Hot-wall Vacuum Evaporation Research Articles

Characterization of AgInSe2 Films Deposited by Hot-Wall Vacuum Evaporation Method

Hot-wall technique was used to prepare AgInSe2 films that work close to thermodynamic equilibrium and, therefore, are considered most suitable for growth at low temperatures. The samples were grown on the glass substrate kept at 135°C. The technique can be described as semiclosed growth reactor consisting of a vertically mounted quartz cylinder heated by three separately temperature-controlled ovens and is closed on the top by the substrate. The first oven heats the source material and controls the growth rate, while the second oven heats the wall between source and substrate, and the substrate temperature is controlled by the third one. The structural and optical properties of AgInSe2 films grown by hot-wall technique were studied. X-ray diffraction (XRD) pattern indicates that the prepared films are highly oriented in the (112) direction. The band gap was found to be 1.19 and 2.09 eV, which is due to the fundamental absorption edge and transition originating from crystal field splitting, respectively. The crystallite size of 47 nm and 94% transparency at 890 nm wavelength was observed for the films.

Structural and Optical Studies of Hot Wall Vacuum Evaporated CdTeSn Thin Films

Bulk compounds of CdTe, Cd0.25Sn0.75Te and Cd0.25Te0.75Sn have been prepared by direct reaction of their high purity (99.9999%) elemental constituents employing rotating furnace. The hot wall system is optimized for the deposition of prepared alloys by using molecular flow studies with Monte Carlo simulation technique. Thin films have been deposited on well cleaned glass substrates using the prepared alloys by the optimized hot wall vacuum evaporation system. The compositions of the prepared bulk and thin films have been identified using energy dispersive X-ray analysis. The compositions are found to be same for both the bulk and thin films as the prepared alloys. The structural properties of the deposited films have been studied using X-ray diffraction technique. The results show that all the films are crystalline in nature and the peaks in the XRD graph of CdTe correspond to cubic zinc blende structure and that of Cd0.25Sn0.75Te and Cd0.25Te0.75Sn compounds to rock salt structure. The lattice parameters and grain sizes of all the films have been evaluated. The surface morphology of the thin films is studied using Scanning Electron Microscope (SEM). The SEM analysis shows that surface of the films are smooth and crystalline in nature. The optical transmittance spectra of thin films were recorded using spectrophotometer in the range of wavelength from 190 nm to 2500 nm. All the films exhibit direct optical band gap and their values are 1.45eV (CdTe), 0.9eV (Cd0.25Sn0.75Te) and 1.1eV (Cd0.25Te0.75Sn). Thicknesses of the thin films have been determined by multiple beam interferometric technique.

Structural and optical characterization of CuInSe 2 films deposited by hot wall vacuum evaporation method

Copper indium diselenide (CuInSe 2) compound was prepared by direct reaction of high-purity elemental copper, indium and selenium. CuInSe 2 thin films were deposited onto well-cleaned glass substrates by a hot wall deposition technique using quartz tubes of different lengths (0.05, 0.07, 0.09, 0.11 and 0.13 m). X-ray diffraction studies revealed that all the deposited films are polycrystalline in nature and exhibit chalcopyrite structure. The crystallites were found to have a preferred orientation along the (1 1 2) direction. Micro-structural parameters of the films such as grain size, dislocation density, tetragonal distortion and strain have been determined. The grain sizes in the films were in the range of 65–250 nm. As the tube length increases up to 0.11 m the grain size in the deposited films increases, but the strain decreases. The film deposited using the 0.13 m long tube has smaller grain size and more strain. CuInSe 2 thin films coated using a tube length of 0.11 m were found to be highly crystalline when compared to the films coated using other tube lengths; it has also been found that films possess the same composition (Cu/In=1.015) as that of the bulk. Scanning electron microscope analysis indicates that the films are polycrystalline in nature. Structural parameters of CuInSe 2 thin films deposited under higher substrate temperatures were also studied and the results are discussed. The optical absorption coefficient of CuInSe 2 thin films has been estimated as 10 4 cm −1 (around 1050 nm). The direct band gap of CuInSe 2 thin films was also determined to be between 1.018 and 0.998 eV.

