CdZnTe Thin Films Research Articles

CdZnTe thin films as proficient absorber layer candidates in solar cell devices: a review

The CdZnTe is an efficient absorber to both single junction and tandem devices. This review presents an overview on associated development routes, characterization tools, developed devices and further recommendations for improving the performance.

Effect of different annealing conditions on CdZnTe thin films for absorber layer applications

The Silicon based single junction and perovskite solar cells have demonstrated high efficiency but Silicon based cells suffer from the drawback of higher cost and perovskite solar cells are not stable and degradable in environment. The thin film based CIGS and CdTe cells have achieved efficiency of more than 22% but, particularly, the CdTe thin film solar cells require a suitable back contact which can be obtained by doping of Zn element in CdTe. The grain growth in CdTe-based films is attained by chloride treatment and therefore, in order to seek applicability of chloride treatment on CdZnTe (CZT) thin films, the present communication reports an evolution of different annealing conditions (in air, CdCl2 and MgCl2 environments) on physical properties to electron beam evaporated CdZnTe thin films. The XRD diffractograms revealed to the dominance of cubic phased peak corresponding to (111) plane and augmentation in grain size with treatment temperature. The optical spectra indicated variation in absorbance and transmittance with temperature of treatments and Tauc's plots demonstrated band gap of all the films in 1.41–1.55 eV range. Current-voltage characteristics divulged Ohmic character of films where conductivity and resistivity are eventually modified with annealing conditions. The FESEM micrographs of pristine films represented bouncy grains with some vacant spaces whereas chloride treated films showed larger grains. The EDS patterns confirmed the deposition of CdZnTe thin films and treatments concerned. The AFM maps demonstrated deep valley-like and hill-like configurations of processed films. The obtained results signify that the films annealed at 300 °C aver their suitability for absorber layer roles in development of efficient solar cells with single junction and tandem architectures.

Forming of CdZnTe thin films grown by hot wall epitaxy and their properties

In this work morphological, X-ray structural and optical studies of CdZnTe films grown by hot wall epitaxy method at relatively low substrate temperatures were performed. Possible mechanisms and processes of self-organization that occur during the growth of such structures are considered. It is shown that at thickness of film more than 130 nm on the surface, large (lateral size 150 - 200 nm, height - up to 10 nm) and small crystals are observed. The thicknesses and energy of the band gap width of the CdZnTe films grown at different growth times were determined. It is shown that the film absorption edge in the transmission spectra depends on the film thickness and the reasons for the shift of the film absorption edge are discussed.

Photoinduced electrical bistability of sputter deposited CdZnTe thin films

CdZnTe thin films were fabricated on glass substrates by sputtering and the potentiality of photoinduced electrical data storage was investigated. As deposited film was annealed to enhance the crystallization and both the as deposited and annealed films were characterized by XRD, RAMAN, UV–vis-NIR, PL, FESEM and EDAX analysis. Optical and electrical properties of the films were investigated in detail. The thin films show interesting photo induced electrical bistability. The electrical bistability was observed with symmetric lateral electrodes under illuminated condition only. In the dark, forward and reverse bias curves traced the same path but under light illumination condition there is a substantial difference in the I–V traces of the forward and reversed biased conditions. After annealing the crystallinity of the as deposited film improved and the ON-OFF ratio of electrical bistability increased considerably. This phenomenom could be useful for light sensitive RAM type devices.

Effect of substrates on structural, optical, electrical and morphological properties of evaporated polycrystalline CdZnTe thin films

It is well known fact that the physical properties of a thin film could be tuned by substrate during deposition process. Therefore, a study on the effect of substrates on structural and opto-electrical properties and surface morphology of CdZnTe thin films (400nm) deposited by electron beam evaporation onto commercial glass, indium tin oxide (ITO) and silicon wafer, has been undertaken. The films exhibited zinc-blende structure and grain size as well as other structural parameters (i.e. internal strain, dislocation density, lattice constant) were found to be affected by the nature of substrates. The optical band gap was found in the range 2.06–2.33eV and depended on the substrates while the electrical conductivity was observed maximum for films on ITO substrate. The surface morphology of films was also found to be uniform and homogeneous.

