Influence of substrate temperature and post-treatment on the properties of ZnO:Al thin films prepared by pulsed laser deposition

Highly transparent conductive Al₂O₃ doped zinc oxide (AZO) thin films have been deposited on the glass substrate by pulsed laser deposition technique. The effects of substrate temperature and post-deposition annealing treatment on structural, electrical and optical properties of AZO thin films were investigated. The experimental results show that the electrical resistivity of films deposited at 240 °C is 6.1 × 10⁻⁴ Ω cm, which can be further reduced to as low as 4.7 × 10⁻⁴ Ω cm by post-deposition annealing at 400 °C for 2 h in argon. The average transmission of AZO films in the visible range is 90%. The optical direct band gap of films was dependent on the substrate temperature and the annealing treatment in argon. The optical direct band gap value of AZO films increased with increasing annealing temperature.     

A comparative study of the physical properties of Sb2S3 thin films treated with N2 AC plasma and thermal annealing in N2

As-deposited antimony sulfide thin films prepared by chemical bath deposition were treated with nitrogen AC plasma and thermal annealing in nitrogen atmosphere. The as-deposited, plasma treated, and thermally annealed antimony sulfide thin films have been characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, scanning electron microscopy, atomic force microscopy, UV-vis spectroscopy, and electrical measurements. The results have shown that post-deposition treatments modify the crystalline structure, the morphology, and the optoelectronic properties of Sb₂S₃ thin films. X-ray diffraction studies showed that the crystallinity of the films was improved in both cases. Atomic force microscopy studies showed that the change in the film morphology depends on the post-deposition treatment used. Optical emission spectroscopy (OES) analysis revealed the plasma etching on the surface of the film, this fact was corroborated by the energy dispersive X-ray spectroscopy analysis. The optical band gap of the films (E_g) decreased after post-deposition treatments (from 2.36 to 1.75 eV) due to the improvement in the grain sizes. The electrical resistivity of the Sb₂S₃ thin films decreased from 10⁸ to 10⁶ Ω-cm after plasma treatments.     

Fabrication and vacuum annealing of transparent conductive Ga-doped Zn0.9Mg0.1O thin films prepared by pulsed laser deposition technique

In this study, highly transparent conductive Ga-doped Zn_0.9Mg_0.1O (ZMO:Ga) thin films have been deposited on glass substrates by pulsed laser deposition (PLD) technique. The effects of substrate temperature and post-deposition vacuum annealing on structural, electrical and optical properties of ZMO:Ga thin films were investigated. The properties of the films have been characterized through Hall effect, double beam spectrophotometer and X-ray diffraction. The experimental results show that the electrical resistivity of film deposited at 200 °C is 8.12 × 10⁻⁴ Ω cm, and can be further decreased to 4.74 × 10⁻⁴ Ω cm with post-deposition annealing at 400 °C for 2 h under 3 × 10⁻³ Pa. In the meantime, its band gap energy can be increased to 3.90 eV from 3.83 eV. The annealing process leads to improvement of (0 0 2) orientation, wider band gap, increased carrier concentration and blue-shift of absorption edge in the transmission spectra of ZMO:Ga thin films.     

Physical properties of vacuum evaporated CdTe thin films with post-deposition thermal annealing

This paper presents the physical properties of vacuum evaporated CdTe thin films with post-deposition thermal annealing. The thin films of thickness 500 nm were grown on glass and indium tin oxide (ITO) coated glass substrates employing thermal vacuum evaporation technique followed by post-deposition thermal annealing at temperature 450 °C. These films were subjected to the X-ray diffraction (XRD),UV-Vis spectrophotometer, source meter and atomic force microscopy (AFM) for structural, optical, electrical and surface morphological analysis respectively. The X-ray diffraction patterns reveal that the films have zinc-blende structure of single cubic phase with preferred orientation (111) and polycrystalline in nature. The crystallographic and optical parameters are calculated and discussed in brief. The optical band gap is found to be 1.62 eV and 1.52 eV for as-grown and annealed films respectively. The I–V characteristics show that the conductivity is decreased for annealed thin films. The AFM studies reveal that the surface roughness is observed to be increased for thermally annealed films.     

