Cadmium Telluride Films Research Articles

The effect of current density on the composition, structure, morphology and optical properties of galvanostatically electrodeposited nanostructured cadmium telluride films

Cadmium telluride (CdTe) films were electrodeposited galvanostatically on fluorine doped tin oxide (FTO) substrates at various applied current densities. The electrodeposited films were characterized using X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM) and optical absorption spectroscopy techniques. The results showed that amount of cadmium in the deposited films were enhanced by increasing the applied current density. It was observed that an applied current density of 18mA/cm2 produced Cd:Te films with 1:1 stoichiometry. Moreover, the maximum optical absorption and the closest band gap value to the expected one for CdTe (1.45ev) were observed in the case of stoichiometric films which had less porosity and larger crystallite size among the films that were electrodeposited at applied current densities other than the optimum value. In addition, those films that were heat treated had less porosity in their structure, larger crystallite size and less band gap energy compared to the films without heat treatment.

Characterization of Thick Films of Cadmium Telluride for Direct Conversion X-ray Detectors

Characterization of Thick Films of Cadmium Telluride for Direct Conversion X-ray Detectors

Numerical study of near-, mid-, and long-infrared photon trapping in crystalline and amorphous HgCdTe metamaterials

Recently, mercury cadmium telluride (HgCdTe) films have been extensively studied for the metamaterial applications. Crystalline HgCdTe (c-HgCdTe) films attracted people’s attention for the spectral detection when used as metasurfaces and demonstrated reduced volume size, increased operating temperature, and improved quantum efficiency. Amorphous HgCdTe (a-HgCdTe) films also have been studied due to their interesting properties. Such properties include film deposition on any substrate, direct growth on device, and higher operating temperatures with the low dark current. Hence, in this work, for the first time, we investigate and compare the use of c-HgCdTe and a-HgCdTe for the photon sorting metasurfaces in near-, mid-, and long-IR spectral range in the sensor applications.

Optical absorption enhancement of CdTe nanostructures by low-energy nitrogen ion bombardment

In this paper we present the fabrication of cadmium telluride (CdTe) nanostructures by means of RF magnetron sputtering followed by low-energy ion implantation and post-thermal treatment. We have thoroughly studied the structural, optical, and morphological properties of these nanostructures. The effects of nitrogen ion bombardment on the structural parameters of CdTe nanostructures such as crystal size, microstrain, and dislocation density have been examined. From x-ray diffractometer (XRD) analysis it could be deduced that N+ ion fluence and annealing treatment helps to form (3 0 0) orientation in the crystalline structure of cadmium-telluride films. Fluctuations in optical properties like the optical band gap and absorption coefficient as a function of N+ ion fluences have been observed. The annealing of the sample irradiated by a dose of 1018 ions cm−2 has led to great enhancement in the optical absorption over a wide range of wavelengths with a thickness of 250 nm. The enhanced absorption is significantly higher than the observed value in the original CdTe layer with a thickness of 3 μm. Surface properties such as structure, grain size and roughness are noticeably affected by varying the nitrogen fluences. It is speculated that nitrogen bombardment and post-annealing treatment results in a smaller optical band gap, which in turn leads to higher absorption. Nitrogen bombardment is found to be a promising method to increase efficiency of thin film solar cells.

The effect of substrate rotation rate on physical properties of cadmium telluride films prepared by a glancing angle deposition method

Physical properties of cadmium telluride thin films, deposited on glass substrates by modified glancing angle deposition (GLAD) technique with various substrate rates of rotation, were investigated in this study. In contrast to obliquely columnar thin films fabricated by the conventional GLAD technique, in which higher columnar angle is coupled to higher degree of porosity, this study introduces obliquely deposited thin films which have packed columnar structures despite their highly tilted columns. Structural and optical properties and surface morphology of the CdTe thin films deposited by this technique were studied using X-ray diffraction, UV–visible spectroscopy and field emission scanning electron microscopy.

Residual strain in cadmium telluride/gallium arsenide (001) heterostructures as a function of temperature

Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E0, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion.

