Some optical properties of Se-Ge-As amorphous chalcogenide glasses

The effects of composition, film thickness, substrate temperature, and annealing of amorphous thin films of Se₇₅Ge_25−x As _x (5⩽x⩽20) on their optical properties have been investigated. X-ray diffraction revealed the formation of amorphous films. The absorbance and transmission of vacuum-evaporated thin films were used to determine the band gap and refractive index. Optical absorption measurements showed that the fundamental absorption edge is a function of glass composition and the optical absorption is due to indirect transition. The energy gap increases linearly with increasing arsenic content. The optical band gap,E _opt, was found to be almost thickness independent. The shapes of the absorption edge of annealed samples displayed roughly the same characteristic as those of the unannealed films, but were shifted towards shorter wavelengths; as a result,E _opt increased andE _e, the width of the band tails, decreases. The increase inE _opt is believed to be associated with void removal and microstructural re-arrangement during annealing. The influence of substrate temperature on the optical parameters is discussed.     

Determination of the optical constants of As–Se–Ag chalcogenide thick films with high precision for optoelectronics applications

Thin films of (As₅₀Se₅₀)_100?xAg_x (with 0?≤?x?≤?25 s) metal-chalcogenide glasses were deposited onto glass substrates by thermal evaporation technique under high vacuum (10^?6 mbar). The optical constants as well as the average thickness of the studied films are determined by the Swanepoel envelope method which is based on the optical transmission spectra measured in the spectral range 300–2500 nm. This method enables the transformation of the optical-transmission spectrum of a thin film of wedge-shaped thickness into the spectrum of a uniform film, whose thickness is equal to the average thickness of the non-uniform layer. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-oscillator model. The optical absorption edge is described using the non-direct transition model proposed by Tauc relation. Analysis of the optical data revealed that an addition of Ag in the range from 0 to 25 at.% to the (As₅₀Se₅₀)_100?x binary alloys affected the optical parameters of the investigated thin films. For instance, the optical band gap decreased from 1.661 to 1.441 eV with increasing the Ag content from 0 to 25 at.%. The results were discussed in terms of Mott and Davis model as well as chemical-bond approach.     

A.c. photoconductivity and optical properties of bulk polycrystalline and amorphous In x Se1−x thin films

The a.c. photoconductivity of bulk polycrystalline In _x Se_1–x and the optical properties of amorphous In _x Se_1–x thin films were investigated. The effect of heat treatment on the absorption coefficient of InSe thin films was also studied. The spectral response of the photoconductivity of polycrystalline In _x Se_1–x related to the near-edge absorption spectrum, shows a well-defined maximum corresponding to the width of the forbidden gap. Polycrystalline In _x Se_1–x realizes the behaviour of photoconductors which are sensitive in a spectral range between 400 and 1700 nm. Optical absorption of amorphous In _x Se_1–x thin films was recorded as a function of photon energy and the data were used to deduce the value of optical energy gap of the films. It was found that the optical energy gap decreased with increasing indium content in both bulk polycrystalline and amorphous thin films. The effect of heat treatment on the optical gap of the film has been interpreted in terms of the density of states model of Mott and Davis.     

Compositional dependence of the optical parameters for Bi5GexSe65−x (30 ≤ x ≤ 45) films

Different compositions of Bi₅Ge_xSe_95−x (x = 30, 35, 40 and 45 at %) thin films were deposited onto cleaned glass substrates by thermal evaporation method. The structural characterization revealed that, the as-prepared films of x = 30, 35 and 40 at. % are in amorphous state but there are few tiny crystalline peaks of relatively low intensity for the film with x = 45 at. %. The chemical composition of the as-prepared Bi₅Ge_xSe_65−x films has been checked using energy dispersive X-ray spectroscopy (EDX). The optical properties for the as-deposited Bi₅Ge_xSe_65−x thin films have been studied. The additions of Ge content were found to affect the optical constants (refractive index, n and the extinction coefficient, k). Tauc’s relation for the allowed indirect transition is successfully describing the mechanism of the optical absorption. It was found that, the optical energy gap (E_g) decreases with the increase in Ge content. These obtained results were discussed in terms of the chemical bond approach proposed by Bicermo and Ovshinsky. The composition dependence of the refractive index was discussed in terms of the single oscillator model.     

