Effect of tellurium doping on the structural, optical, and electrical properties of CdO

Te-doped CdO thin-films (1%, 3%, and 5%) have been prepared by a vacuum evaporation method on glass and silicon-wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV-VIS-NIR absorption spectroscopy, and dc-electrical measurements. Experimental data indicate that Te ions doping slightly stresses the host CdO crystalline structure and changes the optical and electrical properties. The bandgap of the host CdO was suddenly narrowed by about 23% due to a little (1%) doping with Te ions. This bandgap shrinkage was explained by effects of trap levels overlapping with conduction band. The electrical behaviours of the Te-doped CdO films show that they are degenerate semiconductors with a bandgap of 1.7-2.2 eV. The 1% Te-doped CdO film shows increase its mobility by about 5 times, conductivity by ∼140 times, and carrier concentration by ∼27 times, relative to undoped CdO film. From transparent-conducting-oxide point of view, Te is sufficiently effective for CdO doping. Finally, the absorption in the NIR spectral region was studied in the framework of the classical Drude theory.     

Effect of heat treatment on physical properties of CdO films deposited by sol–gel method

CdO film has been deposited by sol–gel spin coating method on the glass substrate and then the film has been annealed at 400, 500, 600 °C for 1 h. Effect of annealing temperature on the structural and optical properties of the film has been investigated. The crystal structure and orientation of the as-grown and annealed CdO films have been investigated by X-ray diffraction method. Annealed CdO films are polycrystalline with (111) preferential orientation. The information on strain and grain size is obtained from the full width-at-half-maximum (FWHM) of the diffraction peaks. Texture coefficient and lattice constant have been calculated. The surface morphology of the films has been analyzed. The optical band gap value decreased with increasing the annealing temperatures.     

Correlated transport and optical phenomena in Ga-doped CdO films

Series of samples of lightly Ga-doped CdO thin films (3%, 6%, and 9%) have been prepared by evaporation method on glass substrate. The prepared films were characterised by X-ray diffraction (XRD), UV–VIS–NIR absorption spectroscopy, and dc-electrical measurements. The investigation shows that Ga doping widens the energygap of CdO. The optical properties were easily explained by using Tauc et al. band-to-band transitions and classical Drude theory. The electrical behaviour of the samples shows that they are degenerate semiconductors. The 6% Ga-doped CdO sample shows increase its mobility by 3.2 times, increase its conductivity by 1.5 times, increase its intrinsic bandgap, and a slight increase its transmittance relative to undoped CdO film. Explanation was given concerning these variations. From transparent conducting oxide (TCO) point of view, Ga is not sufficiently effective for CdO doping comparing to other dopants like In, Sn, Sc, and Y.     

Structural, electrical and optical properties of annealed Al/Sb multiayer films

The multilayer Al/Sb thin films were deposited on quartz glass substrates using magnetron sputtering method and annealed at high temperature to obtain AlSb films. The experimental conditions were optimized to obtain the AlSb single phase. XRD measurements indicate that high-temperature annealing is necessary to form AlSb and is helpful to the grain growth of AlSb polycrystalline. The average grain size of AlSb polycrystalline increases obviously with the increase in annealing temperature when higher than 500 °C. The electrical measurements show that the prepared AlSb films are p-type semiconductors with the conductivity activation energy of 0.21 and 0.01 eV. The optical band gap for a typical AlSb film is 1.76 eV. The obvious photovoltaic effect has been observed in TCO/CdS/AlSb/ZnTe:Cu/Au devices, which demonstrated the potential of AlSb film as the absorber layer in thin film solar cells.     

Influence of dysprosium doping on the electrical and optical properties of CdO thin films

Lightly Dy-doped CdO thin films (molar 0.5%, 1%, 2%, and 2.5%) have been prepared by a vacuum evaporation method on glass and Si wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV-vis-NIR absorption spectroscopy, and dc-electrical measurements. Experimental data indicate that Dy³⁺ doping slightly stretchy-stresses the CdO crystalline structure and changes the optical and electrical properties. The bandgap of CdO was suddenly narrowed by about 20% due to a little doping with Dy³⁺ ions. Then, as the Dy doping level was increased, the energygap was also increased. This variation was explained by the effect of Burstein-Moss energy shift (or bandgap widening effect) together with a bandgap shrinkage effect. The electrical behaviour of the samples shows that they are degenerate semiconductors. However, the 2% Dy-doped CdO sample shows an increase in its mobility by about 3.5 times, conductivity by 35 times, and carrier concentration by 10 times relative to undoped CdO film. From transparent conducting oxide point of view, Dy is sufficiently effective for CdO doping.     