Characterization of SnO2 obtained from the thermal oxidation of vacuum evaporated Sn thin films

Tin oxide has been prepared by thermal oxidation of evaporated tin thin films onto pyrex glass substrates. Films oxidation was achieved in air at a temperature of 600 °C with varied duration from 20min to 3 h. Structural, optical and electrical properties of the films were characterized by means of X-ray diffraction, UV–vis spectroscopy and electrical resistivity measurements respectively. The X-ray analysis revealed the transformation of Sn into SnO₂ with preferential orientation along (101) plans. No intermediate phases such as SnO and Sn₃O₄ were evidenced. It was also found that the SnO₂ crystallites orientation changed with the annealing time due to the strain energy effect. Both band gap energy and electrical resistivity decrease with annealing time due to the crystalline quality improvement and films densification. We have noticed that oxidation at 600 °C for 3 h leads to transparent and conductive films with suitable properties for photovoltaic applications.     

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO₂ thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO₂ thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO₂ thin films. The results show that the TiO₂ thin films crystallize in anatase phase between 400 and 800 °C, and into the anatase-rutile phase at 1000 °C, and further into the rutile phase at 1200 °C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature. The TiO₂ thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 °C, the transmittance of the films was reduced significantly in the wavelength range of 300-800 nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature.     

Optical and electrical properties of plasma-oxidation derived HfO2 gate dielectric films

High-k gate dielectric HfO₂ thin films have been deposited on Si(1 0 0) by using plasma oxidation of sputtered metallic Hf thin films. The optical and electrical properties in relation to postdeposition annealing temperatures are investigated by spectroscopic ellipsometry (SE) and capacitance-voltage (C-V) characteristics in detail. X-ray diffraction (XRD) measurement shows that the as-deposited HfO₂ films are basically amorphous. Based on a parameterized Tauc-Lorentz dispersion mode, excellent agreement has been found between the experimental and the simulated spectra, and the optical constants of the as-deposited and annealed films related to the annealing temperature are systematically extracted. Increases in the refractive index n and extinction coefficient k, with increasing annealing temperature are observed due to the formation of more closely packed thin films and the enhancement of scattering effect in the targeted HfO₂ film. Change of the complex dielectric function and reduction of optical band gap with an increase in annealing temperature are discussed. The extracted direct band gap related to the structure varies from 5.77, 5.65, and 5.56 eV for the as-deposited and annealed thin films at 700 and 800 °C, respectively. It has been found from the C-V measurement the decrease of accumulation capacitance values upon annealing, which can be contributed to the growth of the interfacial layer with lower dielectric constant upon postannealing. The flat-band voltage shifts negatively due to positive charge generated during postannealing.     

Effect of annealing temperature on the optical and electrical properties of amorphous As45.2Te46.6In8.2 thin films

The optical absorption of the As-prepared and annealed As_45.2Te_46.6In_8.2 thin films are studied. Films annealed at temperatures higher than 453 K show a decrease in the optical energy gap (E_o). The value of E_o increases from 1.9 to 2.43 eV with increasing thickness of the As-prepared films from 60 to 140 nm. The effect of thickness on high frequency dielectric constant (?_∞) and carrier concentration (N) is also studied. The crystalline structures of the As_45.2Te_46.6In_8.2 thin films resulting from heat treatment of the As-prepared film at different elevated temperatures is studied by X-ray diffraction. An amorphous-crystalline transformation is observed after annealing at temperatures higher than 453 K. The electrical conductivity at low temperatures is found due to the electrons transport by hopping among the localized states near the Fermi level. With annealing the films at temperatures higher than 473 K (the crystallization onset temperature) for 1 h, the electrical conductivity increases and the activation energy decreases, which can be attributed to the amorphous-crystalline transformations.     

Thickness-dependent electrical and optical properties of Ge8Sb2Te11 thin films

The amorphous Ge₈Sb₂Te₁₁thin films with varying thickness are thermally deposited on well-cleaned glass substrate from its polycrystalline bulk. Absence of any sharp peak confirms the amorphous nature of deposited films. Thickness-dependent electrical and optical properties including dc-activation energy, sheet resistivity, optical band gap, band tailing parameter, etc. of Ge₈Sb₂Te₁₁thin films have been studied. The optical parameters have been calculated from transmission, reflection and absorbance data in the spectral range of 200–1100 nm. It has been found that optical band gap and band tailing parameter decreases with the increase in Ge₈Sb₂Te₁₁thin films thickness. The dc-activation energy and sheet resistivity decreases while the crystallization temperature of the amorphous Ge₈Sb₂Te₁₁ films increases with the increase in thickness of the films. The decrease of the sheet resistivity has been substantiated quantitatively using the classical size-effect theory. These results have been explained on the basis of rearrangements of defects and disorders in the amorphous chalcogenide system.     

