Crystal structure and optical absorption investigations on β-In2S3 thin films

The crystal structure of annealed β-In₂S₃ thin films with different thickness was investigated by X-ray diffraction technique. Lattice parameters, crystallite size and microstrain were calculated. It was found that the lattice parameters are independent on film thickness, while annealing temperatures increase them. Crystallite sizes were increased with the increase of the film thickness and improved by annealing temperatures. In all cases, the microstrains were decreased gradually with the increase in both film thickness and annealing temperatures. Optical properties of β-In₂S₃ thin films were performed in the spectral range from 400 to 2500 nm to determine the optical constants (n and k), the high frequency dielectric constant, ε_∞, the lattice dielectric constant, ε_L, and the energy gap. The optical constants were found to be independent on film thickness in the range from 200 to 630 nm. The high frequency dielectric and lattice dielectric constants of the as-deposited film increased by annealing temperatures. The energy gap for the as-deposited In₂S₃ was found to be 2.60 eV and increased to 2.70 and 2.75 eV by annealing at 423 and 473 K for 1 h, respectively.     

The dielectric functions and optical band gaps of thin films of amorphous and cubic crystalline Mg~ 2NiH~ 4

Magnesium nickel hydride films have earlier been suggested for several optoelectronic applications, but the optical properties and band gap have not been firmly established. In this work, the dielectric functions and the optical band gaps of thin films of Mg₂NiH₄ have been determined experimentally from optical modeling using spectroscopic ellipsometry and spectrophotometry in the photon energy range between 0.7 and 4.2 eV. Samples were prepared by reactive sputtering, resulting in a single-layer geometry that could easily be studied by ellipsometry. Crystalline samples were prepared by annealing the as-deposited amorphous films ex-situ. The resulting films remained in the high temperature cubic Mg₂NiH₄ structure even after cooling to room temperature. Tauc analysis of the dielectric functions shows that Mg₂NiH₄ films exhibit a band gap of 1.6 eV for the amorphous structure and 2.1 eV for the cubic crystalline structure.     

Influence of the annealing in nitrogen atmosphere on the XRD, EDX, SEM and electrical properties of chemical bath deposited CdSe thin films

The influence of annealing in nitrogen atmosphere on the structure, optical and electrical properties of cadmium selenide (CdSe) thin films deposited by chemical bath deposition (CBD) onto glass substrates was studied. The samples were annealed in nitrogen atmosphere at various temperatures. A transition from metastable nanocrystalline cubic to stable polycrystalline hexagonal phase has been observed after annealing. The as-deposited CdSe thin films grow in the nanocrystalline cubic phase with optical band gap 1.93 eV. The electrical resistivity of the thin films has been measured in order of 10⁶ Ω cm. The activation energy of the samples has been found to be 0.26–0.19 eV at low temperature region, and 0.36–0.56 eV at high temperature region. It was also found that the activation energy and the resistivity of the films decrease with the increasing annealing temperature.     

Effect of annealing in a various oxygen atmosphere on structural,optical, electrical and gas sensing properties of MoxOy thin films

Molybdenum oxide thin films were thermally evaporated on a glass substrate and monitored by an annealing process in a variable oxygen atmosphere. The effects of post annealing condition on the microstructural, morphological, optical and electrical properties were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, spectroscopic ellipsometry and impedance spectroscopy. As-deposited amorphous films crystallized into tetragonal metastable phase of Mo₅O₁₄ on annealing at 500 °C in vacuum and air. This structure transformed to stable orthorhombic of MoO₃ with annealing in oxygen environment. The optical parameters such as the refractive index, extinction coefficient, optical band gap energy and the Urbach energy were calculated from Cauchy formalism. Ellipsometric measurements reveal that the samples present optical gap located between 3.24 and 3.90 eV when the atmosphere becomes rich on oxygen. The variation of the conductivity in terms of the temperature shows an electrical behavior with oxygen environment. Finally, it has been found that MoO₃ thin films had high sensitivity to ethanol, which made them as a good candidate for the ethanol sensor.     

Determination and analysis of optical constants for thermally evaporated PbSe thin films

PbSe films have been deposited on glass and quartz substrates at room temperature by thermal evaporation technique. X-ray diffraction patterns of the obtained films showed that they have polycrystalline texture and exhibit cubic FCC structure. The optical constants, the refractive index n and absorption index k were calculated in the spectral range of 400-4000 nm from transmittance and reflectance data using Murmann’s exact equations. Both n and k are practically independent on the film thickness in the range 28 nm to 210 nm. From the analysis of absorption index data, an indirect allowed energy gap of 0.16 eV and direct allowed energy gap of 0.277 eV were obtained. Other direct allowed optical transitions were obtained with energy gap of 0.49 eV and may be due to the splitting of valence band at the Γ point due to the effect of spin-orbit interaction.     