Composition dependence of optical constants of Ge1 − xSe2Pbx thin films

Optical constants of vacuum-evaporated thin films in the Ge_1 − xSe₂Pb_x (x = 0, 0.2, 0.4, 0.6) system were calculated from reflectance and transmittance spectra. It is found that the films exhibit a non-direct gap, which decreases with increasing Pb content. The variation in the refractive index and the imaginary part of the dielectric constant with photon energy is reported. The relationship between the optical gap and chemical composition in chalcogenide glasses is discussed in terms of the average heat of atomization.     

Optical and electrical properties of Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O thin films by sol–gel method

Mg _x Zn_1−x O (0 ≤ x ≤ 0.35) thin films have been deposited by sol–gel technique and the composition related structural, electrical, and optical properties are investigated. All the films have hexagonal wurtzite structure and the separation of MgO phase occurs when x = 0.3 and 0.35. With the increase of Mg content, the densification of the films decrease and band gap values increase. The maximum band gap value reaches 3.56 eV when x = 0.15. After Mg doping the conductivities of the Mg _x Zn_1−x O films are reduced greatly and the electrical current–voltage (I–V) characteristics show nonlinearity for x > 0.15.     

Optical and other physical characteristics of Ge–Se–Cd thin films

Amorphous Ge₂₀Se_80−xCd_x thin films with different compositions (x = 0, 2.5, 5, 7.5 and 10 at.%) were deposited onto glass substrates by thermal evaporation. The reflection spectra, R(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Minkov has been applied to derive the optical constants and the film thickness for the Ge₂₀Se_80−xCd_x thin films. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing cadmium content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 2 to 1.5 eV with increasing cadmium content from 0 to 10 at.%. The chemical-bond approach has been applied successfully to obtain the excess of Se–Se homopolar bonds and the cohesive energy of the Ge₂₀Se_80−xCd_x system.     

Effect of composition and deposition parameters on optical properties of Ge25Sb15−xBixS60 thin films

The optical constants of the Ge₂₅Sb_15−xBi_xS₆₀ (0?x?15) chalcogenide films, either as-deposited or after being annealed at various temperatures have been computed in the spectral wavelength range 400-2400 nm from the transmittance and reflectance measurements of normally-incident light. With the increase in bismuth content, the optical energy gap (which is indirect) decreases, while the refractive index increases. The effects of film thickness, substrate type, deposition rate and γ-radiation on optical properties have been studied. The effect of thermal annealing on the growth characteristics and stability of the films has been studied using X-ray diffraction and scanning electron microscopy. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model.     

Effect of tellurium addition on the optical and physical properties of germanium selenide glassy semiconductors

Thin films of the glasses Ge₁₀ Se_90−x Te_x (0 ≤ x ≥ 40) have been prepared by melt quenching technique; thin films were evaporated at a pressure of ≈10⁻⁴ Pa. The optical absorption behavior of these thin films was studied from the reflection and transmission spectrum in the spectral range 200-1200 nm. The optical constants i.e optical band gap (E_opt), absorption coefficient, refractive index (n) are calculated. The optical band gap has been estimated using Tauc extrapolation and found to decrease with Te content. The Dispersion of refractive index has been studied in terms of Wemple - Di Domenico model. The value of static refraction index has been found to increase with Te content. The distribution of the possible chemical bonds has been calculated. The obtained results were correlated with the character of the chemical bond for the prepared compositions through a study of parameters such as average heat of atomization (Hs), the cohesive energies of the bonds (CE), The mean bond energy <E> and average coordination number (m).     

Optical,structural and electrical investigations on PbTe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

The fabrication of devices with lead salts and their alloys with detecting and lasing capabilities has been an important technological development. The high quality polycrystalline thin films of PbTe_1−x S _x with variable composition (0 ≤ x ≤ 1) have been deposited onto ultra clean glass substrates by vacuum evaporation technique. Optical, structural and electrical properties of PbTe_1−x S _x thin films have been examined. Absorption coefficient and band gap of the films were determined by absorbance measurements in wavelength range 2,500–5,000 nm using FTIR spectrophotometer. Sample nature, crystal structure and lattice parameter of the films were determined from X-ray diffraction patterns. DC conductivity and activation energy of the films were measured in temperature range 300–380 K through I–V measurements.     

