical indirect transtions with band gap energy of 1.89 eV. The variation of the band gap energy as a function of temperature was also studied in the temperature range of 10–300 K. The rate of change of band gap energy (γ = –6.2 × 10^–4 eV/K) and absolute zero value of the band gap (E_gi(0) = 2.01 eV) were reported. The wavelength dependence of the refractive index was analyzed using Wemple and DiDomenico, Sellmeier and Cauchy models to find the oscillator energy, dispersion energy, oscillator strength and zero‐frequency refractive index values. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization and optical properties of CaWO4 and SrWO4

Single crystals of calcium tungstate and strontium tungstate have been grown by double decomposition flux reaction technique using lithium chloride as flux. Growth conditions are optimized to synthesize well faceted crystals. Effect of primary and secondary flux density in the growth charge has been studied. Thermogravimetric study reveals that the grown crystals are highly stable in the temperature range 25 – 1000°C. Analysis of optical absorption normal to the ab‐plane in the spectral range of 200 – 800 nm reveals the true absorption edge, the nature of transition being the allowed indirect one at 4.60 eV and 4.56 eV respectively for CaWO₄ and SrWO₄. The crystals have been characterized by determining useful pertinent optical and dielectric parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical Absorption Edge of Mg + Zn: LiNbO3

The optical transmittance of LiNbO₃ single crystal double doped with MgO and ZnO was measured from the ultraviolet to the visible range. The wavelength dependence of the absorption coefficient α and its root α^1/2 (α versus hv and α^1/2 versus hv, respectively) were calculated and the characteristics of the absorption edge were discussed. The energy gaps E_g and E_g of the crystals which correspond to the direct transition and the indirect transition, respectively, and the energy of phonons taking part in the indirect transition were calculated. The effects of dopants Mg and Zn on the optical absorption properties are discussed. It was found that the energy E_g of our sample which was double doped with MgO and ZnO was smaller than that of congruent LN, causing the indirect transition absorption edge to move towards the infrared.     

Intercalation and anisotropy of optical absorption in MoRe0.005Se1.995 single crystals

The optical energy gap of MoRe_0.005Se_1.995 single crystal has been measured at room temperature near the fundamental absorption method. The incident light was polarized along c‐axis of the crystals. Two and three dimensional models are adopted for generating the results. Direct as well as indirect transitions are involved in the absorption process. The indirect transition was found to be allowed with two phonons involved in the process. The three dimensional model could be used to describe the optical properties of the single crystal of MoRe_0.005Se_1.995. Also, the optical energy gaps depend upon the amount of the intercalating Re material, which show the anisotropy of the chemical bonds. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Doping‐induced‐effects on conduction mechanisms in incommensurate ammonium zinc chloride crystals

The dc conductivity (σ) along the polar b‐axis of ammonium zinc chloride (AZC) crystals in its four high‐temperature phases has been measured as a function of temperature. Doping with Mn²⁺ in different concentrations changed strongly both values of σ at all temperatures and the dependence of ln σ_dc on 1/T in the phase transition regions. The activation energy of conduction was calculated from the linear portions of this dependence in each phase. The results were discussed in the light of the decomposition of (NH₄)₂ in the high‐temperature normal phase, the discommensuaration (DC) formation/annihilation in the incommensurate phase and domain wall motion and stripples nucleation in the commensurate‐ and antiferroelectric‐phases. Pinning of DC's in the crystal lattice and/or by the structural defects and the possibility of dislocation formation was also discussed. The bulk‐ and the electrode‐limited conduction mechanisms were also considered. The current density‐voltage gradient relationship according to the usual Richardson‐Schottky (R‐S) equation shows disagreement between extracted parameters and experimentally measured ones. A modified equation was used to solve this difficulty which, in addition, facilitated the calculation of the electronic mobility (μ), the barrier height (φ) at the electrode‐dielectric interface and the R‐S constant (β_RS). The effect of Mn²⁺‐content on values of μ, φ and β_RS in different phases of AZC was also considered. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparative optical absorption and photoreflectance study of n-type CuInSe2 single crystals

Optical absorption spectra in the photon energy range from 0.4 to 1.2 eV and photoreflectance spectra in the range of the fundamental edge are measured on n-type CuInSe₂ single crystals. Photoreflectance spectroscopy yields the true gap energy while the near-edge absorption spectra are dominated by acceptor–to–conduction band transitions, the acceptor ionisation energy being about 80 meV. Based on intrinsic defect chemistry considerations this acceptor is ascribed to copper vacencies.     