Influence of air annealing on the structural, morphological, optical and electrical properties of chemically deposited ZnSe thin films

Zinc selenide nanocrystalline thin films are grown onto amorphous glass substrate from an aqueous alkaline medium, using chemical bath deposition (CBD) method. The ZnSe thin films are annealed in air for 4 h at various temperatures and characterized by structural, morphological, optical and electrical properties. The as-deposited ZnSe film grew with nanocrystalline cubic phase alongwith some amorphous phase present in it. After annealing metastable nanocrystalline cubic phase was transformed into stable polycrystalline hexagonal phase with partial conversion of ZnSe into ZnO. The optical band gap, E_g, of as-deposited film is 2.85 eV and electrical resistivity of the order of 10⁶-10⁷ Ω cm. Depending upon annealing temperature, decrease up to 0.15 eV and 10² Ω cm were observed in the optical band gap, E_g, and electrical resistivity, respectively.     

Impact of thermal annealing on physical properties of vacuum evaporated polycrystalline CdTe thin films for solar cell applications

A study on impact of post-deposition thermal annealing on the physical properties of CdTe thin films is undertaken in this paper. The thin films of thickness 500 nm were grown on ITO and glass substrates employing thermal vacuum evaporation followed by post-deposition thermal annealing in air atmosphere within low temperature range 150–350 °C. These films were subjected to the XRD, UV‐Vis NIR spectrophotometer, source meter, SEM coupled with EDS and AFM for structural, optical, electrical and surface topographical analysis respectively. The diffraction patterns reveal that the films are having zinc-blende cubic structure with preferred orientation along (111) and polycrystalline in nature. The crystallographic parameters are calculated and discussed in detail. The optical band gap is found in the range 1.48–1.64 eV and observed to decrease with thermal annealing. The current–voltage characteristics show that the CdTe films exhibit linear ohmic behavior. The SEM studies show that the as-grown films are homogeneous, uniform and free from defects. The AFM studies reveal that the surface roughness of films is observed to increase with annealing. The experimental results reveal that the thermal annealing has significant impact on the physical properties of CdTe thin films and may be used as absorber layer to the CdTe/CdS thin films solar cells.     

Effects of H2 ambient annealing in fully 0 0 2-textured ZnO:Ga thin films grown on glass substrates using RF magnetron co-sputter deposition

Gallium doped zinc oxide (ZnO:Ga) thin films were grown on glass substrates using RF magnetron co-sputtering, followed by H₂ ambient annealing at 623 K to explore a possibility of steady and low-cost process for fabricating transparent electrodes. While it was observed that the ZnO:Ga thin films were densely packed c-axis oriented self-textured structures, in the as-deposited state, the films contained Ga₂O₃ and ZnGa₂O₄ which had adverse effect on the electrical properties. On the other hand, post-annealing in H₂ ambient improved the electrical properties significantly via reduction of Ga₂O₃ and ZnGa₂O₄ to release elemental Ga which subsequently acted as substitutional dopant increasing the carrier concentration by two orders of magnitude. Transmittance of the ZnO:Ga thin films were all over 90% that of glass while the optical band gap varied in accordance with the carrier concentrations due to changes in Fermi level. Experimental observation in this study suggests that transparent conductive oxide (TCO) films based on Ga doped ZnO with good electrical and optical properties can be realized via simple low-cost process.     