Change in the type of majority carriers in disordered ln x Se100−x thin-film alloys

Electrical, optical and physico-chemical properties of disordered In_xSe_100–x thin films have been investigated for x ranging from 40–65. The films are found to be p-type for composition ranging from 45–60 at% selenium and n-type for compositions below 40 at% selenium. An increase in the conductivity, together with a decrease of the activation energy and of the optical gap, has also been observed when x varies from 40–65. These results have been interpreted through a theory based on the relative percentage evolution of the In-In and Se-Se chemical bonds and, on the other hand, by a percolation theory due to microcrystalline structures and indium filaments. These two models are discussed in reference to other publications.     

Effect of Al doping and deposition runs on structural and optical properties of In<Subscript>2</Subscript>S<Subscript>3</Subscript> thin films grown by CBD

β-In_2−x Al _x S₃ thin films have been grown on glass substrate by chemical bath deposition for different value of Al concentration y = (([Al])/([In]))_sol (0 ≤ y ≤ 5 at.%). Samples have been characterized using X-ray diffraction, atomic force microscopy and by spectrophotometric measurements. The influence of the increase of y ratio in the structural and optical properties are described and discussed in terms of crystallinity improvement. In order to increase film thickness of β-In_2−x Al _x S₃, we have been realized multi-deposition system. The structural, the surface morphology as well as the optical properties seem to be improved as the film thickness is of about 1200 nm.     

Structural,optical and electrical properties of CdZnS thin films

The thin films of Cd_1-x Zn _x S (x?=?0, 0.2, 0.4, 0.6, 0.8 and 1) have been prepared by the vacuum evaporation method using a mechanically alloyed mixture of CdS and ZnS. The structural, optical and electrical properties have been investigated through the X-ray diffractometer, spectrophotometer and Keithley electrometer. The X-ray diffraction patterns of these films show that films are polycrystalline in nature having preferential orientation along the (002) plane. In the absorption spectra of these films, absorption edge shifts towards lower wavelength with the increase of Zn concentration. The energy band gap has been determined using these spectra. It is found that the energy band gap increases with increasing Zn concentration. The electrical conductivity of so prepared thin films has been determined using a I–V characteristic curve for these films. The result indicates that the electrical conductivity decreases with increasing Zn content and increases with temperature. An effort has also been made to obtain activation energy of these films which increases with increasing Zn concentration in CdS.     

Optical investigation of a—Ga x Se100–x thin films

Abstract

The optical band gap and optical constants have been studied as a function of photon energy for a-Ga _x Se_100–x thin films (where x × 0, 2.5, 5.0, 7.5 and 10.0) in the wavelength region 450–1000 nm. The optical band gap decreases with the increase of Ga concentration in the a-Ga _x Se_100–x system. The refractive index (n) decreases while the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states.     

Thermal and optical properties of zinc halotellurite glasses

Zinc halotellurite glasses were studied with respect to the glass transition, softening temperature, thermal expansion, optical energy gap, Urbach energy, density, molar volume, refractive index, polarizability, molar refraction and third order non-linear optical susceptibility. Thermal characteristic were determined using a dilatometry. The optical absorption in the wavelength range (300–3200 nm) was measured. From the absorption edge studies, the values of optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. Optical parameters viz., color dispersion, dispersion energy, E _d, average oscillator energy, E ₀, and third order non-linear optical susceptibility values are estimated from measuring the refractive index at different wavelength. Results obtained are discussed in terms of the glass structure.     

Annealing effects on the structural and optical characteristics of electron beam deposited Ge-Se-Bi thin films

Measurements of absorbance and transmittance in Ge₂₀Se_80-xBi_x (0 x 10 at %) chalcogenide thin films in the visible range at room temperature were carried out. The dependence of the optical absorption on the photon energy is described by the relation hv=B(hv-E₀)². It was found that the optical energy gap E₀ decreases gradually from 1.93 to 1.205 eV with increasing Bi content up to 10 at %. The rate of the change decreases with increasing Bi content. The composition dependence of the optical energy gap is discussed on the basis of the concentration of covalent bonds formed in the chalcogenide film. The decrease of optical energy gap with increasing Bi content is related to the increase of Bi–Se bonds and the decrease of Se-Se bonds. The effect of thermal annealing for different periods of time on the behavior of optical absorption of the as-deposited films was investigated. The optical gap increases with increasing annealing time. The rate of change decreases with increasing annealing time, then E₀ reaches a steady state. The increase in the values of the optical gap of the amorphous films with heat treatment is interpreted in terms of the density of states model. The structure of the as-prepared and thermally annealed films were investigated using transmission electron microscopy, energy dispersive analysis and X-ray diffraction. It was confirmed that the as-prepared films were in the amorphous state. Phase separation was observed after thermal annealing. The separated crystalline phases were identified.     

