The study of optical parameters and DC conductivity of Se100−xHgx thin films

Se_100−xHg_x bulk samples have been prepared by conventional melt quenching technique. The thin films of the material have been prepared on glass substrate using the thermal evaporation technique. The transmission spectra has been studied to measure the optical constants like absorption coefficient (α), extinction coefficient (K), optical band gap (E_g), Urbach energy (E_e). The DC conductivity (σ_dc) of Se_100−xHg_x has been also studied to find the activation energy (ΔE_a)$(\mathrm{\Delta }{E}_a)$. The optical band gap increases and Urbach energy first increases then decreases with increase in Hg concentration. DC conductivity and activation energy increases with increase in Hg concentration. These materials are found suitable for the optical disk materials and in optoelectronic devices due to their high absorption coefficient and dependence of reflectance on composition.     

Sn1−xBixO2 and Sn1−xTaxO2 (0≤x≤0.75): A first-principles study

The structural, elastic, electronic and optical (x=0) properties of doped Sn_1−xBi_xO₂ and Sn_1−xTa_xO₂ (0≤x≤0.75) are studied using the first-principles pseudopotential plane-wave method within the local density approximation. The independent elastic constants C_ij and other elastic parameters of these compounds have been calculated for the first time. The mechanical stability of the compounds with different doping concentrations has also been studied. The electronic band structure and density of states are calculated and the effect of doping on these properties is also analyzed. It is seen that the band gap of the undoped compound narrowed with dopant concentration, which disappeared for x=0.26 for Bi doping and 0.36 for Ta doping. The materials thus become conductive oxides through the change in the electronic properties of the compound for x≤0.75, which may be useful for potential application. The calculated optical properties, e.g. dielectric function, refractive index, absorption spectrum, loss-function, reflectivity and conductivity of the undoped SnO₂ in two polarization directions are compared with both previous calculations and measurements.     

The structural, electronic and optical properties of CdxZn1 − xSe ternary alloys

The structural, electronic, and optical properties of Cd_xZn_1 − xSe alloys are investigated using the first-principles plane-wave pseudopotential method within the LDA approximations. In particular, the lattice constant, bulk modulus, electronic band structures, density of state, and optical properties such as dielectric functions, refractive index, extinction coefficient and energy loss function are calculated and discussed. Our results agree well with the available data in the literature.     

Bandgap engineering of Cd1−xSrxO

Structural, electronic and optical properties of Cd_1−xSr_xO (0≤x≤1) are calculated for the first time using density functional theory. Our results show that these properties are strongly dependent on x. The bond between Cd and O is partially covalent and the covalent nature of the bond decreases as the concentration of Sr increases from 0% to 100%. It is found that Cd_1−xSr_xO is an indirect bandgap compound for the entire range of x and the bandgap of the alloy increases from 0.85 to 6.00 eV with the increase in Sr concentration. Frequency dependent dielectric functions ε₁(ω), ε₂(ω), refractive index n(ω) and absorption coefficient α(ω) are also calculated and discussed in detail. The peak value of refractive indices shifts to higher energy regions with the increase in Sr. The striking feature of these alloys is that Cd_0.5Sr_0.5O is an anisotropic material. The larger value of the extraordinary refractive index confirms that the material is positive birefringence crystal. The present comprehensive theoretical study of the optoelectronic properties of the material predicts that it can be effectively used in optoelectronic applications in the wide range of spectrum; IR, visible and UV.     

Characterization and properties of ZnO1−xSx alloy films fabricated by radio-frequency magnetron sputtering

A series of ZnO_1−xS_x alloy films (0 ≤ x ≤ 1) were grown on quartz substrates by radio-frequency (rf) magnetron sputtering of ZnS ceramic target, using oxygen and argon as working gas. X-ray diffraction measurement shows that the ZnO_1−xS_x films have wurtzite structure with (0 0 2) preferential orientation in O-rich side (0 ≤ x ≤ 0.23) and zinc blende structure with (1 1 1) preferential orientation in S-rich side (0.77 ≤ x ≤ 1). However, when the S content is in the range of 0.23 < x < 0.77, the ZnO_1−xS_x film consists of two phases of wurtzite and zinc blende or amorphous ZnO_1−xS_x phase. The band gap energy of the films shows non-linear dependence on the S content, with an optical bowing parameter of about 2.9 eV. The photoluminescence (PL) measurement reveals that the PL spectrum of the wurtzite ZnO_1−xS_x is dominated by visible band and its PL intensity and intensity ratio of UV to visible band decrease greatly compared with undoped ZnO. All as-grown ZnO_1−xS_x films behave insulating, but show n-type conductivity for w-ZnO_1−xS_x and maintain insulating properties for β-ZnO_1−xS_x after annealed. Mechanisms of effects of S on optical and electrical properties of the ZnO_1−xS_x alloy are discussed in the present work.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Optical characterization of xTiO2-(60 − x)SiO2-40Na2O glasses: II. Absorption edge, Fermi level, electronic polarizability and optical basicity

