xSe Thin Films Research Articles

Crystal structure and spectral photosensitivity of thermally evaporated ZnxCd1−xSe thin films

Single layers of ZnxCd1−xSe with various compositions (x = 0.39, 0.52, 0.59, 0.69 and 0.8) were prepared by thermal vacuum evaporation. Consecutive deposition of films from ZnSe and CdSe with equivalent thickness of 0.12, 0.25 or 0.37 nm was applied and the composition was varied by alloying ZnSe and CdSe films with different equivalent thicknesses. Single layers from ZnSe were prepared for comparison. X-ray diffraction, Raman scattering and spectral photocurrent measurements were carried out to get information on the film crystallinity and spectral photosensitivity. It was observed that both binary and ternary films are of cubic structure and nanocrystalline. No pure CdSe or ZnSe phases were found in the ZnxCd1−xSe films but the Raman data indicated that Cd-enriched nanosized regions can exist. The Raman scattering and spectral photocurrent results showed that the optical band gap of the ternary films gradually increased with increasing x and achieved a value of around 2.4 eV for the films with x = 0.8.

Synthesis of CIGS absorber layers via a paste coating

CuInxGa1−xSe2 (CIGS) thin films were prepared by a paste coating with the aim of developing a simpler and the lower cost method for fabricating the absorber layers of thin film solar cells. In particular, a paste of a Cu, In, Ga, and Se precursor mixture was first prepared, followed by a reaction between them at elevated temperatures after depositing the paste onto a glass substrate. No apparent change in composition was observed during thermal annealing at 450°C in the absence of a gas-phase selenium source. A pre-annealing process at 250°C under ambient conditions performed before annealing (450°C) under reduction conditions reduced the level of carbon deposition in/on the films without perturbing the stoichiometry of the CIGS thin films.

Structural characterization and optical properties of Cd (1− x) Mn xSe thin films

Stoichiometric bulk ingot materials of the ternary mixture Cd (1− x) Mn x Se (0.05 ≤ x ≤ 0.9) were prepared by direct fusion of the constituent elements in vacuum sealed silica tubes. X-ray diffraction studies indicate that the investigated samples exhibited a hexagonal structure. The lattice parameters varied linearly with Mn content, following Vegard's law. Thin films were deposited by thermal evaporation from the pre-synthesized ingot material, onto glass substrates. X-ray and electron diffraction studies on the as-deposited and annealed films revealed an amorphous-to-crystalline phase transition at T a ≈ 423 K. EDAX studies on the prepared films show that the as-deposited films are nearly stoichiometric. The transmittance and reflectance of the deposited Cd (1− x) Mn x Se films were measured at normal light incident in the wavelength spectral range 500–2500 nm. Analysis of the transmittance spectra in the entire wavelength range allowed the determination of the refractive index. The dispersion parameters have been calculated, from which the static refractive index as well as static dielecric constant were calculated. Analysis of the absorption coefficient of the investigated films revealed the existence of both the allowed direct and forbidden direct optical transition mechanisms. The corresponding energies were estimated.

Preparation and magnetoresistance of Ag2+xSe thin films deposited via pulsed laser deposition.

The preparation of Ag2+xSe thin films with thicknesses between 4 nm and 3 μm by pulsed laser deposition on single crystalline NaCl and MgO substrates is reported. The films are perfectly dense and show a good lateral uniformity with a small number of defects. The microstructure of the films corresponds to a nanoparquet, being composed of two different phases of silver selenide. One phase is identified as the Naumannite low temperature phase of silver selenide, the structure of the other phase has not been reported in detail before and probably represents a metastable phase. Silver-rich films contain silver precipitates with typical sizes on the nanoscale. Their presence and their size appears to be responsible for the large and linear magnetoresistance effect of silver-rich silver selenide.

Structural and optical investigation of electrosynthesized ZnxCd1−xSe thin films

Thin films of ZnxCd1−xSe were electrodeposited from an acidic aqueous solution of selenite (SeO2) and Cd-/Zn-sulphates. The film composition was determined by x-ray diffraction (XRD) and energy dispersive x-ray (SEM/EDX) spectroscopy. In addition, the structural and optical film properties were studied by micro-Raman spectroscopy at room temperature. The Raman spectra were fitted with the modified random element isodisplacement model including interactions to first and second neighbour atoms. It is shown that the dependence of the Raman phonon modes on the molar fraction x can be described in terms of an intermediate-mode behaviour. The results of both the XRD and Raman techniques are in good agreement for most of the prepared films. Deviations were observed for small fractions x due to the presence of an excessive CdSe phase. The dependence of the band-gap energy of the ternary system on the molar fraction is discussed in the light of photoelectrochemical measurements as well as considerations within the virtual crystal approximation.

Structural, electrical and optical properties of copper selenide thin films deposited by chemical bath deposition technique

A low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu2−xSe thin films on glass substrates. Structural, electrical and optical properties of these films were investigated. X-ray diffraction (XRD) study of the Cu2−xSe films annealed at 523 K suggests a cubic structure with a lattice constant of 5.697 Å. Chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). It reveals that absorbed oxygen in the film decreases remarkably on annealing above 423 K. The Cu/Se ratio was observed to be the same in as-deposited and annealed films. Both as-deposited and annealed films show very low resistivity in the range of (0.04–0.15) × 10−5 Ω-m. Transmittance and Reflectance were found in the range of 5–50% and 2–20% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of ∼108 m−1. The band gap for direct transition, Eg.dir varies in the range of 2.0–2.3 eV and that for indirect transition Eg.indir is in the range of 1.25–1.5 eV.

