Optical characterization of pyrolytically deposited ZnxCd1−xS thin films

The precursor bis-(morpholinodithioato-s,s′)–Cd–Zn was prepared and the thin films of Zn_xCd_1−xS deposited on sodalime glass substrate. A direct optical energy gap of 2.63 eV was obtained from the analysis of the absorption spectrum. The photoluminescence spectrum shows shift in the energy of the primary emission peaks as a function of the excitation energy. The shift was explained as due to the quantum size effect in nanometer thick polycrystalline films. Energy Dispersive X-ray Florescence (EDXRF) confirms the elemental composition of both the precursor and the films.     

Synthesis and humidity sensitive properties of nanocrystalline Ba1−xSrxTiO3 thick films

Nanocrystalline Ba_1−xSr_xTiO₃ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) precursors were synthesized using the stearic acid gel method. After the precursors had been calcined at 600–950°C for 0.5–1 h, nanocrystalline powders with the cubic perovskite structure were obtained and these were made into thick films. The powder samples were characterized by differential thermal analysis, X-ray diffraction and transmission electron microscopy, and the thick film samples were characterized by scanning electron microscopy and X-ray diffraction. The humidity-sensitive properties of the nanocrystalline Ba_1−xSr_xTiO₃ thick films were investigated. The results show that these nanocrystalline thick films possess higher humidity sensitivity and lower resistance than those of conventional materials.     

Co-deposition of cobalt disilicide on silicon–germanium thin films

The formation of CoSi₂ on strained epitaxial Si_0.8Ge_0.2/Si(100) films has been studied as a function of the deposition method and annealing temperature. Two types of deposition processes were used: a direct method, where 5 nm of pure Co metal were deposited at room temperature onto a strained 80 nm thick Si_0.8Ge_0.2 layer; and a co-deposition method, where 5 nm Co and 18.2 nm Si were simultaneously deposited in a 1:2 ratio onto a strained Si_0.8Ge_0.2 layer at 450°C. Samples were then annealed at temperatures ranging from 500 to 800°C. Extended X-ray absorbance fine structure spectroscopy (EXAFS) and X-ray diffraction (XRD) were used to characterize the structure of the resulting films. It was found that the samples prepared via the direct deposition method did not convert to CoSi₂ at any annealing temperature up to 800°C, while the co-deposited samples formed epitaxial CoSi₂ at even the lowest annealing temperature of 500°C. These results are discussed in terms of proposed reaction mechanisms of the different deposition methods, based on consideration of the Co–Si–Ge ternary phase diagram.     

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.     

Heteroepitaxial growth of Zn1−xCdxSe on cleavage-induced GaAs (110) surface in ultra high vacuum

Zn_1−xCd_xSe epitaxial growth by molecular beam epitaxy (MBE) on the GaAs (110) surface cleaved in ultra high vacuum (UHV) was investigated. The growth mode of Zn_1−x Cd_xSe on GaAs (110) was not a simple Stranski–Krastanow type. At initial growth stage, growth mode was two-dimensional type. However, as the growth proceeds three-dimensional island growth and two-dimensional growth modes compete. As a result, two kinds of structures were spontaneously formed on the surface, pyramidal-shaped islands and ridge structures aligned to the [1 0] direction. Anisotropic in-plane strain relaxation on (110) is suggested as the formation mechanism of such structures.     

Compositional dependence optical properties study of As40Se60 − xSbx thin films

Thin films were thermally evaporated from the bulk glasses of As₄₀Se_60 − xSb_x (with x = 0, 5, 10, 15 at.%) under high vacuum. We have characterized the deposited films by Fourier Transform Infrared spectroscopy. The relationship between the structural and optical properties and the compositional variation has been investigated. Increasing Sb content was found to affect the thermal and optical properties of these films. Non-direct electronic transition was found to be responsible for the photon absorption inside the investigated films. It was found that, the optical band gap E_o decreases while the width of localized states (Urbach energy) E_e increases.     

Preparation of B-doped a-Si1−xCx:H films and heterojunction p–i–n solar cells by the Cat-CVD method

B-doped a-Si_1−xC_x:H films for a window layer of Si thin film solar cells have been prepared by the Cat-CVD method. It is found that C is effectively incorporated into the films by using C₂H₂ as a C source gas, where an only little C incorporation is observed from CH₄ and C₂H₆ under similar deposition conditions. Using a-Si_1−xC_x:H films grown from C₂H₂, heterojunction p–i–n solar cells have been prepared by the Cat-CVD method. The cell structure is (SnO₂ Asahi-U)/ZnO/a-Si_1−xC_x:H(p)/a-Si:H(i)/μc-Si:H(n)/Al. The obtained conversion efficiency was 5.4%.     

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.     