Determination and analysis of optical constants for thermally evaporated PbSe thin films

PbSe films have been deposited on glass and quartz substrates at room temperature by thermal evaporation technique. X-ray diffraction patterns of the obtained films showed that they have polycrystalline texture and exhibit cubic FCC structure. The optical constants, the refractive index n and absorption index k were calculated in the spectral range of 400–4000 nm from transmittance and reflectance data using Murmann’s exact equations. Both n and k are practically independent on the film thickness in the range 28 nm to 210 nm. From the analysis of absorption index data, an indirect allowed energy gap of 0.16 eV and direct allowed energy gap of 0.277 eV were obtained. Other direct allowed optical transitions were obtained with energy gap of 0.49 eV and may be due to the splitting of valence band at the Γ point due to the effect of spin-orbit interaction.     

Structural and optical properties of In doped Se–Te phase-change thin films: A material for optical data storage

Se_75−xTe₂₅In_x (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10⁻⁶ Torr onto well cleaned glass substrate. a-Se_75−xTe₂₅In_x thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV–Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400–1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se–Te glassy system.     

Phase change and optical band gap behavior of Se0.8S0.2 chalcogenide glass films

Se_0.8S_0.2 chalcogenide glass films have been prepared by thermal vacuum evaporation technique with thickness 583 nm. Annealing process at T ≥ 333 K crystallizes the films and nanostructured films are formed. The crystallite size was increased to 24 nm as the annealing temperature increased to 373 K. Orthorhombic crystalline system was identified for the annealed films. SEM micrographs show that films consist of two parallel surfaces and the thickness was determined by cross section imaging. The optical transmittance is characterized by interference patterns as a result of these two parallel surfaces, besides their average value at longer wavelength decreases as a result of annealing process. The band gap, E_g is red shifted due to crystallization by annealing. As the phase of the films changes from amorphous to crystalline in the annealing temperature range 333–363 K, a non sharp change of the band gap (E_g) is observed. This change was explained by Brus’s model of the energy gap confinement behavior of the nanostructured films. The optical refractive index increases suddenly when the system starts to be crystallized by annealing.     

Role of oxygen in enhancing N-type conductivity of CuInS2 thin films

Post-growth treatments in air atmosphere were performed on CuInS₂ films prepared by the single-source thermal evaporation method. Their effect on the structural, optical and electrical properties of the films was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical reflection and transmission and resistance measurements. The films were annealed from 100 to 350 °C in air. The stability of the observed N-type conductivity after annealing depends strongly on the annealing temperature. Indeed it is shown that for annealing temperatures above 200 °C the N-type conductivity is stable. The resistance of the N-CuInS₂ thin films correlates well with the corresponding annealing temperature. The samples after annealing have direct bandgap energies of 1.45-1.50 eV.     

Optical characterization of vacuum evaporated CdZnTe thin films deposited by a multilayer method

CdZnTe thin films of thickness 450–1400 nm have been evaporated under vacuum onto unheated glass substrates, using a multilayer method. During film deposition, the two evaporation sources, separated by two glass cylinders, were maintained at temperatures of 720 K for Zn and at 925–1200 K for CdTe, respectively. After deposition, the samples were annealed in air up to 775 K. The structural and optical properties of both as-deposited and heat-treated samples were investigated. Depending on the preparation conditions and the annealing temperature, the value of the optical band gap, E_g, of respective films varied between 1.16 and 1.63 eV. The obtained results are discussed in correlation with the structure of the films and the role of Zn atoms in CdTe films.     

Growth and vacuum post-annealing effect on the properties of the new absorber CuSbS2 thin films

Post-growth treatments in vacuum atmosphere were performed on CuSbS₂ films prepared by the single-source thermal evaporation method on glass substrates. The films were annealed in vacuum atmosphere for 2 h in temperature range 130-200 °C. The effect of this thermal treatment on the structural, optical and electrical properties of the films was studied. X-ray diffraction (XRD) patterns indicated that the films exhibited an amorphous structure for annealing temperature below 200 °C and a polycrystalline structure with CuSbS₂ principal phase. For the films annealed at temperatures below 200 °C one direct optical transition in range 1.8-2 eV was found. For the films annealed at 200 °C, two optical direct transitions emerged at 1.3 and 1.79 eV corresponding to the CuSbS₂ and Sb₂S₃ values respectively. The electrical measurements showed a conversion from low resistivities (3.10^− 2-9.10^− 2) Ω cm for the samples annealed at temperatures below 200 °C to relatively high resistivities (2 Ω cm) for the samples annealed at 200 °C. In all cases the samples exhibited p-Ztype conductivity.     