Thermal annealing induced physical properties of electron beam vacuum evaporated CdZnTe thin films

The thermal annealing induced physical properties of electron beam vacuum evaporated CdZnTe thin films have been investigated in the present work. The films of thickness 300nm were deposited on indium tin oxide coated glass substrates followed by thermal annealing at 100°C, 200°C and 300°C. The pristine and annealed films were subjected to the X-ray diffraction, UV–Vis spectrophotometer, Fourier transform infrared (FTIR) spectroscopy, source meter and Energy-dispersive spectroscopy for structural, optical, electrical and elemental composition analysis respectively. The structural analysis revealed that the films have cubic oriented polycrystalline structure with preferred orientation (111). The optical absorbance is found to increase with annealing while optical energy band gap is observed to decrease. Different optical constants like absorption coefficient, refractive index, dielectric constants and energy loss functions have been calculated using Herve-Vandamme model, dielectric theory, Swanepoel model and discussed. The FTIR study indicates the absorption intensity is increased with annealing temperature. The elemental analysis revealed to the presence of Cd, Zn and Te elements in the deposited films. The current-voltage tests show that the electrical conductivity is increased with thermal annealing. The results revealed to a demanding behavior of annealed films for the solar cell applications.

Enhanced structural, electrical and optical properties of evaporated CdZnTe thin films deposited on different substrates

CdZnTe thin films of thickness 300nm were deposited on glass, ITO, FTO and silicon wafer at room temperature employing electron beam vacuum evaporation. These deposited films were analyzed using XRD, source meter and UV–Vis spectrophotometer to investigate structural, electrical and optical properties, respectively. The films are found to be polycrystalline in nature with zinc-blende cubic structure and evaluated structural parameters show significant effect of used substrates. The electrical study shows that the conductivity is maximum for films deposited on FTO substrate. The optical analysis shows wide transmittance with the energy band gap in the range 1.71–2.28eV.

Optimization of structural, optical and electrical properties of CdZnTe thin films with the application of thermal treatment

The structural, optical and electrical properties of electron beam vacuum evaporated CdZnTe thin films have been optimized with the application of thermal treatment. The films of thickness 400nm were deposited on glass and ITO substrates followed by thermal treatment at 150°C, 300°C and 450°C. The XRD study reveals that the films have cubic structure and polycrystalline nature along with predominant reflection (111). The optical transmittance is found to increase with annealing treatment while optical energy band gap is observed to decrease. The current-voltage measurements show ohmic behavior and electrical conductivity is found to increase with thermal treatment. The EDAX patterns exhibit the presence of Cd, Zn and Te elements in the deposited film. The experimental results confirm that the annealed films display a demanding behavior for tandem solar devices.

Effect of thickness on physical properties of electron beam vacuum evaporated CdZnTe thin films for tandem solar cells

The thickness and physical properties of electron beam vacuum evaporated CdZnTe thin films have been optimized in the present work. The films of thickness 300nm and 400nm were deposited on ITO coated glass substrates and subjected to different characterization tools like X-ray diffraction (XRD), UV‐Vis spectrophotometer, source meter and scanning electron microscopy (SEM) to investigate the structural, optical, electrical and surface morphological properties respectively. The XRD results show that the as-deposited CdZnTe thin films have zinc blende cubic structure and polycrystalline in nature with preferred orientation (111). Different structural parameters are also evaluated and discussed. The optical study reveals that the optical transition is found to be direct and energy band gap is decreased for higher thickness. The transmittance is found to increase with thickness and red shift observed which is suitable for CdZnTe films as an absorber layer in tandem solar cells. The current-voltage characteristics of deposited films show linear behavior in both forward and reverse directions as well as the conductivity is increased for higher film thickness. The SEM studies show that the as-deposited CdZnTe thin films are found to be homogeneous, uniform, small circle-shaped grains and free from crystal defects. The experimental results confirm that the film thickness plays an important role to optimize the physical properties of CdZnTe thin films for tandem solar cell applications as an absorber layer.

X-ray DC response of a simple photoconductive detector based on CdZnTe film

CdZnTe thin films were deposited on glass substrates by close-spaced sublimation method and then annealed in vacuum. The structure, morphology and composition are investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, respectively. The CdZnTe film detector shows a linear X-ray response, which is about one order of magnitude lower than that of the single crystalline CdZnTe sample. The mobility and lifetime product of the CdZnTe film is estimated to be around 6.5 × 10−6cm2/V. Time dependent X-ray response shows the increase of photocurrent with time at the first 20 min for the film detector, indicating a filling of traps in the forbidden gap.

