Some physical properties of ZnO thin films prepared by RF sputtering technique

ZnO thin films were deposited with RF sputtering using pure Zn target. In order to generate oxidation process of Zn, Ar:O₂ gas mixing in (9:1), (7:3) and (5:5) ratios of Ar:O₂ was used. To characterize ZnO thin films thickness and transparency were measured using optical method, and refractive index and band gap energies were calculated. Electrical conductivity of the ZnO thin films was also determined. AFM images were used to determine surface morphology of produced ZnO thin films.     

Angular selective optical properties of thin films: Measurement of polar and azimuthal transmittance

A new instrument for automatic recordings of angular dependent transmittance through thin film samples is described. The output is a polar plot giving a schematic and convenient overview of the optical performance. The usefulness of the instrument and procedures was documented by measurements on an angular selective Cr film prepared by oblique angle evaporation.     

Optical properties and thermal stability of TiAlN/AlON tandem absorber prepared by reactive DC/RF magnetron sputtering

Spectrally selective TiAlN/AlON tandem absorbers were deposited on copper and stainless steel substrates using a reactive DC/RF magnetron sputtering system. The compositions and thicknesses of the individual component layers were optimized to achieve high absorptance (α=0.931-0.942) and low emittance (ε=0.05-0.06) on copper substrate. The experimental spectroscopic ellipsometric data have been fitted with the theoretical models to derive the dispersion of the optical constants (n and k). In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) for different durations and temperatures. The tandem absorber deposited on Cu substrates exhibited high solar selectivity (α/ε) of 0.946/0.07 even after heat treatment in air up to 600 °C for 2 h. At 625 °C, the solar selectivity decreased significantly on Cu substrates (e.g., α/ε=0.924/0.30). The tandem absorber on Cu substrates was also stable in air up to 100 h at 400 °C with a solar selectivity of 0.919/0.06. Studies on the accelerated aging tests indicated that the activation energy for the degradation of the tandem absorber is of the order of 100 kJ/mol.     

Optical characterization of a-C:N thin films deposited by RF sputtering

In this work we present the main results of the optical properties study of amorphous carbon nitride (a-C:N) thin films prepared by reactive radio frequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto glass substrates, from a graphite target, in a pure nitrogen plasma. During the deposition, the pressure of nitrogen and the power density were maintained at 10⁻² mbar and 0.79 W cm⁻², respectively. Optical properties of these films were deduced from optical transmission spectra in the ultraviolet–visible–near infrared (UV–Vis–NIR) range. The refractive index follows well the Cauchy law with an extrapolated value of 1.68 in the far IR region. The optical band gap of the a-C:N films is about 1.2 eV. This value is relatively high in comparison with that of amorphous carbon films (0.8 eV) obtained in similar conditions. The incorporation of nitrogen in the amorphous carbon network leads to an increase of the optical band gap.     

Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering A comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°C, were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb₂0₅. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and K = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb₂O₅ + x Li⁺ + x e⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ films exhibits electrochemical reversibility beyond 1200 cycles without change in performance. “In situ” optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < λ < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

Optical properties of TiO2 thin films deposited by DC sputtering and their photocatalytic performance in photoinduced process

TiO₂ thin films were deposited by DC Sputtering varying the deposit time. These films were characterized by XRD, AFM, photoluminescence, UV–Vis, ellipsometry and XPS. The optical properties of TiO₂ thin films with different thickness, influenced their photocatalytic behavior in two photoinduced process. When TiO₂ thin films were irradiated with a UV light, midgap states were generated and the electrons were placed in lower energies than its band gap, favoring the photocatalytic hydrogen production and CO₂ photoreduction. From PL technique analyses it was observed that electrons occupied midgap states between the bands, with lower energies than the band gap. With these results it was possible to propose an energy diagram in order to correlate with photoinduced processes results. The presence of Ti³⁺ species was reconfirmed by means of XPS analyses. These species could be found in the midgap states, generated by the interaction between the UV irradiation and the film surface, which contributed to the photocatalytic activity of the films. The hydrogen production was similar for all the thin films studied (33–35 μmol) associated to the presence of similar energy midgap states. In the case of CO₂ photoreduction, all films produced CH₂O (8951 and 6252 μmol/g) and the films with a thickness of 330 and 420 nm generated CH₃OH (970 and 292 μmol/g). The extinction coefficient confirmed the XRD results for the film with greatest deposited time, which exhibited the highest crystallinity. All photocatalytic results did not show any dependence with the thin film thickness.     

