A. Ashour

On the electrical and optical properties of CdS films thermally deposited by a modified source

Abstract CdS films were prepared by a physical vacuum deposition technique on an amorphous (glass) substrate using a modified evaporation source. The effect of the processing conditions (the film thickness, substrate temperature and the deposition rate) were considered in comparison with the normal evaporation source. A van der Pauw four-probe arrangement was used for measuring the electrical resistivity, which varies between 0.1 × 10 4 and 8.18 × 10 4 Ωcm. It decreases with increasing film thickness, and decreasing deposition rate and substrate temperature. This was attributed to the effect of crystallite size, the degree of preferred orientation, internal microstrain and the stoichiometry. From spectrophotometric measuring and using an approximate formula, the optical parameters ( n , k , α) were evaluated. The values of the refractive indices and the energy gaps were evaluated and correlated with the preparation parameters.

Structural study of ZnS thin films prepared by spray pyrolysis

Abstract Thin films of ZnS were prepared by spray pyrolysis. The effect of substrate temperature as well as deposition time and annealing in air and in a nitrogen atmosphere on some structural features was investigated by X-ray diffraction. At a substrate temperature of 300 °C, ZnS appears almost in amorphous form. With rising substrate temperature, the crystallinity was improved. At 550 °C, a well-crystallized cubic phase of ZnS was obtained. The films were preferably oriented with the 〈111〉 direction perpendicular to the surface. Annealing in air created ZnO, no evidence for oxides was found when annealing was carried out in a nitrogen atmosphere. Therefore, using a spray pyrolysis technique with a substrate temperature of 500 °C and annealing in a non-oxidizing atmosphere for about 120 min, one can obtain well-crystallized single-phase cubic ZnS thin films.

On the Van der Pauw method of resistivity measurements

Abstract The Van der Pauw method is one of the most effective and widely used methods of the four-probe mode of determining the resistivity of materials in the form of thin films. The right version of the suggested equations for the correction function that fit the given graph was confirmed by numerical calculations. Then, the equation was experimentally verified by designing samples of different shapes and contact configurations. For easy, fast and accurate estimation of the correction factor, tables of the numerical values are given with steps of 0.1 and 1.0 for the range of R ′/ R ″ ratio from 1 to 200. It is recommended that these tables be used rather than an inaccurate estimation from the graph or recalculation of the factor.

Structural dependence of d.c. electrical properties of physically deposited CdTe thin films

Abstract The structural and electrical properties of physical vapour deposition thin films of cadmium telluride of thickness up to 800 nm were investigated. X-ray diffraction studies showed that the films were polycrystalline with a crystallite size of 82–129 nm and a preferred orientation of the [111] fibre texture. The degree of preferred orientation increased as the film thickness increases. Capacitance measurements indicated that the films have a relative permittivity, ∈ r , of approximately 8.91. The current density-voltage characteristics showed ohmic conducti in the lower voltage range and space-charge-limited conductivity in the higher voltage range. Further evidence for this conduction process was provided by the linear dependence of V t on t 2 and of log J on log t . Analysis of the results yielded an electron concentration n 0 = (1.29–6.28) × 10 10 m −3 , which is correlated with the crystallite size. The ratios of the free charge to the trapped charge, Θ = (1.49–8.59) × 10 −11 , were estimated and the traps concentration, N T , has been correlated with the degree of preferred orientation.

Effect of some spray pyrolysis parameters on electrical and optical properties of ZnS films

Abstract Polycrystalline ZnS films of cubic phase were prepared by spray pyrolysis on amorphous substrate. The films were found to be uniform and to adhere well to the substrate. The effects of deposition time (10–30 min) and substrate temperature (300–500 °C) on their electrical and optical properties were studied. The resistivity was measured at room temperature using the square four-probe method. The dark resistivity was measured as a function of deposition time and substrate temperature. Optical data were recorded in the wavelength range 200–1000 nm. The effect of deposition time (thickness) on resistivity proved the assumption that the resistivity decreases as the film grows owing to the increase in crystallinity and preferred orientation. The dark resistivity increased as the substrate temperature increased. This may be owing to the decrease in film thickness and improvement in stoichiometry or the decrease in electron density and mobility. Increasing thermal mismatch is another important factor. A considerable difference in transmittance was observed for films prepared at different substrate temperatures. The absorption coefficient was measured and then correlated with the photon energy to estimate the energy gap, which was about the value for the cubic phase with a direct transition.