Ellipsometric Analysis of Cadmium Telluride Films’ Structure

Ellipsometric analysis of CdTe films grown on Si and CdHgTe substrates at the “hot-wall” epitaxy vacuum setup has been performed. It has been found that ellipsometric data calculation carried out by using a simple one-layer film model leads to radical distortion of optical constants spectra: this fact authenticates the necessity to attract a more complicated model that should include heterogeneity of films. Ellipsometric data calculation within a two-layer film model permitted to conclude that cadmium telluride films have an outer layer that consists of the three-component mixture of CdTe, cavities, and basic matter oxide. Ratio of mixture components depends on the time of deposition, that is, on the film thickness. The inner layer consists of cadmium telluride.

Effect of sodium chloride on the electrochemistry activity of electrodeposited CdTe films with various morphologies

In this paper, a cadmium telluride (CdTe) film is successfully synthesized on Ni foils via a simple electrochemical deposition method.

Physical properties of electron beam evaporated CdTe and CdTe:Cu thin films

In this paper, we report on physical properties of pure and Cu doped cadmium telluride (CdTe) films deposited onto corning 7059 microscopic glass substrates by electron beam evaporation technique. X-ray diffraction study showed that all the deposited films belong to amorphous nature. The average transmittance of the films is varied between 77% and 90%. The optical energy band gap of pure CdTe film is 1.57 eV and it decreased to 1.47 eV upon 4 wt. % of Cu addition, which may be due to the extension of localized states in the band structure. The refractive index of the films was calculated using Swanepoel method. It was observed that the dispersion data obeyed the single oscillator of the Wemple-Didomenico model, from which the dispersion energy (Ed) parameters, dielectric constants, plasma frequency, and oscillator energy (Eo) of CdTe and CdTe:Cu films were calculated and discussed in detail with the light of possible mechanisms underlying the phenomena. The variation in intensity of photoluminescence band edge emission peak observed at 820 nm with Cu dopant is due to the change in surface state density. The observed trigonal lattice of Te peaks in the micro-Raman spectra confirms the p-type conductive nature of films, which was further corroborated by the Hall effect measurement. The lowest resistivity of 6.61 × 104 Ω cm was obtained for the CdTe:Cu (3 wt. %) film.

Photo-responsivity characterizations of CdTe films for direct-conversion X-ray detectors

We have fabricated and investigated thin, polycrystalline, cadmium-telluride (CdTe) films in order to utilize them for optical switching readout layers in direct-conversion X-ray detectors. The polycrystalline CdTe films are fabricated on ITO glasses by using the physical vapor deposition (PVD) method at a slow deposition rate and a pressure of 10−6 torr. CdTe films with thicknesses of 5 and 20 μm are grown. The electrical and the optical characteristics of the CdTe films are investigated by measuring the dark-current and the photo-current as functions of the applied field under different wavelengths of light. Higher photo-currents are generated at the longer wavelengths of light for the same applied voltage. When a higher electrical field is applied to the 20 μm-thick CdTe film, a higher dark-current, a higher photo-current, a larger number of charges, and a higher quantum efficiency are generated.

Nondestructive Monitoring of Defect Evolution in Epitaxial CdTe Thin Layers Grown on Si(111)

Reduction of crystalline defect density and size is a major issue in optimization of thin film for industrial scale applications. It is particularly hard to achieve defect minimization due to the lack of statistically relevant techniques where defect-induced strain information is also available. In this work, we use a combination of high-resolution transmission electron microscopy and synchrotron X-ray diffraction to investigate the evolution of defects during the growth of highly mismatched cadmium telluride (CdTe) films on silicon(111) as a function of growth temperature and layer thickness. High-resolution transmission electron microscopy was employed to show that the grown layers are epitaxial, following the [111] substrate orientation, and to identify the main defect type (double twins). Detailed reciprocal space maps obtained by synchrotron X-ray diffraction were used to identify the presence of additional intensities near the (113) and (002) CdTe bulk reciprocal space positions caused by these defe...