Microstructure and optical properties of Si–Ag nanocomposite films prepared by co-sputtering

Novel composite thin films consisting of nanosized Ag particles embedded in an amorphous Si matrix were made with Ag contents from 0 at% to 61 at% by radio frequency co-sputtering of Si and Ag. The microstructure and optical properties of the films were characterized by conventional and high resolution transmission electron microscopy and spectrometry in the wavelength range from 200 to 1500 nm. It was found that the films consist of nanosized Ag particles (2.8–6.0 nm) particle and their clusters embedded in an amorphous Si matrix. The optical absorption spectra of the films up to 40 at% Ag exhibit characteristics similar to the amorphous semiconductor Si. At higher Ag contents two absorption maxima at 350 nm and 700 nm appear. Effective medium theories were examined to predict the optical properties of the films and it was found that the predictions from the Sheng Ping theory with a modified dielectric function of bulk Ag (taking into account the mean free path limitation of Ag particle boundaries) qualitatively agree with the measured absorption spectra. The two absorption maxima are accounted for as interfacial plasma resonance absorption associated with the silver particle/silicon matrix interfaces.     

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.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

Structural characterization and novel optical properties of defect chalcopyrite ZnGa2Te4 thin films

Stoichiometric thin film samples of the ternary ZnGa₂Te₄ defect chalcopyrite compound were prepared and characterized by X-ray diffraction technique. The elemental chemical composition of the prepared bulk material as well as of the as-deposited film was determined by energy-dispersive X-ray spectrometry. ZnGa₂Te₄ thin films were deposited, by conventional thermal evaporation technique onto highly cleaned glass substrates. The X-ray and electron diffraction studies revealed that the as-deposited and the annealed ZnGa₂Te₄ films at annealing temperature t_a ≤ 548 K are amorphous, while those annealed at t_a ≥ 573 K (for 1 h), are polycrystalline. The optical properties of the as-deposited films have been investigated for the first time at normal incidence in the spectral range from 500 to 2500 nm. The refractive index dispersion in the transmission and low absorption region is adequately described by the Wemple–DiDomenico single oscillator model, whereby, the values of the oscillator parameters have been calculated. The analysis of the optical absorption coefficient revealed an in-direct optical transition with energy of 1.33 eV for the as-deposited sample. This work suggested that ZnGa₂Te₄ is a good candidate in solar cell devices as an absorbing layer.     

Study of the Al-grading effect in the crystallisation of chalcopyrite CuIn1−xAlxSe2 thin films

Chalcopyrite CuIn_1−xAl_xSe₂ (CIAS) thin films with an atomic ratio of Al/(In + Al) = 0.4 were grown by a two-stage process onto soda-lime glass substrates. The selenisation was carried out at different temperatures, ranging from 400 °C to 550 °C, for metallic precursors layers evaporated with two different sequences. The first sequence, C1, was evaporated with the Al as the last layer, while in the second one, C2, the In was the last evaporated element. The optical, structural and morphological characterisations led to the conclusion that the precursors sequence determines the crystallisation pathway, resulting in C1 the best option due to the homogeneity of the depth distribution of the elements. The influence of the selenisation temperature was also studied, finding 540 °C as the optimum one, since it allows to achieve the highest band gap value for the C1 sequence and for the given composition.     

Phase formation process of IrxSi1−x thin films Structure and electrical properties

Experimental data on the phase formation process of amorphous Ir_xSi_1−x thin films with 0.30 ≤ x ≤ 0.41 are presented and discussed in relation to electric transport properties. The structure formation process at temperatures from 300 K up to 1223 K was investigated by means of X-ray diffraction. Distinct phases were observed in the final stage in dependence on the initial composition: Ir₃Si₄, Ir₃Si₅, and IrSi₃. An unknown metastable phase was found in films with a silicon concentration of 61 at.% to 64 at.% after annealing above the crystallization temperature T = 970 K. The crystal structure of this phase was determined by the combined use of X-ray diffraction and electron diffraction. It was found to be monoclinic, basic-face centred with lattice constants a = 1.027 nm, b = 0.796 nm, c = 0.609 nm, and γ = 113.7°. Additionally, microstructure and morphology of the films were investigated by transmission electron microscopy (TEM). The annealing process was studied by means of mechanical stress investigations as well as by electrical resistivity and thermopower measurements. Correlations between the structure, the phase formation and the electrical transport behaviour are discussed on the basis of conduction mechanism.     

Composition and substrate temperature dependence of structural and optical studies on ZnSexCdS1−x alloy films

Thin (t−0.60 μm) films of ZnSe_xCdS_1−x were formed by vacuum evaporation on glass substrates held at 350 and 470 K. XRD studies showed that all the films were polycrystalline in nature. Films with x0.70 were hexagonal whereas films with x0.80 were cubic in structure. The structural transition was in the range 0.70<x<0.80. The lattice parameter was higher in films formed at a higher temperature. The lattice parameter followed Vegard's law. Grain size increased with substrate temperature. From the optical transmission spectra recorded in the wavelength range 300–2500 nm, the extinction coefficient, refractive index and band gaps were obtained. Band gap values showed a downward bowing with ‘x' with a bowing parameter of 0.40 eV.     

