Controlling the phase composition of cadmium sulfide films during pulsed laser deposition

Thin cadmium sulfide films grown by pulsed laser deposition on crystalline and amorphous substrates have been shown to consist of a mixture of a cubic (sphalerite structure) and a hexagonal (wurtzite structure) phase. We have demonstrated the possibility of controlling the percentages of the hexagonal and cubic phases in cadmium sulfide films by varying pulsed laser deposition parameters. Varying the deposition parameters allows one to control the optical and structural parameters and surface morphology of thin cadmium sulfide films.     

Structural and optical properties of vacuum evaporated CdxZn1−xS thin films

II–VI semiconductors are of great importance due to their applications in various electro-optic devices. Sulphides of zinc and cadmium have been utilized effectively in various opto-electronic devices. We have prepared vacuumed CdZnS films by the vacuum evaporation method. Wide band gap binary films have wide application in solar cells. The structural and optical properties of these films have been studied. The band gap of these films is studied by absorption spectra in the wavelength range 400–650 nm. The films have a direct band gap, which varies from 3.50 eV for zinc sulphide to 2.44 eV for cadmium sulphide. The X-ray diffraction pattern of these films for structural analysis is also reported.     

Plasma oxidation of electron beam evaporated cadmium thin films

Thin films of pure cadmium have been deposited using electron beam evaporation technique. Effect of radio frequency (RF) plasma oxidation on structural, optical and electrical properties of cadmium thin films has been investigated. It was found that the RF plasma treatment affects on the physical properties of the oxidized cadmium films. Transmittance values of 87% in the visible region and 90% in the near infrared region have been obtained for cadmium oxide (CdO) film oxidized at a plasma-processing power of 600 W. The optical energy gap, E_g, was found to increase as the RF plasma-processing power increases. The resistivity values of 3 × 10^− 3 and 5 × 10^− 3 (Ω cm) have been obtained for CdO films oxidized at RF plasma-processing powers of 550 and 600 W respectively.     

Effect of air annealing on structural, optical, microscopic, electrical properties of cadmium selenide thin films

Cadmium selenide films have been deposited on glass substrate dip method. The resultant films were annealed upto 473 K temperature. The structural properties of cadmium selenide thin films have been investigated by X-ray diffraction techniques. The X-ray diffraction spectra showed that cadmium selenide thin films are polycrystalline. As deposited sample shows cubic phase whereas sample annealed at 473 K shows hexagonal phase. The optical properties showed direct band gap values were found to be in the region of 1.82–1.55 eV. The electrical studies shows conductivity increases with increase in annealing temperature. The optoelectric and structural data are discussed from the point of applications based on achieving high performance devices.     

A study of size dependent structure, morphology and luminescence behavior of CdS films on Si substrate

Size tunable cadmium sulfide (CdS) films deposited by a dip coating technique on silicon (100) and indium tin oxide/glass substrates have been characterized using X-ray diffraction, X-ray reflectivity, transmission electron microscopy, atomic force microscopy and photoluminescence spectroscopy. The structural characterization indicated growth of an oriented phase of cadmium sulfide. Transmission electron microscopy used to calculate the particle size indicated narrow size dispersion. The tendency of nanocrystalline CdS films to form ordered clusters of CdS quantum dots on silicon (100) substrate has been revealed by morphological studies using atomic force microscopy. The photoluminescence emission spectroscopy of the cadmium sulfide films has also been investigated. It is shown that the nanocrystalline CdS exhibit intense photoluminescence as compared to the large grained polycrystalline CdS films. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the observed photoluminescence behavior of CdS is substantially enhanced when the nanocrystallites are assembled on silicon (100) substrate.     

Optical properties of CdS sintered film

Chemical method has been used to prepare cadmium sulphide by using cadmium, hydrochloric acid and H₂S. The reflection spectra of covered and uncovered sintered films of CdS have been recorded by ‘Hitachi spectrophotometer’ over the wavelength range 300–700 nm. The energy band gaps of these films have been calculated from reflection spectra. It is found that the energy band gap of both films is same as 2.41 eV. It is indicated that energy band gap of these films does not change. This value of band gap is in good agreement with the value reported by other workers. The measurement of photocurrent has also been carried out using Keithley High Resistance meter/ Electrometer. This film shows the high photosensitivity and high photocurrent decay. Thus so obtained films are suitable for fabrication of photo detectors and solar cells.     

