Spectral dependence of the optical absorption temperature coefficient of CdS1−xSex nanocrystallites embedded in a silicate glass

The absorption temperature coefficient of CdS_1−xSe_x nanocrystallites embedded in a silicate glass has been studied in the temperature range above room temperature at different technological regimes and sizes of nanocrystals. To understand the optical properties of silicate glasses with semiconductor nanocrystallites, especially that at the initial stage of their formation, it is necessary to include the structural changes occurring in the nanocrystals during the heat treatment.     

Studies on growth and characterization of ternary CdS1−xSex alloy thin films deposited by chemical bath deposition technique

Ternary alloyed CdS_1−xSe_x thin films of variable composition ‘x’ were grown by the simple and economical chemical bath deposition technique. The as-grown thin films were characterized for structural, compositional, surface morphological, optical and electrical studies. The X-ray diffraction (XRD) patterns of the sample indicated that all the samples were polycrystalline in nature with hexagonal structure. Scanning electron microscopy (SEM) micrographs showed uniform morphology with spherical shaped grains distributed over entire glass substrate. EDAX studies confirmed that the CdS_1−xSe_x films were having approximately same stoichiometry initially as well as finally. Room temperature optical measurements showed that band gap engineering could be realized in CdS_1−xSe_x thin films via modulation in composition ‘x’. Electrical resistivity of CdS_1−xSe_x thin films for various compositions was found to be low. The broad and fine tunable band gap properties of ternary CdS_1−xSe_x thin films have potential applications in opto-electronic devices.     

Formation and optical properties of CdSSe semiconductor nanocrystals in the silicate glass matrix

Technological conditions providing the formation of CdS _x Se_1−x semiconductor crystal grains with sizes ranging from 2 to 8 nm in a silicate glass matrix have been determined. As the temperature of forming annealing increases, the size of crystal grains increases without changes in their crystal structure and composition. The observed short-wavelength shift of the optical absorption edge indicates that the quantum confinement affects the energy band structure of the nanocrystals. Intense luminescence of the samples is due to radiative transitions involving defects at the semiconductor nanocrystal-silicate matrix interface or intrinsic defects of nanocrystals.     

Compositional dependence of the optical properties of amorphous semiconducting glass Ge10AsxSe(90−x) thin films

Optical properties of ternary chalcognide amorphous Ge₁₀As_xSe_(90−x) (with 10?x?25 at%) thin films prepared by thermal evaporation have been measured in visible and near-infrared spectral region. The straightforward analysis proposed by Swanepoel has been successfully employed, and it has allowed us to determine the average thickness , and the refractive index, n, of the films, with high accuracy. The refractive index, n and the average thickness has been determined from the upper and lower envelopes of the transmission spectra measured at normal incidence, in the spectral range 400-2500 nm. The absorption coefficient α, and therefore extinction coefficient k, have been determined from the transmission spectra in the strong-absorption region. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple-DiDomenico model, and the optical absorption edge is described using the ‘nondirect transition’ model proposed by Tauc. Likewise, the optical energy gap is derived from Tauc's extrapolation. The relationship between the optical gap and chemical composition in Ge₁₀As_xSe_(90−x) amorphous system is discussed in terms of the average heat of atomization H_s and average coordination number N_c. Finally, the chemical bond approach has been also applied successfully to interpret the decrease of the glass optical gap with increasing As content.     

Optical and physical properties of different composition of InxSe1−x thin films

Thin film binary alloys of In_xSe_1−x (0.05?x?0.30) have been prepared by the thermal evaporation technique. The optical transmission and reflection spectrum of these films were measured in the range 300-1100 nm. Both refractive index, n and extinction coefficient k have been determined from transmission and reflection measurements in terms of Murmann's equations. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The width of band tail is determined and the optical absorption edge is described using the ‘non-direct transition’ model proposed by Tauc. Finally, the relationship between the optical gap and chemical composition in In_xSe_1−x amorphous system is discussed in terms of the average heat of atomization H_s and average coordination number N_c. The results of these calculations can be used rationalize the observed optical properties of these materials. Finally, the chemical bond approach has been also applied to interpret the decrease of the glass optical gap with increasing In content.     

Quantum confinement in CdSxSe1−x spherical nanocrystals in a fluorophosphate glass matrix

Differential (wavelength-modulated) absorption of sulfoselenide solid-solution nanocrystals has been studied in a glassy fluorophosphate matrix near the fundamental absorption edge at 360–620 nm. The observed oscillations in the absorption are attributed to size quantization of electrons and holes under strong quantum-confinement conditions. The sulfur content in the mixed semiconductor has been refined from Raman scattering spectra in CdS_xSe_1?x samples with x=0.30, and the nanocrystal size (R?30 Å) has been derived from low-frequency Raman scattering spectra. These data were used to calculate the energies of electron-hole transitions in nanocrystals of mixed composition, and their subsequent comparison with experiment. The calculations were found to be in a good agreement with the observed experimental absorption spectrum for nanocrystals about 45 Å in size. The applicability of band-structure simulation for a nonspherical nanocrystal grown in a fluorophosphate glass matrix is discussed.     