Optical properties of electrodeposited CdTe thin films

The absorption coefficient spectra of the elctrodeposited CdTe thin films were analyzed and compared with that of the single crystal. Pinhole-free thin films facilitated the analysis of the high-energy regions of the absorption coefficient spectra. The various allowed direct and indirect transitions were detected successively by subtracting the extrapolated values of the lower-energy transitions. The effect of heat treatment on the optical transitions were analyzed with films annealed at 300°C in air, argon and CdCl₂.The direct band gap of the electrodeposited films decreased with increasing film thickness and approaches the value of the single crystal. The films annealed at different environments show slightly lower value for the band gap. Annealing in argon caused significant change in the optical transition spectra.     

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.     

Raman and XPS characterization of vanadium oxide thin films deposited by reactive RF sputtering

In this paper we report on Raman and XPS characterization of vanadium oxide thin films deposited by RF-sputtering. The samples were deposited by using a vanadium target in different oxygen fluxes, so that the stoichiometry (O/V ratio) of the oxide was varied. Several physical parameters of the films indicate a strong structural difference between the sample deposited at lower oxygen flux (1 scc m) and those obtained with higher flux (from 1.25 to 9 scc m). The increase of O/V ratio corresponds to a lower crystallinity of the thin films as indicated by the initial lowering and the final disappearance of the characteristic Raman mode of V₂O₅ (crystal) at about 140 cm⁻¹. For the highest flux samples new broad bands develop, typical of amorphous materials, both in polarized as well as in depolarized Raman spectra.     

Optical and photoelectrical properties of Bi2S3 thin films obtained by spray pyrolysis technique

Polycrystalline films of Bi₂S₃ compound have been prepared, at substrate temperature of 570 K, from Bismuth chloride (BiCl₃) and Thiourea (CS (NH₂)₂) solutions. The structural characterisation has been carried out by the X-ray diffraction analysis (XRD). Study of optical properties shows that Bi₂S₃ compound has an allowed indirect transition at 1.43 eV energy. The photoelectrical parameters (diffusion length and lifetime) have been determined from the study of the photoconductivity.     

Investigation of sol–gel derived thin films of titanium dioxide doped with vanadium oxide

Thin films of titanium dioxide and titanium–vanadium oxide were obtained by a sol–gel method. The coatings are uniform, smooth with very good optical properties. The solutions of both kinds are stable for more than a year. Structure and vibrational properties were studied with the help of X-ray diffraction (XRD) analysis and infrared spectroscopy (IR). The refractive indices and film thicknesses were measured by an ellipsometer at a wavelength of 638.2 nm, as a function of annealing temperature. The optical properties were investigated by ultraviolet–visible (UV–VIS) spectroscopy.     

Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films

Optical, structural and electronic properties of amorphous and crystalline molybdenum oxide thin films have been investigated. As-deposited amorphous films got crystallized into a layered orthorhombic phase on annealing at 350°C. Refractive index (n) and extinction coefficient (k) of as-deposited films and films annealed at 150°C, 240°C and 350°C have been calculated using reflectance and transmittance data. Spectral dependence of absorption coefficient has been explained on the basis of charge transfer transition mechanism. Optical band gap of amorphous MoO_3−x is 3.16 eV and it has increased by 0.11 eV on crystallization. XPS core level analysis reveals the presence of Mo⁺⁴, Mo⁺⁵ and Mo⁺⁵ oxidation states in amorphous films, proving oxygen deficiency in as-deposited films. Same studies on crystalline films show the presence of only Mo⁺⁶ states. Valence band spectrum of amorphous films reveal emission from Mo4d levels, which is absent in crystalline films. Complete correlation is seen between the optical properties and XPS data.     