Formation and properties of vacuum-evaporated high conductivity n-type CdTe films

High dark conductivity CdTe films have been prepared by co-evaporating CdTe and cadmium. The structural, electrical and optical properties were investigated. The dark conductivity of the film increased monotonically with an increase in the amount of co-evaporated cadmium. The highest dark conductivity of the films obtained in this experiment was 1.4x10^-2Ω^-1 cm^-1. The film structure was of the zinc blende type with a preferential orientation of the (111) planes parallel to the substrate and fibrous. The crystallinity of the films was similar to that of films without cadmium doping. The dark conductivity vs. the reciprocal temperature characteristics showed regular aspects. High dark conductivity films will be useful for CdTe thin film device applications.     

Microstructure of nanocrystalline CdS powders and thin films by Electrostatic Assisted Aerosol Jet Decomposition/Deposition method

Nanocrystalline CdS powders and thin films have been prepared by an Electrostatic Assisted Aerosol Jet Decomposition/Deposition (EAAJD) method from an aqueous solution of cadmium chloride and thiourea. The microstructure of the powders and films was characterised by a combination of transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. The results showed that nanocrystalline powders and films were produced with a crystallite size less than 35 nm. A predominant hexagonal structure with a strong preferred orientation was formed in the films.     

Preparation of CdTe films by electrodeposition

Single-phase CdTe thin films have been prepared by depositing sequentially a layer of tellurium and a layer of cadmium on a molybdenum substrate followed by a short thermal treatment. Deposition of tellurium films was done in an aqueous solution containing TeO₂ at a current density of ≈ 1 mA/cm². An aqueous solution containing cadmium sulfate was used for cadmium deposition with a current density of ≈1 mA/cm². Solution temperature was ≈ 95°C for tellurium film deposition and was 50°C for cadmium deposition. It was found that after a heat treatment at ≈ 370°C for 10 min the deposited Te/Cd layers were converted to CdTe thin films with a cubic structure. Compositional uniformity of the films was also investigated by electron probe microanalysis.     

Thin CdSe films obtained by the method of chemical deposition with selenurea

The conditions for obtaining thin polycrystalline CdSe films from the chemical interaction in water of a cadmium salt and selenurea have been studied. The influences of the concentrations of the reagents, the pH of the reaction mixture and the temperature on the thickness of the deposited films, their adhesion to the substrate, and their composition and properties have been investigated.     

Electrochemical synthesis of thin CdS films

Thin cadmium sulfide (CdS) films have been electrochemically synthesized on metal substrates and the related photosensitive surface barrier structures have been obtained. The proposed method can be used in the technology of thin-film photoelectric converters with large areas.     

Investigation of passivation processes for HgCdTe/CdS structure for infrared application

The effect of chemical and in-situ electrochemical etching on the morphology of the mercury cadmium telluride (HgCdTe) surface, prior to growth of the cadmium sulfide (CdS) films, has been examined. The CdS films have also been characterized using grazing angle X-ray diffraction and optical absorption spectroscopy. Preferential growth of (002) planes of CdS has been observed on the HgCdTe substrate. The effectiveness of the surface treatment and CdS growth methodology for passivation of the HgCdTe surface has been examined using capacitance–voltage studies of Au/HgCdTe/CdS/Au devices at 77 K. A negative fixed charge density  10¹⁵/m² was obtained.     

An electroanalytical study of chemiplated thin films of copper sulphide

Thin films of copper sulphide have been deposited from aqueous solution onto glass, platinum and cadmium sulphide by two new methods: first, chemiplated Cu₂O films have been subjected to anion exchange with S_2- ions and second, copper (I) thiourea complexes have been hydrolysed in sodium tetraborate solution. The copper sulphide films Cu _x S have been analysed electrochemically andx has been found to have values within the range 1.83 to 1.93. The dominance of the digenite phase (Cu_1.8S) in these films has been confirmed by optical measurements.     

Growth of thin CdS films on glass substrates via reaction of thiourea with cadmium acetate in aqueous solution

Thin CdS films have been produced by chemical surface deposition from aqueous solutions of cadmium acetate, Cd(CH₃COO)₂. We have studied the morphology and optical properties of the CdS films and calculated the mechanical stress induced by the difference in linear thermal expansion coefficient between the film and substrate materials.     