Compositional effects on the optical properties of Gex Sb40−x Se60 thin films

Ge_x Sb_40−x Se₆₀ (x = 0, 2.42 and 23.41 at.%) thin chalcogenide films were deposited on glass and quartz substrates by the conventional thermal evaporation technique at 300 K. The chemical composition of the bulk material and as-deposited films were determined by energy dispersive analysis X-ray spectrometry (EDAX). X-ray diffraction pattern (XRD) of Ge_x Sb_40−x Se₆₀ (x = 0, 2.42 and 23.41 at.%) thin films indicates that they have amorphous structure. The optical transmission and reflection spectra were measured in the range of 500 to 2500 nm. The optical absorption coefficient spectra were studied for deposited samples. It is observed that the optical absorption edge shift to higher energy range, as the germanium content, x, increases in the film. The type of electronic transition, responsible for the optical properties, is indirect allowed transition. It is found that the optical band gap increases as the Ge content increases.The average coordination number (N_c) in Ge_x Sb_40−x Se₆₀ films increases, but the number of chalcogenide atoms remains constant. The number of Ge - Se bonds and the average bond energy of the system increase with the increase of the average coordination number. The optical band gap, E_g, increases with the increase of the average coordination number, (N_c). Also the energy gap, E₀₄, is discussed in terms of its relation to the chemical composition. The dispersion of the refractive index (n) is discussed in terms of the Single Oscillator Model (SOM) (Wimple - Didomenico model). The single oscillator energy (E₀), the dispersion energy (E_d) and the optical dielectric constant (?_∞) are also estimated.     

Effect of gallium on the electrical properties and photoresponse of CuIn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2</Subscript> films

Semiconducting films of CuIn_{1 − x} Ga _x Se₂ solid solutions have been produced on CdS by selenizing metallic layers in flowing nitrogen. We have optimized the film growth conditions and investigated the structure, electrical properties, energy band structure, and photoconversion quantum efficiency of the films. With increasing gallium content, the band gap of the CuIn_{1 − x} Ga _x Se₂ (x = 0, 0.2, 0.25, 0.30) films increases, reaching 1.27 eV. The films are potentially attractive for use in high-efficiency solar cells.     

Microstructure, optical and dielectric properties of compositional graded (Ba,Sr)TiO3 thin films derived by RF magnetron sputtering

Thin films of compositional graded Ba_1−x Sr _x TiO₃ (BST) (x decreasing from 0.3 to 0) were prepared on fused quartz and Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. The microstructure of the graded BST thin films was characterized by X-ray diffraction (XRD). It indicates that the films were crystallized with peroveskite structure and (100) + (111) preferred orientation. The refractive index and the band gap were determined at room temperature in the wavelength 200–1100 nm from spectrophotometric measurements of the transmittance. The average value of the refractive index is found to be 2.17 for the graded BST films in the wavelength 400–1000 nm. The optical band gap of the graded BST film was 3.77 eV. The dielectric measurement showed that the dielectric constant and loss factor of the graded BST film was 318.04 and 0.028 at 100 KHz and room temperature.     

Compositional dependence of the optical parameters for Bi5GexSe65−x (30 ≤ x ≤ 45) films

Different compositions of Bi₅Ge_xSe_95−x (x = 30, 35, 40 and 45 at %) thin films were deposited onto cleaned glass substrates by thermal evaporation method. The structural characterization revealed that, the as-prepared films of x = 30, 35 and 40 at. % are in amorphous state but there are few tiny crystalline peaks of relatively low intensity for the film with x = 45 at. %. The chemical composition of the as-prepared Bi₅Ge_xSe_65−x films has been checked using energy dispersive X-ray spectroscopy (EDX). The optical properties for the as-deposited Bi₅Ge_xSe_65−x thin films have been studied. The additions of Ge content were found to affect the optical constants (refractive index, n and the extinction coefficient, k). Tauc’s relation for the allowed indirect transition is successfully describing the mechanism of the optical absorption. It was found that, the optical energy gap (E_g) decreases with the increase in Ge content. These obtained results were discussed in terms of the chemical bond approach proposed by Bicermo and Ovshinsky. The composition dependence of the refractive index was discussed in terms of the single oscillator model.