The effects of CTAB concentration on the properties of electrodeposited cadmium telluride films

The effects of cetyltrimethylammonium bromide (CTAB) concentrations on the properties of electrochemically deposited cadmium telluride films have been investigated using SEM, EDX, XRD and photoelectrochemical measurements.

Optical and structural properties of cadmium telluride films grown by glancing angle deposition

Cadmium telluride films were grown by the glancing angle deposition (GLAD) technique. The samples were prepared under different incident deposition flux angles (α = 0°, 20° and 70° measured from the normal to the substrate surface). During deposition, the substrate temperature was maintained at room temperature. The structural study was performed using an x-ray diffraction diffractometer. The samples were found to be poly-crystalline with cubic structure for those deposited at α = 0° and 20° and hexagonal structure for the one deposited at 70°. The images of samples obtained by the field emission scanning electron microscopy technique showed that the GLAD method could produce a columnar layer tilted toward the incident deposition flux. The optical properties study by the UV–Vis spectroscopy technique showed that the use of this growth technique affected the optical properties of the films. A higher absorption coefficient in the visible and near-IR spectral range was observed for the sample deposited at α = 70°. This is an important result from the photovoltaic applications point of view where absorber materials with large absorption coefficients are needed. Also, it seems that the sample with a high incident deposition flux angle has the capability of making an n-CdTe/p-CdTe homo-junction.

Structural and Optical Properties of Thermally Evaporated CdTe Film

Abstract. Cadmium Telluride (CdTe) films were thermally evaporated on to glass substrates at room temperature. By varying the amount of source material, thin films of thickness ranging from 90 nm – 635 nm have been prepared. Film of thickness 200 nm was annealed to 400°C for different durations of time and also subjected to alternate heating - cooling cycle. X-ray diffraction study was carried out to study the effect of film thickness, annealing duration and alternate heating-cooling cycle on the structural properties of the films. The transmittance spectra recorded using a UV-Vis-NIR spectrophotometer was used to study the change in optical properties of the films with respect to film thickness, annealing duration and alternate heating-cooling cycle.

NUMERICAL SIMULATION OF THE DEPOSITION PROCESS AND THE EPITAXIAL GROWTH OF CADMIUM TELLURIDE THIN FILM IN A MOCVD REACTOR

Metalorganic Chemical Vapour Deposition (MOCVD) is an attractive method for depositing thin films of cadm ium telluride (CdTe) and other group II-VI compound materials. It has been known that the growth rate of CdTe thin film is sen sitive to the substrate temperature and the reactant partial pres sures, indicating that the deposition process is kinetical ly controlled and affected by many conditions. In the deposition process, heterogeneous reactions play an important role in f ilm formation and the process is further complicated by the coupling of gas and surface reactions via desorptio n of the reactive intermediates. A detailed understanding of the deposition mechanism and kinetics will be crucial f or the design, optimization and scale-up of II-VI MOCVD reactors. This paper presents the results of CFD modelling of the deposition process in an inline MOCVD reactor, taki ng into account the heat transfer and mass transport of the chemical species. The numerical simulations have been conducted using the CFD code, ANSYS FLUENT. The influence of the process controlling parameters such as total flow rate, rea ctor pressure and substrate temperature on the deposition behavio ur has been assessed. In the present study, dimethylcadmium (DMCd) and diisopropyltelluride (DiPTe) have been used as prec ursors while H 2 is acting as the carrier gas and N 2 as the flushing gas. The capabilities of using the developed CFD models for revealing the deposition mechanisms in MOCVD have been demonstrated. The simulations have been conducted in both mass transport and kinetics regimes at the temperat ure range of 355-455 ° to match the experimental conditions.