In the present work different optical properties of xTiO₂-(60 − x)SiO₂-40Na₂O (wt%) optical glasses are determined. The characterization is done over a wide energy range, 0.41-6.2 eV. The refractive index and the extinction coefficient data are used to measure the absorption coefficient of the different glass compositions. Studying the UV-absorption edge, both direct and indirect allowed transitions with their optical energy gaps are carried out. In the same time, the Urbach energy is evaluated. From the extinction coefficient data, the Fermi energy of the glasses is calculated. The molar refraction, electronic polarizability and the optical basicity are obtained using the measured glass refractive indices.     

Optical and electrical properties of nonstoichiometric a-Ge1−xCx films prepared by magnetron co-sputtering

Amorphous non-hydrogenated germanium carbide (a-Ge_1−xC_x) films have been prepared by magnetron co-sputtering method in a discharge of Ar. The dependence of structural and chemical bonding properties on the Ge/C ratio (R) has been investigated by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. The relationship between the chemical bonding and the optical and electrical properties of the a-Ge_1−xC_x films has also been explored. It has been shown that the refractive index of the films increases from 2.9 to 4.4 and the optical gap decreases from 1.55 to 1.05 eV as R increases from 1.22 to 5.67. Moreover, the conductivity σ increases clearly and the activation energy E_a decreases with the increasing R owing to the reduction of sp³ CGe bonds. The a-Ge_1−xC_x films exhibit refractive index and optical gap values changing with x in a wide range, which may make a-Ge_1−xC_x films good candidates in the fields of protection coatings for IR windows and electronic devices.     

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.     

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.     

Optical properties of MgxZn1−xO thin films deposited on silicon and sapphire substrate by rf magnetron sputtering

The index dispersion at UV–VIS range for polycrystalline Mg_xZn_1−xO films on silicon with different Mg concentration was obtained by spectroscopic ellipsometry (SE) method. It decreases with the increase of the Mg content. Above the relative peak wavelength, they are well fitted by the first-order Sellmeier relation. The band gap of films on sapphire of different Mg content was determined from transmission measurements. Photoluminescence (PL) illustrated that for Mg_xZn_1−xO films every PL peak corresponded to a special excitation wavelength. The wavelength of the PL peak was proportional to the special excitation wavelength. A strong peak was obtained in the blue band for the films due to the large amount of oxygen vacancies caused by excess Zn and Mg atoms, while weak peak at ultraviolet band.     

Size dependent effective band gap,optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn_1−xMg_xS/ZnS/Zn_1−xMg_xS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn_1−xMg_xS quantum wells.     

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.     

Electronic structure and optical properties of TbNi5−xCux

In this paper we present theoretical investigation of optical conductivity for intermetallic TbNi_5−xCu_x series. Within the framework of LSDA+U calculations, electronic structure for x=0, 1, 2 is calculated and additionally optical conductivity is obtained. Disorder effects of Cu for Ni substitution on a level of LSDA+U densities of states (DOS) are taken into account via averaging over all possible Cu ion positions in the unit cell for given doping level x. Gradual smoothing of optical conductivity structure at 2 eV together with simultaneous intensity growth at 4 eV corresponds to increase of Cu and decrease of Ni content.     

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.     

First principles investigation of optical properties of zinc-blende AlxGa1−xAs1−yNy materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al_xGa_1−xAs_1−yN_y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al_xGa_1−xAs and Al_xGa_1−xAs_1−yN_y with x≤0.375 and y up to 4% are presented. Al_xGa_1−xAs on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al_xGa_1−xAs has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C_MN=2.32 and a N level of E_N=(1.625+0.069x) eV. The results show that Al_xGa_1−xAs can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al_xGa_1−xAs_1−yN_y could be an alternative to Al_xGa_1−xAs when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.     