Synthesis and optical characterization of n-ZnO and p-Cu2ZnSnS4 nanocrystalline thin films for low cost solar cells

High quality ZnO/Cu₂ZnSnS₄ thin films as a window/absorber layers were successfully synthesized via spin coating the sol-gel precursor of each composition without using any vacuum facilities. In this study, the impact of annealing temperature (400 °C, 3 h) on the ZnO window layer and different thickness (3 and 5 layers) of the Cu₂ZnSnS₄ (CZTS) absorber layer were investigated. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and UV–vis–NIR spectroscopy were used for the structural, compositional, morphological and optical absorption analysis of each layer. ZnO exhibits wurtzite hexagonal crystal structure with particle size equals to 8.60 and 28.59 nm for fresh and annealed films, respectively. Micro-strain and dislocations density decreased with the annealing temperature. X-ray diffraction patterns for CZTS films show small peak at (112) according to the kesterite structure with particle size in nano-scale for the two thicknesses. ZnO films demonstrated direct optical band gap of 3.23 and 3.21 eV for fresh and annealed films, respectively. CZTS films (3 and 5 layers) also have direct optical band with optimum value (1.51 eV) for thickness of 5 layers. The J-V characteristics of the CZTS-based thin film solar cells (CZTS/ZnO/ZnO:Ag) were measured under air mass AM 1.5 and 100 mW/cm² illumination. The values of the short circuit current (J_sc), open circuit voltage (V_oc) and fill factor (FF) also have been obtained.     

Effects of deposition time and temperature on the optical properties of air-annealed chemical bath deposited CdS films

CdS films of over 1-μm thickness were deposited onto glass substrates by chemical bath deposition (CBD). Deposition temperature and time were varied from 40 °C to 60 °C and from 30 min to 4 h, respectively. The highest deposition rate, 6.39 nm/min, was obtained with samples deposited for 90 min at 60 °C. The films deposited at 60 °C for 4 h were found to have the best adhesion and without defects. The optical properties, in particular the optical band gap, depended on film thickness, the deposition and annealing temperatures. Annealing in air resulted in a shift of the absorption edge towards higher wavelengths, i.e., a decrease in the gap value from 2.45 eV to 2.38 eV. The optical band edges of the films were not constant but depended on the annealing temperature. The refractive index, calculated by applying the envelope method on the transmission of the films in wavelengths from 550 nm to 850 nm, was in the range 1.95-2.26. The resistivity determined from dark conductivity measurement, as a function of the annealing temperature, was found to be in the order of 10⁵ Ω cm for samples annealed in air at 250 °C, 3 h, and the activation energy was about 0.22 eV.     

Annealing effects on the structural and optical characteristics of electron beam deposited Ge-Se-Bi thin films

Measurements of absorbance and transmittance in Ge₂₀Se_80-xBi_x (0 x 10 at %) chalcogenide thin films in the visible range at room temperature were carried out. The dependence of the optical absorption on the photon energy is described by the relation hv=B(hv-E₀)². It was found that the optical energy gap E₀ decreases gradually from 1.93 to 1.205 eV with increasing Bi content up to 10 at %. The rate of the change decreases with increasing Bi content. The composition dependence of the optical energy gap is discussed on the basis of the concentration of covalent bonds formed in the chalcogenide film. The decrease of optical energy gap with increasing Bi content is related to the increase of Bi–Se bonds and the decrease of Se-Se bonds. The effect of thermal annealing for different periods of time on the behavior of optical absorption of the as-deposited films was investigated. The optical gap increases with increasing annealing time. The rate of change decreases with increasing annealing time, then E₀ reaches a steady state. The increase in the values of the optical gap of the amorphous films with heat treatment is interpreted in terms of the density of states model. The structure of the as-prepared and thermally annealed films were investigated using transmission electron microscopy, energy dispersive analysis and X-ray diffraction. It was confirmed that the as-prepared films were in the amorphous state. Phase separation was observed after thermal annealing. The separated crystalline phases were identified.     

Optical properties of post-annealed ZnO:Al thin films studied by spectroscopic ellipsometry

In this paper, effects of the thermal annealing on the structural, electrical, and optical properties of Al-doped ZnO (ZnO:Al) thin films prepared by reactive radio-frequency sputtering were investigated. From the X-ray diffraction observations, the orientation of ZnO:Al films was found to be a c-axis in the hexagonal structure. The optical properties of the films were investigated by optical transmittance and spectroscopic ellipsometry characterization. Based on Tauc–Lorentz model, the optical constants of ZnO:Al films were extracted in the photon energy ranging from 1.0 to 4.5 eV. Our result showed that the refractive index and extinction coefficient of the films changed consistently with annealing temperature.     