Characterization of copper selenide thin films deposited by chemical bath deposition technique

A low-cost chemical bath deposition (CBD) technique has been used for the preparation of Cu2−xSe thin films onto glass substrates and deposited films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and UV–vis spectrophotometry. Good quality thin films of smooth surface of copper selenide thin films were deposited using sodium selenosulfate as a source of selenide ions. The structural and optical behaviour of the films are discussed in the light of the observed data.

Deposition of single-phase Cu(In,Ga)Se2 thin films by a novel two-stage growth technique

Homogeneous single-phase quaternary CuInxGa1−xSe thin films were prepared with a reproducible two-step growth technique. The growth process is based on the controlled selenization of sputtered metallic CuIn0.75Ga0.25 alloys in a H2Se reactor at atmospheric pressure. The reaction velocities of the binary selenide phases were carefully controlled to prevent the formation of stable group I–III–VI ternary alloys during a first selenization step. The composite alloys were subsequently annealed in an inert atmosphere, followed by a second selenization step to promote the homogeneous alloying of gallium with CuInSe2. Glancing incident angle x-ray diffraction (GIXRD) at incident angles between 0.5° and 10° revealed virtually no shift in d-spacing with sample depth, which confirmed the monophasic nature of the quaternary alloys. This observation was confirmed by x-ray fluorescence (XRF) studies, revealing a high degree of in-depth compositional uniformity.

Structural and optical properties of CuInxGa1−xSe2 thin films prepared by flash evaporation

CuInxGa1−xSe2 (CIGSe) thin films with x = 0.5 and 0.75 were deposited by a single source vacuum thermal evaporated method ‘Flash’ on molybdenum coated glass. Some of these films were annealed in selenium atmosphere. X-ray diffraction and energy dispersive x-ray measurements have indicated that these films contain exclusively a phase with a chalcopyrite structure. The morphology, grain distributions and composition of the CIGSe films were studied and compared for x = 0.5 and x = 0.75. The optical studies have shown that the films are highly absorbing (7 × 104 cm−1). The absorption edge is shifted to high photon energies when the gallium concentration is increased. After selenization, the bandgaps increase from 1.00 eV to 1.18 eV for x = 0.75 and from 1.16 eV to 1.28 eV for x = 0.5.

Structural and optical properties of ZnxCd1−xSe thin films

Thin ZnxCd1−xSe films with a thickness ranging from 100 to 300nm are prepared by thermal evaporation under a vacuum of 4.0×10−4Pa. Films deposited at substrate temperatures ranging from 570 to 625K were found to be of polycrystalline nature, with sphalerite structure for x⩾0.6 and wurtzite structure for x<0.6. The optical constants n and k of ZnxCd1−xSe thin films of different composition were determined in the spectral range 400–2000nm. The analysis of the accuracy of the adopted technique gave ±1.0% and ±0.5% for the real part of the refractive index n and the extinction coefficient k, respectively. The band gap showed a non-linear variation with the value of x and obeyed the empirical relationEg(x)=1.66+0.65x+0.25x2.The effective masses of the carriers and the band-gap bowing parameters of ZnxCd1−xSe thin films were estimated.

Optical absorption and photosensitivity of CuInxGa1-x Se2 thin film structures

Thin polycrystalline films of CuInxGa1−xSe2 (0⩽x⩽1) were fabricated by pulsed laser evaporation. Results of measurements of the optical properties, photocurrent polarization indicatrices, and spectral dependence of the photoconversion quantum yield of In-p-CuInxGa1−xSe2 structures are discussed. A window effect in the photosensitivity has been observed, and it is concluded that it is possible to use CuInxGa1−xSe2 thin films as photoconverters of solar radiation.

Structural, electrical and optical properties of Cdx Zn1?xSe thin films

X-ray diffraction and electron diffraction techniques indicate that Cdx Zn1−xSe thin films on glass substrates have a polycrystalline nature, with sphalerite structure for x≤0.5 and wurtzite structure for x≥0.6. The crystalline size in each composition increases with increasing the film thickness. The room temperature dark resistivity π varies from one composition to another showing a transition at x=0.55

Chalcopyrite CuGaxIn1−xSe2 semiconducting thin films produced by radio frequency sputtering

CuGaxIn1−xSe2 thin films have been deposited by rf sputtering from three targets with different (Ga,In) content (x=0.25, x=0.5, and x=0.75). A structural, compositional, optical, and electrical study has been carried out for films grown at substrates temperatures higher than 350 °C. We have successfully obtained chalcopyrite single phase stoichiometric films. Very sharp absorption edges are obtained, with band gaps of 1.12, 1.35, and 1.51 eV for x=0, x=0.5, and x=0.75, respectively.

The properties of chemically deposited Cu 2−xSe thin films

The properties of chemically deposited Cu 2−xSe thin films

Electrical properties of Cu 2− xSe thin films and their application for solar cells

Cu 2− x Se thin films were evaporated and deposited on glass substrates by a vacuum evaporation method. The films showed a low resistivity p-type conduction. Hole mobility measurements performed on films prepared on substrates at 40 and 200°C gave values of 2–5 cm 2V -1s -1 and 8–15 cm 2V -1s -1 respectively. The films prepared at 40°C have a carrier density of 10 22 cm -3 which is one order of magnitude higher than the carrier density of the films prepared at 200°C. In order to employ the Cu 2− x Se films as a window material in solar cells, the film was deposited on n-type silicon. The electrical and photovoltaic properties of the junction were mainly determined by the corresponding properties of the substrate material. A maximum solar conversion efficiency of 8.8% was obtained.