Effect of sulfur substitutions on optical, electrical and structural properties of Ge(SxSe1−x)2 system

Glasses in the system Ge-Se-S were prepared with different Se/S ratios in order to investigate the compositional dependence of selected physical properties. We report the results of a systematic study examining the UV-vis transmission, dc electrical conductivity and X-ray diffraction of the system Ge(S_xSe_1−x)₂ with x=0, 0.1, 0.4 and 1.0 where replacement of S by Se was made. The changes in the optical energy gap, E_g, (from 1.95 to 2.43 eV) and band tail width, E_e, (from 103 to 243 meV) behave contrarily to the change in refractive index, n, (from 2.3 to 2) with the progressive replacement of S by Se. This behavior was discussed and interpreted with the changes in cohesive energy. The analysis of defects in the prepared films was carried out by the examination of activation energies obtained from dc electrical conductivity. The analysis of the X-ray diffraction pattern revealed a remarkable reduction in the intensity of the first and second diffraction peaks with the progressive replacement of S content, which confirms a change in the intermediate range order structure: reorganization of the structural properties.     

Influence of composition on electrical and optical properties of new chalcogenide thin films from Ge-Se-Tl system

Bulk Ge₂₀Se_80−xTl_x (x ranging from 0 to 15 at%) chalcogenide glasses were prepared by conventional melt quenching technique. Thin films of these compositions were prepared by thermal evaporation, on glass and Si wafer substrates at a base pressure of 10⁻⁶ Torr. X-ray diffraction studies were performed to investigate the structure of the thin films. The absence of any sharp peaks in the X-ray diffractogram confirms that the films are amorphous in nature. The optical constants (absorption coefficient, optical band gap, extinction coefficient and refractive index) of Ge₂₀Se_80−xTl_x thin films are determined by absorption and reflectance measurements in a wavelength range of 400-900 nm. In order to determine the optical gap, the absorption spectra of films with different Tl contents were analyzed. The absorption data revealed the existence of allowed indirect transitions. The optical band gap showed a sharp decrease from 2.06 to 1.79 eV as the Tl content increased from 0% to 15%. It has been found that the values of absorption coefficient and refractive index increase while the extinction coefficient decreases with increase in Tl content in the Ge-Se system. These results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. DC electrical conductivity of Ge₂₀Se_80−xTl_x thin films was carried out in a temperature range 293-393 K. The electrical activation energy of these films was determined by investigating the temperature dependence of dc conductivity. A decrease in the electrical activation energy from 0.91 to 0.55 eV was observed as the Tl content was increased up to 15 at% in Ge₂₀Se_80−xTl_x system. On the basis of pre-exponential factor, it is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges.     

Influence of doping of mercury atom(s) on optoelectronic properties of binary cadmium chalcogenides - A density functional theory based investigation with different exchange-correlation functionals and including spin-orbit coupling

Influence of doping of mercury atom(s) on optoelectronic properties of binary cadmium chalcogenides have been investigated theoretically by designing the mercury doped cadmium chalcogenide ternary alloys in B3 phase at some specific Hg-concentrations and studying their optoelectronic properties using DFT based FP-LAPW methodology. The structural properties are computed using WC-GGA, while spin-orbit coupling included electronic and optical properties are computed using TB-mBJ, EV-GGA, B3LYP and WC-GGA exchange-correlation functionals. In addition, electronic properties of mercury chalcogenides are calculated precisely using the GGA+U functionals. The concentration dependence of lattice parameter and bulk modulus of each of the Hg_xCd_1−xS, Hg_xCd_1−xSe, Hg_xCd_1−xTe alloy systems show almost linearity. For each of the alloy systems, band gap decreases almost linearly with increase in Hg-concentration in the unit cell and contribution from charge exchange to the band gap bowing is larger than that from for each of the volume deformation and structural relaxation. Also, covalent bonding exists between different constituent atoms in each compound. Optical properties of each specimen are computed from their spectra of dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity, optical absorption coefficient and energy loss function. Several calculated results have been compared with available experimental and other theoretical data.     

Structural,electronic and optical properties of CdxZn1 − xS alloys from first-principles calculations

Structural, electronic and optical properties as well as structural phase transitions of ternary alloy Cd_xZn_1 − xS have been investigated using the first-principles calculations based on the density functional theory. We found that the crystal structure of Cd_xZn_1 − xS alloys transforms from wurtzite to zinc blende as Cd content of x=0.83$x=0.83$. Effect of Cd content on electronic structures of Cd_xZn_1 − xS alloys has been studied. The bandgaps of Cd_xZn_1 − xS alloys with wurtzite and zinc blende structures decrease with the increase of Cd content. Furthermore, dielectric constant and absorption coefficient also have been discussed in detail.