Thermal stability of sputtered zirconium oxide films

In this work, the effect of post-growth annealing on the structural and optical properties of sputtered zirconium oxide films has been investigated. The temperature dependence of structure, density, and optical constants has been systematically studied by X-ray diffraction, X-ray reflectometry, atomic force microscopy (AFM) and optical spectroscopy. X-ray diffraction studies show no variation in the crystalline phase upon annealing except grain growth. X-ray reflectivity measurements determine a density increase of approximately 11% and a simultaneous thickness reduction of 10% upon annealing. The surface roughness of the films increases upon annealing as determined by XRR and confirmed by AFM measurements. Optical spectroscopy measurements confirm that the refractive index n of the films decreases with increasing annealing temperature. At the same time the optical band gap E_g of the films increases from 4.58 to 4.97 eV annealing at 900°C. The surprising decrease of refractive index upon annealing is attributed to both the intermixing of Si with ZrO₂ and the increasing surface roughness of the films.     

Characterization and photocatalytic activity of TiO2 thin films prepared by RF magnetron sputtering

TiO₂ nano-structured thin films were prepared by RF magnetron sputtering and annealed in ambient air for 1 h at 400, 600, 800, 1000, and 1200 °C, respectively. Their phase structure, surface topography, and energy gap were characterized by X-ray diffractometer, atomic force microscope, and fluorescence spectrometer. Photocatalytic activity of the films was evaluated by light induced degradation of methyl orange (C₁₄H₁₄N₃NaO₃S) solution using a high pressure mercury lamp as lamp-house. The relation of photocatalytic activity and annealed temperature was studied in detail. It is found that the crystal phase transforms from amorphous to anatase, and rutile structure with annealing temperature increasing from room temperature to 1200 °C. Energy gap varies with annealing temperature. Photocatalytic activity is dependent on energy gap and grain size. Suitable energy gap from 2.97 to 3.07 eV is favorable for creation of electronic-hole pairs that make the films show excellent photocatalytic activity.     

Optical properties and hydrophilic behaviors of TaOxNy thin films with and without rapid thermal annealing

TaO_xN_y thin films were prepared using DC reactive sputtering of Ta target with the variation of (O₂ + N₂)/Ar ratio, followed by rapid thermal annealing (RTA) at 800 °C for 5 min. During RTA, the amorphous structure of the as-deposited films would transform to crystallized phases that might contain either TaON, or Ta₂O_5, or both. With the increase of (O₂ + N₂)/Ar ratio, Ta₂O₅ phase becomes more dominant, while TaON was formed in an opposite way. Elemental analysis also shows the same trend. Hydrophilicity and optical properties of these films were found to be dependent on phases formed after annealing. The optical band gap was measured and calculated to be in the range of 2.43 eV (510 nm) to 3.94 eV (315 nm). The films with low band gap values exhibited super hydrophilicity behavior under visible light irradiation, mainly due to their light absorption had been extended to visible light range. Clearly, it was due to the existence of TaON phase.     

Optical and structural properties of sol-gel prepared MgZnO alloy thin films

Mg_xZn_1−xO (x = 0-0.5) alloy thin films were prepared by a sol-gel dip-coating method. Mg_0.1Zn_0.9O and Mg_0.5Zn_0.5O films prepared were annealed in the range of 400-900 °C to investigate their thermal stability and temperature-dependent optical properties. The Mg_0.1Zn_0.9O films were thermally stable in the investigated annealing temperature range and exhibited the maximum ultraviolet emission at 800 °C. The segregation of MgO occurred in the Mg_0.5Zn_0.5O films, and the near-band-edge ultraviolet emission of this alloy was enhanced with increasing annealing temperature. The Mg saturation content in the sol-gel prepared MgZnO alloys was found to be about 0.23 where the band gap extended to 3.48 eV.     

Influence of thermal annealing on optical properties and surface morphology of NiOx thin films

NiO_x thin films were deposited by reactive DC-magnetron sputtering from a nickel metal target in Ar + O₂ with the relative O₂ content of 5%. Thermal annealing effects on optical properties and surface morphology of NiO_x films were investigated by X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscope and optical measurement. The results showed that the changes in optical properties and surface morphology depended on the temperature. The surface morphology of the films changed obviously as the annealing temperature increased due to the reaction NiO_x → NiO + O₂ releasing O₂. The surface morphology change was responsible for the variation of the optical properties of the films. The optical contrast between the as-deposited films and 400 °C annealed films was about 52%. In addition, the relationship of the optical energy band gap with the variation of annealing temperature was studied.