Substrate temperature effect on the optical properties of amorphous Sb2S3 thin films

Sb₂S₃ amorphous thin films were prepared by thermal evaporation of corresponding powder on thoroughly cleaned glass substrates held at temperature in the range 300‐473 K. X‐ray diffraction and atomic force microscopy have been used to order to identify the structure and morphology of surface thin films. The optical constants of the deposition films were obtained from the analysis of the experimental recorded transmission data over the wavelength range 400‐1400 nm. An analysis of the absorption coefficient values revealed an optical indirect transition with the estimation of the corresponding band gap values. It was found that the optical band gap energy decrease with substrate temperature from 1.67 eV at 300 K to 1.48 eV at 473K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitation intensity and temperature‐dependent photoluminescence and optical absorption in Tl4Ga3InSe8 layered crystals

Photoluminescence (PL) spectra of Tl₄Ga₃InSe₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 600‐750 nm and in the temperature range of 17‐68 K. A broad PL band centered at 652 nm (1.90 eV) was observed at T = 17 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.13 to 55.73 mW cm^‐2 range. Radiative transitions from donor level located at 0.19 eV below the bottom of conduction band to shallow acceptor level located at 0.03 eV above the top of the valence band were suggested to be responsible for the observed PL band. From X‐ray powder diffraction and optical absorption study, the parameters of monoclinic unit cell and the energy of indirect band gap were determined, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,electrical, thermal and optical properties of the nonlinear optical crystal L‐Arginine Fluoride

Single crystals of L‐Arginine Fluoride (LAF) have been grown by the slow evaporation technique, and the crystalline perfection was studied by HRXRD. Optical absorption studies reveal the lower cut off wavelength (280 nm) and the band gap (5.1 eV). The dielectric constant and dielectric loss have been measured as a function of frequency (42 Hz–5 MHz) and temperature (307‐368K) and the activation energy is 77 μeV. The thermal transport properties such as thermal conductivity (0.88 ± 0.02 W/mK) and specific heat capacity (482±24 J/kg/K) have been estimated by the photopyroelectric technique. The nonlinear refractive index n₂, is found to be of the order of 10⁻¹³ cm²/W by the Z‐scan technique.     

Preparation and investigation of (CuInSe2)x(2ZnSe)1‐x and (CuInTe2)x(2ZnTe)1‐x solid solution crystals

The (CuInSe₂)_x(2ZnSe)_1‐x and (CuInTe₂)_x(2ZnTe)_1‐x solid solution crystals prepared by Bridgman method and chemical vapor transport have been studied. The nature of the crystalline phases, the local structure homogeneity and composition of these materials have been investigated by X‐ray diffraction (XRD) and Electron Probe Microanalysis (EPMA) methods. The analysis revealed the presence of chalcopyrite‐sphalerite phase transition between 0.6 ≤ X ≤ 0.7. Lattice constants, value of σ position parameter and bond length between atoms were also calculated. It was found that the lattice parameters exhibit a linear dependence versus composition. The transmission spectra of solid solution crystals in the region of the main absorption edge were studied. It was established that the optical band gap of these materials changes non‐linearly with the X composition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bestimmung der Energielücke von (Pb1–x*Snx)1–yTe y-Einkristallen. Vergleich indirekter mit direkten Methoden

For applications of Pb_1–xSn_xTe in infrared light emitting and light detecting devices it is necessary to control the energy gap. For determining the gap in as-grown Bridgman crystals indirect methods have to be used (measurement of lattice constant and density). The results of these indirect methods are compared with results obtained by more direct measurements (spectral photoresponse, intrinsic absorption edge).     

Effect of temperature and isomorphic atom substitution on optical absorption edge of TlInS2xSe2(1‐x) mixed crystals (0.25 ≤ x ≤ 1)

The optical properties of the TlInS_2xSe_2(1‐x)mixed crystals (0.25 ≤ x ≤ 1) have been investigated through the transmission and reflection measurements in the wavelength range of 400–1100 nm. The optical indirect band gap energies were determined by means of the analysis of the absorption data. It was found that the energy band gaps decrease with the increase of selenium atoms content in the TlInS_2xSe_2(1‐x)mixed crystals. The transmission measurements carried out in the temperature range of 10–300 K revealed that the rates of change of the indirect band gaps with temperature are γ = –9.2×10^–4 eV/K, –6.1×10^–4 eV/K, –4.7×10^–4 eV/K and –5.6×10^–4 eV/K for TlInS₂, TlInS_1.5Se_0.5, TlInSSe and TlInS_0.5Se_1.5 crystals, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)