Characterization of Nanoscale Chips and a Novel Model for Face Nanogrinding on Soft-Brittle HgCdTe Films

A novel ceramic bond ultrafine diamond wheel was developed to acquire nanoscale chips for face nanogrinding. The ceramic bond consists of silicon carbide, alumina, silica, sodium chloride, and sodium bicarbonate. Damage-free subsurfaces of soft-brittle mercury cadmium telluride films were obtained directly by face nanogrinding using the developed ultrafine diamond wheel. Twins and nanocrystals with random orientations were observed. The thickness of the nanoscale chips varied from 21 to 30.2 nm, and their cross-section was triangular. A novel model for maximum undeformed chip thickness for face nanogrinding was constructed. The calculated results obtained from the model are in good agreement with those found experimentally. The surface roughness and peak-to-valley ratio induced by face nanogrinding were approximately stable at around 1.9 and 16 nm, respectively.

Studies on CdTe films deposited by pulsed laser deposition technique

Abstract Polycrystalline cadmium telluride films were successfully deposited on glass substrates by ablating a CdTe target by pulsed Nd–YAG laser. Microstructural studies indicated an increase in the average crystallite size from 15 nm to ∼50 nm with the increase in substrate temperature during deposition. The films deposited here were slightly tellurium rich. X-ray diffraction pattern indicated that the films deposited at 300 K had wurtzite structure while those deposited above 573 K were predominantly of zinc blende structure. Residual strain in the films deposited at 300 K was quite low as compared to those deposited at higher temperatures. PL spectra of all the CdTe films were dominated by a strong peak at ∼921 nm (∼1.347 eV) followed by a low intensity peak at ∼863 nm (∼1.438 eV). Characteristics Raman peaks for CdTe indicated a peak at ∼120 cm −1 followed by peaks located at ∼140 cm −1 and 160 cm −1 .

Injection photodiode based on p-CdTe film

Photosensitive large-block p-type cadmium telluride (p-CdTe) film with a resistivity of ρ ≈ 106−107 Ω cm has been used to create an Al-p-CdTe-Mo structure with a Schottky barrier, which exhibits the properties of an injection photodiode. Being switched in the forward current passage direction (with “+” on the Mo contact) at high illumination levels, the proposed structure exhibits a responsivity of Sλ ≈ 2.6 A/W at a wavelength of λ = 0.625 μm, which is about five times the spectral response of the ideal photodetector for this wavelength range.

Impact of CdTe surface composition on doping and device performance in close Space sublimation deposited CdTe solar cells

ABSTRACTCool‐down after close‐space sublimation growth of cadmium telluride films has been shown to lead to the formation of a Cd‐rich surface that can block the subsequent in‐diffusion of cadmium chloride that is used in post‐growth doping of solar cell devices. Pre‐etching of the as‐grown surfaces using a nitric–phosphoric acid etch prior to doping leads to increase in chlorine and oxygen in‐diffusion, and an associated improvement in device efficiency from <3% to 12% in controlled trials. Capacitance‐voltage analysis of such devices revealed an increase in the acceptor concentration from 3.76 × 1013 cm−3 to 5.6 × 1014 cm−3, whereas thermal admittance spectroscopy revealed fewer deep states at energies >0.4 eV for etched devices. Impedance analysis showed the improved cadmium chloride treatment—resulting from the additional etching step—removed the grain boundary component from the equivalent circuit, indicating a grain boundary passivation effect. Copyright © 2011 John Wiley & Sons, Ltd.

Study of molecular beam epitaxy of cadmium telluride on sapphire

The effect of thermal treatment of (0001) sapphire substrates on the structure of cadmium telluride films grown by molecular beam epitaxy was investigated. The growth process was carried out on a laboratory facility equipped with a mass spectrometer and electron diffractometer. It was established that no annealing or annealing in vacuum (P 1000°C. Electron diffraction patterns showed that they have a mosaic structure and contain twins with the 〈111〉 twinning axis. Atomic-force microscopic images revealed CdTe crystallites with lateral sizes of ∼50 nm arranged along the steps on the sapphire substrate surface. These results allowed the conclusion to be drawn that the growth of CdTe on sapphire substrates can occur by the formation of the three-dimensional nucleation centers according to the Volmer-Weber mechanism.