An in-situ chemical reaction deposition of nanosized wurtzite CdS thin films

Nanocrystalline CdS thin films were deposited on glass substrates by an ammonia-free in-situ chemical reaction synthesis technique using cadmium cationic precursor solid films as reaction source and sodium sulfide based solutions as anionic reaction medium. Effects of ethanolamine addition to the cadmium cationic precursor solid films, deposition cycle numbers and annealing treatments in Ar atmosphere on structure, morphology, chemical composition and optical properties of the resultant films were investigated by X-ray diffraction, field emission scanning electron microscope, energy dispersive X-ray analysis and UV-Vis spectra measurements. The results show that CdS thin films deposited by the in-situ chemical reaction synthesis have wurtzite structure with (002) plane preferential orientation and crystallite size is in the range of 16 nm-19 nm. The growth of film thickness is almost constant with deposition cycle numbers and about 96 nm per cycle.     

Effects of CdCl2 in CdTe on the properties of sintered CdS/CdTe solar cells

Sintered CdS films on glass substrates with low electrical resistivity and high optical transmittance have been prepared by a coating and sintering method. All-polycrystalline CdS/CdTe solar cells with different microstructures and properties of the CdTe layer were fabricated by coating a number of CdTe slurries, which consisted of cadmium and tellurium powders, an appropriate amount of propylene glycol and various amounts of CdCl₂, on the sintered CdS films and by sintering the glass-CdS-(Cd + Te) composites at various temperatures. The presence of more than 5 wt% of CdCl₂ in the (Cd + Te) layer enhances the sintering of the CdTe film and the junction formation by a liquid-phase sintering mechanism. A low sintering temperature results in poor densification of the CdTe layer and the CdS-CdTe interface, whereas a high sintering temperature results in a deeply buried homojunction. The optimum temperature for the sintering of the CdTe layer and for junction formation decreases with increasing amount of CdCl₂. All-polycrystalline CdS/CdTe solar cells with an efficiency of 10.2% under solar irradiation have been fabricated by a coating and sintering method using cadmium and tellurium powders for the CdTe layer.     

Investigation of tin-based alternatives for cadmium in optoelectronic thin-film materials

Increasing legislation has steadily been introduced throughout the world to restrict the use of heavy metals, particularly cadmium (Cd) and lead (Pb) in high temperature pigments, ceramics, and optoelectronic material applications. Removal of cadmium from thin-film optical and semiconductor device applications has been hampered by the absence of viable alternatives that exhibit similar properties with stability and durability. We describe a range of tin-based compounds that have been deposited and characterized in terms of their optical and mechanical properties and compare them with existing cadmium-based films that currently find widespread use in the optoelectronic and semiconductor industries.     

Optical properties of Langmuir-Blodgett films investigated by a photoacoustic technique

A photoacoustic (PA) technique based on a four-layer prism configuration has been developed for nondestructive determination of the optical properties of Langmuir-Blodgett (LB) films, in which the PA signal amplitudes are measured as functions of the incident angle of light. With this technique the experimental PA angular spectra of manganese stearate and cadmium stearate LB films were obtained. Based on the electromagnetic theory and taking into account the anisotropic properties of LB films, as well as use of the numerical optimization method, we have simultaneously determined the optical constants and thicknesses of films based on experimental PA angular spectra.     

Electric field assisted growth of self-organized CdS films: size dependent structural and optical properties

This paper presents a systematic study of electric field assisted growth of self-organized cadmium sulphide (CdS) quantum dots (Q-CdS). CdS thin films of self-organized quantum dot like structure with different particle size have been successfully deposited simply by varying the concentration of surfactant in the reaction matrix. The model to describe the self-organization is also discussed. The size of CdS nanoparticles can be altered from 68 nm (corresponding to bulk) to 2 nm. The structural, optical, and morphological properties of Q-CdS films have been investigated. A blue shift has been observed in optical absorption and photoluminescence spectra. The strained growth of Q-CdS films has been observed. The microstructural strain calculated from peak broadening reveals an increase in strain with decreasing particle size. This study may provide a convenient method to deposit size selective and organized nanocrystalline semiconductor thin films.     

Processing of CdTe thin films by the stacked elemental layer method: compound formation and physical properties

Cadmium telluride (CdTe) thin films have been deposited using the stacked elemental layer (SEL) technique. This process consists of sequentially depositing tellurium and cadmium layers and then annealing the stacks in order to synthesize the compound. The films were characterized using X-ray diffraction (XRD), optical transmittance and reflectance, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The evolution of the thin film reaction and compound formation were studied using X-ray data. The results show that the growth is diffusion-controlled and the activation energy is (82±2) kJ/mol. The effect of the conventional post-synthesis CdCl₂ heat treatment on the physical properties of the films produced is also reported.