Preparation and characterization of transparent conducting ZnTe:Cu back contact interface layer for CdS∕CdTe solar cell for photoelectrochemical application

FSEC Photovoltaic Materials Laboratory has developed a photoelectrochemical (PEC) cell using multiple band gap tandem of thin-film photovoltaic (PV) cells and a photocatalyst for hydrogen production by water splitting. CdS∕CdTe solar cell, a promising candidate for low-cost, thin-film PV cell, is used as one of the thin-film solar cells in a PEC cell. The back contact has been developed for a CdS∕CdTe solar cell which involves the deposition of a primary p-type ZnTe:Cu back contact interface layer followed by the deposition of transparent and conducting ZnO:Al and Ni–Al outer metallization layer. This article presents preparation and characterization of ZnTe:Cu back contact interface layer deposited by hot wall vacuum evaporation (HWVE) technique. HWVE technique has produced highly stoichiometric ZnTe:Cu thin films with cubic phase having {111} texture orientation and produced better transparency in the near infrared region on a glass substrate.

Structural and optical characterizations of CdTe on CdS grown by hot-wall vacuum evaporation

We report on characterizations of polycrystalline CdTe on CdS grown by hot-wall vacuum evaporation. The CdTe film grown on CdS/SnO 2/glass was compared with the other two CdTe films, which were grown directly on #7059 glass and on SnO 2-coated glass. The grown CdTe/CdS film is composed of a grain size 3–15 μm and have a close-packed structure compared to other CdTe films. A weak excitonic peak at 1.59 eV as well as two kinds of donor–acceptor pair emission bands has been observed in PL spectrum of CdTe/CdS film. On the other hand, the excitonic peak cannot be detected in other CdTe films.

Growth kinetics and structural characterization of polycrystalline CdTe films grown by hot-wall vacuum evaporation

A systematic growth of CdTe films on two kinds of substrates, Corning 7059 glass and SnO 2-coated glass, has been performed by hot-wall vacuum evaporation (HWVE). From the dependence of deposition rate on source temperature, the growth in the HWVE system is limited by the transport of CdTe vapor from source to substrate. The deposition rate of CdTe films grown on either 7059 glass or SnO 2-coated glass gradually decreases with increasing substrate temperature, while the grain sizes increase with substrate temperature. X-ray diffraction showed that all of the CdTe films on either Corning 7059 glass or SnO 2-coated glass deposited at substrate temperature below 275°C have a preferential orientation of the (111) planes parallel to the substrate. However, the degree of the preferential orientation in the films at substrate temperature of 300°C reduced remarkably. High-energy shift of absorption edge in the grown CdTe films were also observed in transmittance spectra.

CdS/CdTe thin film solar cell fabricated by hot wall vacuum evaporation

Abstract CdS/CdTe heterojunctions have been made on glass/ITO or glass/ITO/ZnO substrate by the hot wall vacuum evaporation method, and their photovoltaic properties have been investigated. The cell performances have been improved by using a ZnO layer since the ZnO layer suppressed the interdiffusion of indium and tin from ITO to the CdS layer. In addition, the back side contacts to the CdTe film have been studied by doping acceptor impurities such as Cu, Ag, and P. The best photovoltaic cell shows an efficiency of 10.3% under the condition of AM 1.0 and 100 mW/cm 2 .

Grain boundary effect in polycrystalline ZnTe films

Electrical and optical properties of ZnTe films, deposited at different substrate temperatures (300–630 K) by hot-wall vacuum evaporation, were studied. The experimental data were analysed in the light of the grain boundary trapping model in polycrystalline films. The crystallites were found to be partially depleted of carriers. The barrier height along with the carrier concentration in the films were determined from the optical reflectance measurements. The effect of deposition temperature on the film properties has been critically discussed. The density of trap states in the intercrystalline region of the films was found to decrease with increasing deposition temperature. The built-in electric field in the crystallites was evaluated for different sizes of the crystallites.

The influence of the substrate temperature and the source temperature on the structure of thin Cd 1 − xMn xTe films

An analysis of transmission electron microscopy studies of thin Cd 1 − x Mn x Te films prepared by the hot wall vacuum evaporation method on NaCl substrates is presented. The effect of the substrate temperature (varied from room temperature to 450 °C) and the source temperature (varied from 430 °C to 1000 °C) has been investigated and it was found that the substrate temperature had the most obvious influence on the crystalline structure and on the average grain size. The results are discussed on the basis of known theories of nucleation.

Preparation and characterization of ZnTe/CdSe solar cells

All thin film solar cells were prepared and investigated by using n-CdSe in conjunction with p-ZnTe. Both the layers were deposited by hot wall vacuum evaporation technique. The devices thus produced were characterized by I-V, C-V and spectral response measurements. The open circuit voltage, the short circuit current and the fill factor were obtained as 450–575 mV, 7–11 mA/cm −2 and 0.30 respectively. The width of the depletion layer and the diffusion potential were calculated to be 0.25 μm and 1.4–1.6 V respectively. Surface photovoltage measurements indicated the diffusion length to be ∼0.1 μm. The photovoltaic performances of the cells produced above were observed to depend critically on the evaporation conditions of both the layers.