Dependence of resistivity and photoconductivity in evaporated cadmium sulphide thin films on deposition and annealing conditions

The resistivity of evaporated thin films of CdS has been measured as a function of deposition and annealing conditions. Resistivity was found to be thermally activated with a high temperature activation energy of approximately 0-5 eV, which was identified with the height of the inter-crystalline barrier in the Petritz model. Resistivity decreased with increasing film thickness and annealing temperature and increased with increasing deposition rate and substrate temperature during deposition. It is proposed that these variations are related to previously reported changes in the microcrystallite grain size and the degree of preferential orientation in the [002] direction. In addition resistivity is also dependent on compositional changes during the deposition process and mobility variations due to surface scattering. Preliminary results indicate that the films are weakly photoconductive, with lower photosensitivity for the thicker films.

The electrical properties of CdTe films of different preparation conditions in correlation with microstructure changes

Abstract Polycrystalline films of CdTe have been thermally deposited on glass substrates. Using the Van der Pauw technique, the dark resistivities have been measured at different temperature (298–523 K). The resistivity, ϱ, was found to decrease markedly in the range of thickness less than 400 nm and was then subjected to a slight decrease at larger thicknesses. The behaviour was found to fit properly with the Fuchs and Sondheimer relation with parameters ϱ = 4.39 × 10 5 Ω cm and mean free path, l = 734 nm. Raising the substrate temperature the decreasing the deposition rate were found to increase the resistivity monotonically. Annealing the prepared films caused a significant decrease in the resistivity particularly for films of small thicknesses (t

Variation of optical constants of cadmium telluride thin films with deposition conditions

Abstract Polycrystalline CdTe films have been thermally deposited on quartz substrates under different conditions. The reflectance and transmittance have been measured at normal incidence, and the complex refractive index has been determined in the spectral range 0.4–3.0 μm. The accuracy of the adopted technique has been analyzed, and has been found to be ± 1.0% and ± 0.5% for the real refractive index n and extinction coefficient k respectively. The effect of the deposition conditions on the optical parameters has been studied separately. The results showed thickness-dependent and substrate temperature-dependent properties, while it was not possible to find any correlation with the rate of deposition. An interpretation of the results, in correlation with corresponding microstructural parameters, the internal microstrain and crystallite size, is presented.

Thermal effects on structural characterization of evaporated CdTe films during and after deposition

Abstract CdTe films were thermally deposited onto amorphous substrates at different temperatures (25–250°C). Post-annealing under vacuum at 300°C for 2 h has been also carried out. Using X-ray diffraction, the structural characteristics (preferential orientation, stoichiometry, microstructural properties) have been studied. Due to the high degree of preferred orientation, Voigt analysis of single reflection was used to determine the microstructural properties (crystallite size and microstrain). Raising the substrate temperature was observed to lead to a decrease in both integrated intensity and degree of preferred orientation as well as an increase in crystallite size and internal microstrain associated with improving the film stoichiometry. Post-annealing was found to increase the integrated intensity, the crystallite size and the degree of preferred orientation. On the other hand, it resulted in a decrease of FWHM and microstrain. The relative change in such parameters decreases as the film thickness increased with a pronounced change in thinner films.

Effect of modified evaporation source on the structural characteristics of CdS films

CdS films were deposited using a modified evaporation source under different preparative conditions. Pieces of CdS which pile up and adhere to the film were not observed. The structural features and morphology were investigated using both XRD and SEM. The results showed highly oriented films and the crystal perfection was improved at lower deposition rate, at high substrate temperature and for thicker films. The results were compared with those for film prepared with normal deposition source.