Characterization of ZnO films obtained by ultrasonic spray pyrolysis technique

In this work, ZnO film was deposited onto microscope glass substrates at 300 ± 5 °C by ultrasonic spray pyrolysis technique to investigate its application potential in photovoltaic solar cells. Optical, surface, structural and electrical properties of the film were investigated. Transmittance and absorbance spectra were taken to examine the optical properties, and band gap was calculated by optical method. Scanning electron microscopy micrographs and X-ray diffraction pattern were used to investigate the surface and structural properties of the film, respectively. Temperature dependent current measurements were performed by two-probe method to analyze electrical properties, and electrical conductivity at room temperature and activation energy values were calculated. After all investigations, application potential of ZnO film in photovoltaic cells as transparent conducting oxide contact was searched.     

TiO2 thin films as protective material for transparent-conducting oxides used in Si thin film solar cells

Nb-doped TiO₂ films have been fabricated by RF magnetron sputtering as protective material for transparent-conducting oxide (TCO) films used in Si thin film solar cells. It is found that TiO₂ has higher resistance against hydrogen radical exposure, utilizing the hot-wire CVD (catalytic CVD) apparatus, compared with SnO₂ and ZnO. Further, the minimum thickness of TiO₂ film as protective material for TCO was experimentally investigated. Electrical conductivity of TiO₂ in the as-deposited film is found to be 10⁻⁶ S/cm due to the Nb doping. Higher conductivity of 10⁻² S/cm is achieved in thermally annealed films. Nitrogen treatments of Nb-doped TiO₂ film have been also performed for improvements of optical and electric properties of the film. The electrical conductivity becomes 4.5×10⁻² S/cm by N₂ annealing of TiO₂ films at 500 °C for 30 min. It is found that the refractive index n of Nb-doped TiO₂ films can be controlled by nitrogen doping (from n=2.2 to 2.5 at λ = 550 nm) using N₂ as a reactive gas. The controllability of n implies a better optical matching at the TCO/p-layer interface in Si thin film solar cells.     

High quality ZnO and Ga:ZnO thin films grown onto crystalline Si (1 0 0) by RF magnetron sputtering

Undoped and 2% Ga-doped ZnO films have been deposited by RF magnetron sputtering onto single crystal Si (1 0 0) substrates equivalent to the commercial Si solar cells. The same films were also grown on amorphous silica substrates to complete their characterization. The films have been characterized by X-ray diffraction, electrical and optical measurements, X-ray photoelectron spectroscopy, Raman microspectroscopy and scanning and high-resolution transmission electron microscopy. Films present a very good quality crystalline wurtzite structure with the c-axis perpendicular to the substrate, with continuity of the (0 0 0 2) planes along the whole film, as shown by transmission electron microscopy. The doped sample shows an increase of two orders of magnitude of the electrical conductivity, an optical transmittance bigger than 85% along the visible spectrum, a diminution of the grain size in the direction parallel to the substrate and a lower surface roughness. The Ga-cations act only as substitutional impurities, they are homogeneously distributed in the whole film, maintaining the wurtzite structure and increasing the carrier density. The formation of any spurious phase or segregation of Ga₂O₃ clusters that can act as carrier traps can be discarded. The characterization results allow us to conclude that the doped film has improved electrical and optical properties with respect to the undoped one. Therefore, the Ga-doped films are very suitable candidates as transparent conducting electrodes for solar cells, displays and other photoelectronic devices.     

Wide range tuning of electrical conductivity of RF sputtered CdO thin films through oxygen partial pressure variation

The effect of oxygen partial pressure variation on the electrical conductivity and the optical transparency of CdO thin films, deposited through RF magnetron sputtering were studied in detail. Thin films of CdO have been deposited through radio frequency magnetron sputtering of a prefabricated CdO target at a fixed pressure 0.1 mbar and at a substrate temperature 523 K. It was found that the electrical conductivity of the CdO films could be varied over three decades for a variation of oxygen partial pressure of 0–100%, without introducing any extrinsic dopants. X-ray diffraction (XRD) studies showed that the films were polycrystalline in nature with a preferential orientation along (1 1 1) plane. Compositional information was obtained by X-ray photoelectron spectroscopic studies. This wide range of variation of electrical properties was explained through the oxygen vacancies formation.     

Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering

Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar+O₂ atmosphere. The deposited films were characterized by studying their structural, electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (1 1 1) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (>5%) the conductivity was found to decrease.     

Electrical,optical, and structural characterization of arsenic–tin oxidized transparent conducting films

A comprehensive structural, optical, electrical, and optoelectronic study of arsenic-doped SnO₂ was conducted. Several arsenic-doped SnO₂ thin films with different arsenic content have been prepared on glass and silicon substrates by a vacuum thermal evaporation technique. The structural, electrical and optical study show that some of As⁵⁺ ions occupied locations in interstitial positions of SnO₂ lattice. The prepared oxidized pure tin film is found to be consisting of orthorhombic and tetragonal SnO₂ structure. The optical properties show that arsenic-doped SnO₂ films are good transparent oxides. The bandgap of arsenic-doped SnO₂ varies with arsenic content following the Moss–Burstein rule. The electrical behaviors show that the prepared arsenic-doped SnO₂ films are degenerate semiconductors and might transform into insulators with increasing arsenic doping level. The electrical properties (resistivity, mobility, and carrier concentration) vary depending on the arsenic doping level. The SnO₂ film doped with wt. 0.6% arsenic shows utmost dc electrical conductivity parameters: resistivity of 4.6 × 10^?2 Ω cm, mobility of 6.0 cm²/V s, and carrier concentration of 2.25 × 10¹⁹ cm^?3. From transparent-conducting-oxide (TCO) point of view, low arsenic concentration (less that 1%) is effective for SnO₂ donor doping but not emulate doping with other dopant like Sb.     

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO₃) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm² C⁻¹ (pure WO₃) to 27 cm² C⁻¹ for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO₃ films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.     

Hydrogen absorption–desorption,optical transmission properties and annealing effect of Mg thin films prepared by magnetron sputtering

We prepared Pd-capped Mg thin films by direct current (DC) magnetron sputtering. We investigated their structural, optical and electrical properties, as well as the effect of annealing on these films during hydrogenation and dehydrogenation. The hydrogen absorption behaviors at both room temperature and 353 K, and dehydrogenation at 353 K, were studied. The Mg film annealed at 473 K for 2 h exhibited the best absorption kinetics and superior switchable mirror properties among all the samples annealed at different temperatures. We concluded that annealing temperature is a crucial parameter in forming high-quality Mg films. The improved hydriding kinetics can be attributed to the favorable morphology and structure induced by the annealing process.     

Doping, vacuum annealing, and thickness effect on the physical properties of zinc oxide films deposited by spray pyrolysis

Gallium, aluminum, and indium-doped ZnO (ZnO:Ga, ZnO:Al, and ZnO:In) films have been deposited by the chemical spray method on sodocalcic substrates. The effect of different dopant elements, a post-annealing treatment in vacuum, and the film thickness on the electrical, optical, structural, and morphological properties of the films has been investigated. The best electrical properties were observed in the thickest indium-doped ZnO films; the lowest electrical resistivity was of the order of 10⁻³ Ω cm. In general, the optical transmittance value in the visible spectrum oscillated around of 87% in the thinnest films. The structural and morphological properties of ZnO:Ga and ZnO:Al films are similar, as in both cases the (0 0 2) orientation is dominant on the rest of the peaks, and both surfaces have a rough appearance. In the case of ZnO:In films, the (1 0 1) was the preferential growth orientation, and the surfaces seem to be smoother than the corresponding ZnO:Ga and ZnO:Al films.     

Properties of fluorine doped ZnO thin films deposited by magnetron sputtering

Fluorine doped ZnO (FZO) films were deposited on Corning glass by radio frequency (rf) magnetron sputtering of pure ZnO target in CF₄ containing gas mixtures, and the compositional, electrical, optical, and structural properties of the as-grown films as well as the vacuum-annealed films were investigated. The fluorine content in FZO films increased with increasing CF₄ content in sputter gas. FZO films deposited at elevated temperature of 150 °C had considerably lower fluorine content and showed a poorer electrical properties than the films deposited at room temperature. Despite high fluorine contents in the films, for all the FZO films, the carrier concentration remained below 2×10²⁰ cm⁻³, leading to fairly low doping efficiency level. Vacuum-annealing of the FZO films deposited at room temperature resulted in substantial increase of Hall mobilities, reaching as high as 43 cm²/Vs. This was attributed partly to the removing of oxygen vacancies and/or the forming chemical bonds with interstitial zinc atoms by fluorine interstitials and partly to the passivation effect of excess fluorine atoms by filling in the dangling bonds at the grain boundaries. For all the films with thickness of around 300 nm, the optical transmissions in visible were higher than 80%, and increased with increasing fluorine content up to 85% for the film with highest fluorine content.     