Control of thermochromic spectrum in vanadium dioxide by amorphous silicon suboxide layer

Vanadium dioxide (VO₂) is a promising candidate for various thermochromic devices. Layering VO₂ film with amorphous silicon suboxide (a-SiO_x) is an effective approach for controlling the thermochromic spectrum efficiently, since a-SiO_x possesses an easily adjustable refractive index over a wide range. However, the crystalline quality of VO₂ is so sensitive to under- and/or over-coated materials that the a-SiO_x coatings may be disadvantageous with respect to material suitability. In this study, the controllability of the thermochromic spectrum of VO₂ by employing a-SiO_x layer was evaluated under several fabrication conditions and, as a result, it was found that an a-SiO_x layer can control thermochromism well without any decline in the stability of VO₂ crystalline growth, which strongly suggests the possibility of expansion in the optical design for efficient thermochromic control.     

Optical and electrical properties of flash-evaporated amorphous CuInSe2 films

Amorphous films of CuInSe₂ were deposited on glass substrate by flash evaporation of source materials. The films were found to be p-type semiconductors. The direct optical band-gap energy was obtained to be 1.21–1.41 eV. The film DC conductivity ranged from 1.2–5.7 S cm⁻¹ at 285 K for different film thickness with corresponding activation energy of 55.5–301 meV. From temperature dependence of conductivity, the carrier transport was interpreted to be due to band conduction above 270 K.     

Unexpected large hydrogen adsorption by Nb cluster films under mild conditions of pressure and temperature

Hydrogen absorption in several cluster assembled films was investigated by the optical and electrical response of the films when modified by hydrogen adsorption. The films were grown by deposition of niobium clusters and by co-deposition of niobium clusters with manganese or palladium clusters on sapphire and then capped by a thin Pd film. Unusual high adsorption of hydrogen by Nb clusters and thin metal oxide reductions at room temperature can be inferred by changes in optical transmission and resistance of the film. On the other hand, no hydrogen absorption can be inferred in the same experiments with co-deposited clusters of Nb and Pd and co-deposited clusters of Nb and Mn. The technique used in this work allows fast screening and detection of potential perm-selective materials for hydrogen.     

Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based metal oxides

Thin films of tungsten oxide, molybdenum oxide and mixed MoO₃–WO₃ oxides were obtained by atmospheric pressure chemical vapor deposition (CVD). All the films were prepared using identical technological parameters and through investigation of the optical properties of as deposited and annealed at 400°C a comparative study is reported. Raman, IR and VIS spectrophotometry and spectral ellipsometry methods were used. The mixed MoO₃–WO₃ films have higher optical absorption with maxima at a closer position with respect to the human eye sensitivity peak at 2.5 eV. The observed electrochromic effect is better expressed in the mixed films; the electrical charge inserted is higher.     

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.     

Growth and characterization of ZnO thin films prepared by electrodeposition technique

ZnO thin films were deposited on either indium tin oxide-coated glass or copper substrate by the electrodeposition process, using zinc chloride and flowing air as precursors. The effect of pH on the structural and morphological ZnO films was studied and the optimum deposition conditions have been outlined. The kinetics of the growth of the films have been investigated. We note that the rate of deposition of ZnO in an acidic solution was larger than in a basic solution. The structure of the films was studied using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The surface morphology and thickness of the films were determined using scanning electron microscopy. The X-ray diffraction analysis shows that the films are polycrystalline with hexagonal crystal structure (zincite) at pH 4. The optical transmittance of ZnO decreases with varying film thickness. The optical energy bandgap was found to be 3.26 eV.     

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.