Effects of Pb addition and heat treatment on optical properties of thermally evaporated Ge1−xSe2Pbx thin films

Thin chalcogenide films of Ge_1−xSe₂Pb_x (x=0, 0.2, 0.4, 0.6 and 0.8) have been prepared by the thermal evaporation technique, from previously synthesized bulk samples. The X-ray diffraction showed the amorphous nature for the as-deposited films and the partially crystalline for the annealed films. The optical constants (the refractive index, n, and absorption index k) were determined for as-deposited and annealed Ge_1−xSe₂Pb_x films of different thicknesses by using spectrophotometeric measurements of the transmittance and reflectance at normal incidence in the spectral range 200-2500 nm. The obtained values of both n and k were found to be independent of the film thickness. The optical absorption edges are described using both the Urbach rule and the indirect transition. In transparent region, the spectral dependences of refractive index were interpreted in the frame of a single oscillator model.     

Thermal annealing effect of on optical constants of vacuum evaporated Se75S25−xCdx chalcogenide thin films

Chalcogenide glasses are interesting materials due to their infrared transmitting properties and photo induced effects exhibited by them. Thin films with thickness of 3000 Å of the glasses Se₇₅S_25−xCd_x with x=6, 8 and 10 at% prepared by melt quench technique were evaporated by thermal evaporation onto glass substrates under a vacuum of 10⁻⁶ Torr. The optical constants (absorption coefficient, refractive index and extinction coefficient) of as-prepared and annealed films have been studied as a function of photon energy in the wave length region 400-1000 nm. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It has been found that the absorption coefficient and optical band gap increase with increasing annealing temperatures. The refractive index (n) and the extinction coefficient (k) were observed to decrease with increasing annealing temperature.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Characterization of CdS1−xSex thin films by chemical bath deposition technique

The cadmium sulfo selenide CdS_1?xSe_x thin films were chemical bath deposited in aqueous media onto coated glass substrates. As-deposited CdS_1?xSe_x thin films were annealed at 350 °C in air for 30 min. The structural, morphological, compositional and optical properties of deposited CdS_1?xSe_x thin films were studied using X-ray diffractometer (XRD), scanning electron microscopy (SEM), Energy dispersive analysis by X-ray (EDAX), and UV-Vis-NIR spectrophotometer respectively. X-ray diffraction patterns of CdS_1?xSe_x thin films reveal the polycrystalline nature and hexagonal structure. The microstructural parameters such as crystallite size (D), micro strain (?), and dislocation density were calculated and found to depend on compositions. The surface morphology and grain size are found to be influenced with the annealing temperature. The presence of Cd, S and Se of the CdS_1?xSe_x thin films and the composition of CdS_1?xSe_x thin film are estimated by EDAX analysis. The optical transmittance and absorption spectra were recorded in the range 400–2500 nm. The band gap of the CdS_1?xSe_x thin films is found to decrease from 2.5 eV to 1.75 eV.     

Effect of Cu deficiency on the optical properties and electronic structure of CuIn1−xGaxSe2

Spectroscopic ellipsometry measurements of CuInSe₂ (CIS) and CuIn_1−xGa_xSe₂ (CIGS) over a range of Cu compositions reveal that there are important differences in electronic and optical properties between α-phase CIS/CIGS and Cu-poor CIS/CIGS. We find a reduction in the imaginary part of the dielectric function ?₂ in the spectral region, 1-3 eV. This reduction can be explained in terms of the Cu-3d density of states. An increase in band gap is found for Cu-poor CIS and CIGS due to the reduction in repulsive interaction between Cu-3d and Se-4p states. We also characterize the dielectric functions of polycrystalline thin-film α-phase CuIn_1−xGa_xSe₂ (x=0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn_1−xGa_xSe₂. The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.     

Thermodynamic and kinetic properties of Ho1−xTixCo2-hydrogen system

The hydrogen absorption behavior of Laves phase Ho_1−xTi_xCo₂ (x=0.1-0.6) alloys has been investigated by pressure-concentration (PC) isotherms and cyclic-, temperature- and pressure-dependent absorption kinetics. The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.01-1 bar and temperature range 50-200 °C using Sievert's-type apparatus. The drastic changes in the induction period and particle size during the activation process have been discussed based on the kinetics of repeated hydrogenation cycles and scanning electron microscopy (SEM) images of the hydrides at different hydriding cycles, respectively. The experimental results of kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model, and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions in Ho_1−xTi_xCo₂-H have been identified from the different slope regions of the first-order-type kinetic plots. The dependence of the reaction rate parameter on hydriding pressure and temperature in the (α+β)-phase region has been discussed.     