Investigation of lateral parameter variations of Al-doped zinc oxide films prepared on glass substrates by rf magnetron sputtering

Sputter-deposited Al-doped zinc oxide (ZnO : Al) is an interesting transparent conductive oxide (TCO) material for application in electronic devices and thin-film solar cells. A phenomenon in the planar magnetron sputtering of the ZnO : Al films that is not well investigated as yet are the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material, whereby the lateral distribution of the film properties depends strongly on the sputtering parameters. In this work, the lateral distributions of the electrical, optical, and surface structure properties of the ZnO : Al films prepared by the rf magnetron sputtering on glass substrates are investigated across a distance of 64 mm using the four-point probe technique, optical transmission and reflection measurements, X-ray diffraction, and scanning electron microscopy. We find that the lateral variations of the parameters of the ZnO : Al films prepared by the rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. Hence, it seems that the sputter-deposited ZnO : Al is a promising TCO material for large-area thin-film solar cells.     

Synthesis of nano-crystalline Zr-M (M=Ni,Co, Fe,Cu) bilayer films and their thermodynamics of hydrogen uptake by resistance measurement

Nano-crystalline thin metal films for hydride formation for small amount of hydrogen storage is an emerging field of research for portable applications e.g. thin film fuel cells. Nano-crystalline films of Zr/ M (M = Ni, Co, Fe, Cu) bilayer systems were synthesized using ion beam sputtering technique in argon atmosphere which were characterized using GIXRD and AFM techniques. In thin film metal hydride it is difficult to measure P-C-T isotherm because of the small amount of hydrogen present and the same difficulty is to study thermodynamics of such systems. Hence in the present work change in electrical resistance with hydrogen pressure in temperatures range 298 to 573 K has been used to investigate thermodynamic properties and found that resistance of film increases with the absorption of hydrogen and decreases due to hydrogen desorption.     

Highly textured ZnO thin films doped with indium prepared by the pyrosol method

Indium doped ZnO thin films have been prepared on heated Corning 7059 glass by the pyrosol spray method. It was found that indium doping has an important role in grain growth at high substrate temperature. Indium also was used to improve the electrical properties, acting as an N type dopant, and we obtained highly conductive ZnO:In thin films with a resistivity of 3.0 × 10⁻³ Ω cm. At substrate temperatures from 425°C to 475°C, the deposited ZnO:In thin films have clear hexagonal crystallites and, therefore, a highly textured surface showing optical haze phenomena due to the crystallites. The haze ratio of ZnO:ln thin films can be controlled from 10% to 50% at the wavelength of 550 nm by varying the substrate temperature from 375°C to 475°C.     

Optoelectronic properties of doped layers for a-Si solar cells prepared using helium dilution

This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH₄) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360Å, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm)⁻¹ and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.     

The effects of various annealing regimes on the microstructure and physical properties of ITO (In2O3:Sn) thin films deposited by electron beam evaporation for solar energy applications

The goal of this study has been to investigate the influence of various post-deposition heat treatments on the microstructure, electrical and optical properties of In₂O₃:Sn (ITO) thin films deposited by electron beam evaporation. We have shown that electron beam evaporated ITO thin films deposited onto substrates kept upto 150 °C, have poor electrical properties and low optical transmission in the visible range, due to their amorphous structure. As the microstructure changes from amorphous to polycrystalline it was observed that the film resistivity decreases and it is simultaneously related to an improvement in the optical transmission. From comparisons of several annealing processes it has been observed that oxygen plays an important role in doping as well as the presence of Sn in the target material. Furthermore we have shown that high quality ITO thin films can be reproducibly prepared with optical transmission being enhanced by an annealing in air and the electrical characteristics being improved by a further annealing in a reducing atmosphere. Superior electrical and optical properties could be correlated with annealed films that exhibited a cubic bixbyte structure and large crystallite dimensions larger than 50 nm.