The effect of annealing temperature on structural, morphological and optical properties of CdZnTe thin films

Cadmium zinc telluride (CdZnTe) thin films were grown on glass substrates by the chemical bath deposition. The films were annealed in air for 1h at various temperatures (300°C, 400°C and 500°C). The grown films were analyzed by X-ray diffraction, scanning electron microscopy and UV spectra. X-ray diffraction studies reveal that films are polycrystalline in nature with rhombohedral structure. The structural parameters such as grain size, dislocation density and micro strain were evaluated. SEM analysis at high temperature indicates the hexagonal face like rod structure. Optical transmittance study shows the wide transmittance with the band gap energy decreasing, from 2 to 1.75eV.

Effects of Substrate Temperature on the Microstructure and Morphology of CdZnTe Thin Films

The effects of substrate temperature on the microstructure and morphology of CdZnTe thin films were investigated in detail. The CdZnTe thin films were deposited on glass substrates at 200°C, 300°C and 400°C by radio frequency magnetron sputtering and annealed at 450°C for an hour under N2 ambient at atmospheric pressure. The microstructure and morphology of the CdZnTe films were analyzed by using x-ray diffractometry, scanning electron microscopy, energy dispersive x-ray spectroscopy and atomic force microscopy. The effects of substrate temperature on transmission spectra of the CdZnTe films were also investigated. The experimental results show that the optimum morphology and crystalline thin film structures were achieved at 400°C growth temperature. At higher substrate temperatures, atomic mobility and diffusion are promoted, which can stabilize the uniformity and crystallite size. The crystalline grains enlarge and the surface morphology becomes smoother due to growth of grains in the CdZnTe thin films. In addition, the transmission spectra of the films are consistent with the morphological changes. It may be concluded that substrate temperature in growing CdZnTe films has a substantial effect on morphological characteristics, and desired quality of the CdZnTe thin films may be fabricated at higher substrate temperatures.

Effect of Al-Induced Crystallization on Structure of CdZnTe Thin Films Deposited by Radio Frequency Magnetron Sputtering

Different substrate temperature of CdZnTe films and one with Al-induced crystallization have been investigated by XRD, AFM and UV-spectrophotometry. It was shown that, as the substrate temperature varied from room temperature to 400 °C, improving the substrate temperature helps to enhance the grain size of CdZnTe film, and favours the preferential orientation (111). Moreover, Al-induced crystallization method can effectively reduce the crystallization temperature of the CdZnTe film so that the film can be for high-quality oriented crystallization in a lower temperature. In addition, Al-induced crystallization can make the surface roughness of the film at a small value that facilitates the later application of the film.

The dependence of Zn content on thermal treatments for Cd1−xZnxTe thin films deposited by close-spaced sublimation

Abstract CdZnTe thin films were deposited on FTO (SnO2: F)-coated glass substrates by close-spaced sublimation method and then annealed under different conditions. The influences of the three thermal treatments on the structure, morphology, composition and optical properties of the CdZnTe thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), photoluminescence (PL) and Raman spectroscopy. The better crystalline quality can be obtained after any of three thermal treatments as evidenced by XRD, SEM, PL and Raman spectroscopy. The composition is homogeneous in depth for the as-deposited films and those upon annealing after CdCl2, while for films upon annealing both directly in vacuum and with ZnCl2 treatment, the inhomogeneity of Zn composition in depth, due to the loss of Cd atom at surface, can be evidenced by XRD, EDS and PL results. After annealing with ZnCl2 treatment, the Zn composition can be increased most notably by 1.5–2 times to around 7–10%.

Effect of Al-induced crystallization on CdZnTe thin films deposited by radio frequency magnetron sputtering

High quality polycrystalline CdZnTe films were prepared by aluminum induced crystallization (AIC) and radio frequency (r.f.) magnetron sputtering. The crystallinity, morphology and optical property of both as-deposited and AIC samples were investigated by X-ray diffraction (XRD), and atomic force microscopy (AFM), as well as Raman and ultraviolet–visible spectrometry. The results of XRD showed that AIC favours the preferential orientation [111], and promotes the crystallinity of the CdZnTe films. AFM micrographs show that the grain size was increased from 50nm to 300nm after AIC. In the Raman spectrum of CdZnTe films, the intensity of CdTe-like TO mode is enhanced after AIC. From the optical transmittance and absorption coefficient, the value of the band gap varied from 1.53eV to 1.65eV.