Optical properties of ZnTe films

Optical properties of p-type ZnTe films, deposited by hot-wall vacuum evaporation, were studied extensively in the range of incident photon energy 0.6–2.6 eV. Variations of refractive index, absorption and extinction coefficients with incident photon energy are reported in this communication.

Relative carrier densities and trap effects on the properties of CdS/CdTe

It is shown that the open-circuit voltage of CdS/CdTe heterojunction solar cells is a maximum when the carrier densities in the CdS and CdTe are approximately equal, because of the effects of a level lying 0.45 eV below the conduction band in CdTe on the junction transport. Cells investigated include those prepared by vacuum evaporation of CdS on thin-film CdTe, deposited by hot-wall vacuum evaporation on graphite substrates and on single-crystal p-CdTe:P substrates, and on single-crystal p-CdTe:P substrates. The existence of a level in the CdTe lying 0.45 eV below the conduction band is identified by measurements of the extrinsic photoresponse of CdS/CdTe junctions, and a model using this level defining junction transport is consistent with the dark and light current versus voltage relationships observed. This model involves two recombination paths: (1) depletion-layer recombination in the CdTe through the 0.45-eV level, which dominates when the electron density in the CdS in much larger than the hole density in the CdTe, and (2) tunneling of electrons from the CdS conduction band to the 0.45-eV level in the CdTe, followed by recombination, which dominates in other cases.

Twinned structures in hot-wall grown Cd 1 − xMn xTe films with x ⩽ 0.02

Transmission electron microscopy and diffraction have been used in a study of semimagnetic-semiconductor Cd 1 − x Mn x thin films with x ⩽ 0.02 prepared by a hot-wall vacuum evaporation technique on rock-salt substrates. The films had a preferred (001) orientation containing many { 111 } microtwins. Rarely they had a (111) orientation twin related to double positioning structure.

Thin film CdTe solar cells

This paper reviews some of the most promising CdTe-based solar cells, with efficiencies in excess of 10%, prepared by various film deposition methods including close-spaced vapor transport, close-spaced sublimation, hot-wall vacuum evaporation, electrodeposition and screen-printing. One of the major problems with CdTe films has always been the difficulty of achieving high p-type doping in order to provide a low resistance frontal layer as well as to solve the ohmic contact problem through tunneling effects in a Schottky-barrier contact. Heavily p-type doping experiments with phosphorus and arsenic impurities are described. The results of the diffusion treatment on homoepitaxial CdTe films, grown by the close-spaced vapor transport method are investigated by measuring the concentration and profile of the impurities, and by analyzing the excitation spectra of luminescence at 1.8 K; it is shown that the doping efficiency is related to the structural location and the electronic activity of the impurity.

Cadmium telluride films and solar cells

CdTe thin films for solar cell applications have been deposited by close-spaced vapor transport and by hot-wall vacuum evaporation. As-deposited films are p-type with hole densities that increase to values of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with increasing substrate temperature. A variety of experimental results can be interpreted either in terms of doping by native defects such as cadmium vacancies or doping by diffusion from the graphite substrate, with evidence for self-compensation. Many CdS/CdTe/graphite solar cells have been prepared by vacuum evaporation of CdS onto thin-film CdTe, which have low values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_{O} \sim 10^{-9}</tex> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and high values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_{SC} \sim 17</tex> mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The open-circuit voltage is low at 0.48 V for CdS deposition at 300° C, but increases with decreasing CdS deposition temperature. The highest efficiency prepared to date is 6.4 percent. Tile efficiency is limited at present by the fill factor, associated with a total series resistivity in the light of the order of 10 Ω-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Supporting research on low-resistance contacts to p-type CdTe, grain boundary properties and passivation in p-type CdTe bicrystals and thin films, and high-resolution transmission electron microscopy of junction interfaces is briefly described.

Grain boundary phenomena in n-type CdTe films grown by hot wall vacuum evaporation

Films of n-type CdTe doped by coevaporation of indium have been deposited by hot wall vacuum evaporation on 7059 Corning glass and BaF2 single crystal substrates. Layers deposited on glass show a dark resistivity of the order of 105 Ω cm and a light resistivity of 500 Ω cm under AM1.5; photoexcitation increases the electron density but does not affect the electron mobility. Layers deposited on BaF2 show a dark resistivity of about 3 Ω cm and a light resistivity of about 2 Ω cm, corresponding to an electron density of 3.9×1016 cm−3 and an electron mobility of 48 cm2/V s; illumination of these layers on BaF2 increases the electron mobility but not the electron density. A quantitative model for grain boundary transport in polycrystalline materials is shown to give good agreement with the experimental data.