Structural investigation and improvement of photoluminescence properties in Ba(ZrxTi1−x)O3 powders synthesized by the solid state reaction method

In this paper, Ba(Zr_xTi_1−x)O₃ powders with different (x) compositions were synthesized by the solid state reaction method and their structure and improvement of photoluminescence (PL) properties with the Ti substitution by Zr were discussed. The structural investigation of these powders was performed by means of X-ray diffraction (XRD) and Fourier transform Raman (FT-Raman) spectroscopy. Their optical properties were monitored by ultraviolet–visible (UV–vis) absorption spectroscopy and PL measurements. XRD patterns indicated that the powders with x = 0 and 0.1 have a tetragonal structure while compositions with x ≥ 0.2 exhibit cubic structure. FT-Raman spectra revealed that the replacement of Ti by Zr significantly reduced the intensity of the Raman active modes. This behavior is related to the increase of undistorted [ZrO₆] clusters in the global matrix at short range and decrease in local concentration of distorted octahedral [TiO₆] clusters. UV–vis absorption spectra shown the presence of intermediary energy levels between the valence band (VB) and the conduction band (BC). These intermediary electronic levels are mainly related to 2p orbitals of O atoms, 4d orbitals of Zr atoms and 3d orbitals of Ti atoms between the VB and CB. A significant improvement in PL properties of Ba(Zr_xTi_1−x)O₃ powders was observed with an increase of undistorted [ZrO₆] clusters in the lattice. Finally, we propose possible wideband models based on intermediary energy deep and shallow levels to explain the PL behavior at room temperature.     

The structure and optical properties of ZnO films on various LiNbO3 substrates

ZnO films thin films have been deposited on glass and three different LiNbO₃ (LNO) substrates at room temperature using radio frequency magnetron sputtering. The structure and optical properties of the films were investigated by X-ray diffraction (XRD), optical transmission spectroscopy and spectro-photometry. XRD analysis shows that all the films are hexagonal wurtzite structure, and there is compressive strain in the films. Typical optical transmittance values in the order of 80% were obtained for all the films, and the band gaps are in the range of 3.273-3.282 eV. The Photo-Luminescence (PL) spectra results indicate that the type of substrate affects the photoluminecence of ZnO films significantly, and the films on rotated Y-cut 128° LNO substrates have strong UV emission at room temperature.     

Characterization of CuInS2 thin films for solar cells prepared by spray pyrolysis

We have made a study of the chemical composition, the electrical, the optical and the structural properties of polycrystalline CuInS₂ thin films prepared by spray pyrolysis to be used for thin film solar cells. These films were deposited starting from aqueous solutions with different chemical compositions ([Cu]/[In] and [S]/[Cu] ratios) and at different substrate temperatures. In all cases, the material is p-type with grains preferentially oriented in the (112) direction of the sphalerite structure. The electro-optical properties show a very strong dependence on the [Cu]/[In] ratio in the solution. Films with copper excess have smaller resistivity and better crystallinity than those which are stoichiometric or have indium excess. The results obtained in this work show the possibility of having CuInS₂ thin films with a wide range of resistivity, a fact that could be important for making solar cells based on this material.     

Initial study on the structure and optical properties of Zn1−xFexO films

Zn_1−xFe_xO (x = 0, 0.052, 0.103, 0.157 and 0.212) films were prepared by the radio-frequency magnetron sputtering technique on Si (111) substrates and the microstructure of which was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The samples had a preferential c-axis orientation and the position of (002) diffraction peak shifted to the lower degree side with increasing Fe component. In order to investigate the optical transmittance properties of Zn_1−xFe_xO films, we prepared the films on Al₂O₃ (001) substrates simultaneity and the UV-VIS optical transmittance spectra showed that the band gap energy of Zn_1−xFe_xO films decreased with increase of Fe concentration. Photoluminescence spectra of the samples were observed at room temperature.     

Formation of CuIn1 − xAlxSe2 thin films studied by Raman scattering

CuIn_1 − xAl_xSe₂ (CIAS) thin films (x = 0.06, 0.18, 0.39, 0.64, 0.80 and 1) with thicknesses of approximately 1 μm were formed by the selenization of sputtered Cu―In―Al precursors and studied via X-ray diffraction, inductively coupled plasma mass spectrometry and micro-Raman spectroscopy at room temperature. Precursor films selenized at 300, 350, 400, 450, 500 and 550 °C were examined via Raman spectroscopy in the range 50-500 cm^− 1 with resolution of 0.3 cm^− 1. Sequential formation of In_xSe_y, Cu_2 − xSe, CuInSe₂ (CIS) and CIAS phases was observed as the selenization temperature was increased. Conversion of CIS to CIAS was initiated at 500 °C. For all CuIn_1 − xAl_xSe₂ products, the A₁ phonon frequency varied nonlinearly with respect to the aluminum composition parameter x in the range 172 cm^− 1 to 186 cm^− 1.