Effect of composition and thermal annealing on the absorption and electrical conduction of Se85−xTe15Sbx glasses

The optical absorption of the as-prepared and thermally annealed Se_85−xTe₁₅Sb_x (0≤x≤9) thin films was measured. The mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap (E₀) decreased from 1.12 to 0.84 eV with increasing Sb content of the as-prepared films from 0 to 9 at.%. The as-prepared Se₇₆Te₁₅Sb₉ films showed an increase in (E₀) with increasing the temperature of annealing in the range above T_g (363 K). The electrical conductivity of the as-prepared and annealed films was found to be of Arrhenius type with temperature in the range 300-360 K. The activation energy for conduction was found to decrease with increasing both the Sb content and temperature of annealing. The results were discussed on the basis of the lone-pair electron effect and of amorphous crystalline transformation.     

Composition dependence of thermal stability, micro-hardness and compactness in glassy Se90In10−xGex alloys

In the present work, the effect of Ge incorporation is studied on the thermal stability, micro-hardness and compactness of glassy Se₉₀In₁₀ alloy.Thermal stability of glassy Se₉₀In_10−xGe_x alloys has been studied using differential scanning calorimetric technique. Micro-hardness of glassy Se₉₀In_10−xGe_x alloys is measured at room temperature. The compactness of the structure of Se₉₀In_10−xGe_x alloys is also determined from the measured densities. The variation in micro-hardness and compactness with composition has been discussed.     

Compositional dependence of the optical properties of the amorphous GexTe1−x system

Optical absorption at room temperature of vacuum evaporated Ge_xTe_1−x (0.125?x?0.225) amorphous thin films has been studied as a function of composition. It was found that the optical absorption is due to a direct transition. The real and imaginary parts of the dielectric constant were determined. The single-oscillator energy (E_o) and the energy dispersion parameter (E_d) have been calculated and discussed in terms of the Wemple and Di-Domenico model. The relationship between the optical gap, the average heat of atomization and the coordination numbers has been determined. The optical properties of the amorphous thin films do not change monotonically with increasing Ge content. The observed behavior of the optical properties in the range of compositions studied are explained on the basis of the Bond Constraint Theory (BCT) and rigidity theory, which provide a powerful framework for understanding the structure and properties of amorphous materials. The non-monotonic variation of the optical properties indicates that a transition from floppy to rigid occurs in the Ge_xTe_1−x films.     

Fröhlich electron-phonon interaction in CdSxSe1-xnanocrystals

We demonstrate that the polar electron-phonon coupling in CdS_xSe_1-x nanocrystals increases with decreasing crystallite size. This result is supported by the size dependence of the Huang-Rhys parameter measured by absorption and photoluminescence spectroscopy and by microprobe resonant Raman measurements which allow us to determine the size dependence of the electron-LO phonon coupling and the relative strength between Fröhlich and deformation potential Raman polarizabilities.     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

Selected properties of EuCo2(Si1−xGex)2 alloys

EuCo₂(Si_1−xGe_x)₂, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x?0.2 and x?0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.     

Dielectric properties of valence compensated Ca1−xBixTi1−xCrxO3 perovskite prepared using the sol-gel process

Ca_1−xBi_xNb_1−xCr_xO₃ (x=0.01-0.5) ceramic powders were synthesized using the sol-gel process. The single-phase solids can be presented at x=0.01 and 0.03. The coexistence of orthorhombic perovskite and the secondary phase of BiCrO₃ was verified, as presented for x=0.05-0.5. Grains with a micro-cube topography were obtained for x=0.3-0.5. The average grain size is about 0.4 and 1.1 μm for x=0.3 and 0.5, respectively. The highest dielectric constant peak was measured at around 55 °C for x=0.5 and at 75 °C for x=0.3. The high dielectric constant was caused by the formation of barrier layers at the interface of the bi-phase mixed ceramics. Space charge polarization contributed to the observed behavior.     

The structural, electronic, elastic and optical properties of AlCu(Se1−xTex)2 compounds from first-principle calculations

The structural, electronic structure, elastic and optical properties of the AlCu(Se_1−xTe_x)₂ compounds have been investigated by using a first-principles method based on density functional theory. The lattice constants of the quaternary compounds AlCu(Se_1−xTe_x)₂ increase with the increasing of Te composition. The calculated lattice constants for the ternary compounds i.e. AlCuSe₂ and AlCuTe₂ are in good agreement with the experimental data. The band structures show that the compounds have direct band gap and the band gaps are found to vary nonlinearly with composition. The total and part density of states of the quaternary AlCu(Se_1−xTe_x)₂ compounds are discussed. The calculated elastic constants indicate that all of the AlCu(Se_1−xTe_x)₂ compounds are mechanically stable. The bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν can be obtained by using the Voigt-Reuss-Hill averaging scheme. The B/G ratios of the AlCu(Se_1−xTe_x)₂ compounds indicate that AlCu(Se_0.8Te_0.2)₂ is ductile and the others are brittle. The Debye temperature of the AlCu(Se_1−xTe_x)₂ compounds decreases a little with increasing Te content except the compound with x = 0.4. The dielectric functions, refractive index, extinction coefficient, absorption spectrums and energy-loss function of the AlCuSe₂ and AlCuTe₂ are also calculated and discussed in this work.