An Investigation of CdZnTe Thin Films for Photovoltaics

Thin film solar cells are widely investigated for terrestrial and space photovoltaic and offer a variety of choices in terms of the device design and fabrication. A variety of substrates, such as flexible or rigid, metal or insulator, can be used for deposition of different layers using different techniques, including physical vapor deposition (PVD), chemical vapor deposition, electrochemical deposition, plasma-based, hybrid, etc. For large-area devices required for realistic applications, thin-film device fabrication becomes complex and requires proper control over the entire process sequence. Detailed investigation of thin-film deposition processes can help in achieving high-efficiency devices over large areas for different solar cells. PVD techniques were extensively used to deposit thin film by using a few targets simultaneously. In this study, single target CdZnTe is used for deposition on glass. Two hundred nm thin films were fabricated. Structural and optical properties of CdZnTe thin films are investigated.

Analytical model for the photocurrent of solar cells based on graded band-gap CdZnTe thin films

Solar cells based on CdTe have reached a maximum efficiency of 16.5%, which has remained almost unchanged for a long time. Physical reasons for this fact have been given by different authors, but not much advance has been achieved in this regard for about 15 years. Therefore, new concepts have to be developed for a further increase of the efficiency of this kind of solar cells. One of the possibilities is to use a graded band-gap absorbing semiconductor causing an associated quasi-electric field. The effects of this quasi-electric field upon the minority carrier drift-diffusion length and the back surface recombination velocity may induce a larger carrier collection at the junction with the corresponding increase of the illumination current density. Therefore, based on these concepts, the author has recently proposed structures based on CdZnTe with a variable Zn content profile. In this work, an analytical model is developed for taking into account the quasi-electric field, the carrier back surface recombination velocity and the “graded” absorption in a solar cell using a variable band-gap semiconductor in the base. The conditions to be satisfied in order to have a real improvement of the illumination current density are discussed, in particular for a graded CdZnTe solar cell.

Effect of Film Thickness on the Structural and Physical Properties of CdZnTe Thin Films

Different thickness of CdZnTe films were deposited onto glass substrates by RF magnetron sputtering from Cd0.9Zn0.1Te crystals target. Their structural characteristics were studied by X-ray diffraction (XRD). The XRD experiments showed that the films are polycrystalline and have a zinc-blende (cubic) structure. The crystallite size and micro-strain were calculated. It is observed that the crystallite size increases and micro-strain decreases with the film thickness. The optical measurements showed that the average transmittance of all the samples have is less than 50% in the visible range. The possible optical transition in these films is found to be allowed direct transition with energy gap increase from 1.53 to 1.75 eV. For the electrical properties, the sheet resistivity decreased from 2.582×108 to3.069×107 Ohm/sq when the thickness increased from 307 to 823 nm; while the carrier concentration seems to be less affected by the film thickness. This behaviour in electrical properties was explained by the crystallinity and the grain size evolution.

Optical characterization of vacuum evaporated CdZnTe thin films deposited by a multilayer method

CdZnTe thin films of thickness 450–1400 nm have been evaporated under vacuum onto unheated glass substrates, using a multilayer method. During film deposition, the two evaporation sources, separated by two glass cylinders, were maintained at temperatures of 720 K for Zn and at 925–1200 K for CdTe, respectively. After deposition, the samples were annealed in air up to 775 K. The structural and optical properties of both as-deposited and heat-treated samples were investigated. Depending on the preparation conditions and the annealing temperature, the value of the optical band gap, E g, of respective films varied between 1.16 and 1.63 eV. The obtained results are discussed in correlation with the structure of the films and the role of Zn atoms in CdTe films.

Deposition and characterization of CdZnTe thin films for gas microstrip detectors

Deposition, characterization, fabrication and performance of gas microstrip detectors (GMDs) using CdZnTe thin films to control surface resistivity are reported. CdZnTe thin films deposited on glass or on flexible plastic substrates are attractive for GMDs since CdZnTe bulk resistivity varies in the range 10/sup 9/ to 10/sup 11/ ohm-cm and CdZnTe films with good uniformity can be deposited over large areas using inexpensive deposition techniques. Metalorganic chemical vapor deposition was used to form CdZnTe thin films on glass and flexible plastic substrates. The sheet resistance for 1 /spl mu/m thick CdZnTe film on glass varied in the range 10/sup 11/ to 10/sup 13/ ohm/square depending on the growth conditions. GMDs were fabricated using CdZnTe films deposited on glass substrates and tested for energy response to /sup 55/Fe X-rays. The well-resolved peaks (5.9 keV X-ray and 2.9 keV escape peak) clearly show the ability to successfully fabricate CdZnTe thin films on GMDs.