Optical properties of hexagonal boron nitride thin films deposited by radio frequency bias magnetron sputtering

The optical properties of hexagonal boron nitride (h-BN) thin films were studied in this paper. The films were characterized by Fourier transform infrared spectroscopy, UV--visible transmittance and reflection spectra. h-BN thin films with a wide optical band gap E_g (5.86 eV for the as-deposited film and 5.97 eV for the annealed film) approaching h-BN single crystal were successfully prepared by radio frequency (RF) bias magnetron sputtering and post-deposition annealing at 970~K. The optical absorption behaviour of h-BN films accords with the typical optical absorption characteristics of amorphous materials when fitting is made by the Urbach tail model. The annealed film shows satisfactory structure stability. However, high temperature still has a significant effect on the optical absorption properties, refractive index n, and optical conductivity σ of h-BN thin films. The blue-shift of the optical absorption edge and the increase of E_g probably result from stress relaxation in the film under high temperatures. In addition, it is found that the refractive index clearly exhibits different trends in the visible and ultraviolet regions. Previous calculational results of optical conductivity of h-BN films are confirmed in our experimental results.     

Effect of process parameters and post-deposition annealing on the microwave dielectric and optical properties of pulsed laser deposited Bi<Subscript>1.5</Subscript>Zn<Subscript>1.0</Subscript>Nb<Subscript>1.5</Subscript>O<Subscript>7</Subscript> thin films

Bismuth Zinc niobate (Bi_1.5Zn_1.0Nb_1.5O₇) thin films were deposited by pulsed laser deposition (PLD) method on fused silica substrates at different oxygen pressures. The structural, microwave dielectric and optical properties of these thin films were systematically studied for both the as-deposited and the annealed films at 600°C. The as-deposited films were all amorphous in nature but crystallized on annealing at 600°C in air. The surface morphology as studied by atomic force microscopy (AFM) reveals ultra-fine grains in the case of as-deposited thin films and cluster grain morphology on annealing. The as-deposited films exhibit refractive index in the range of 2.36–2.53 (at a wavelength of 750 nm) with an optical absorption edge value of 3.30–3.52 eV and a maximum dielectric constant of 11 at 12.15 GHz. On annealing the films at 600°C they crystallize to the cubic pyrochlore structure accompanied by an increase in band gap, refractive index and microwave dielectric constant.     

Influence of indium content on the optical, electrical and crystallization kinetics of Se100−xInxthin films deposited by flash evaporation technique

Thin films of Se _100−xIn_x (x=10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se_100−x In_x films has been investigated by the optical characterization. The optical band gap values of the Se_100−x In_x thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, E_g, of the Se_100−x In_x films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303-433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se_100−x In_x films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se_100−xIn_x films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization.     

Influence of temperature annealing on optical properties of SrTiO3/BaTiO3 multilayered films on indium tin oxide

We have prepared SrTiO₃/BaTiO₃ thin films with multilayered structures deposited on indium tin oxide (ITO) coated glass by a sol-gel deposition and heating at 300-650 °C. The optical properties were obtained by UV-vis spectroscopy. The films show a high transmittance (approximately 85%) in the visible region. The optical band gap of the films is tunable in the 3.64-4.19 eV range by varying the annealing temperature. An abrupt decrease towards the bulk band gap value is observed at annealing temperatures above 600 °C. The multilayered film annealed at 650 ° C exhibited the maximum refractive index of 2.09-1.91 in the 450-750 nm wavelength range. The XRD and AFM results indicate that the films annealed above 600 ° C are substantially more crystalline than the films prepared at lower temperatures which were used to change their optical band gap and complex refractive index to an extent that depended on the annealing temperature.     

Optimization of the optical and electrical properties of electron beam evaporated aluminum-doped zinc oxide films for opto-electronic applications

Aluminum-doped zinc oxide (AZO) thin films have been deposited by electron beam evaporation technique on glass substrates. The structural, electrical and optical properties of AZO films have been investigated as a function of annealing temperature. It was observed that the optical properties such as transmittance, reflectance, optical band gap and refractive index of AZO films were strongly affected by annealing temperature. The transmittance values of 84% in the visible region and 97% in the NIR region were obtained for AZO film annealed at 475 °C. The room temperature electrical resistivity of 4.6×10⁻³ Ω cm has been obtained at the same temperature of annealing. It was found that the calculated refractive index has been affected by the packing density of the thin films, whereas, the high annealing temperature gave rise to improve the homogeneity of the films. The single-oscillator model was used to analyze the optical parameters such as the oscillator and dispersion energies.     

Laser-induced changes on optical band gap of amorphous and crystallized thin films of Se75S25−xAGx

Optical band gap of amorphous, crystallized, laser induced amorphous and laser induced crystallized films of Se₇₅S_25−xAg_x (x=4, 6 and 8) glassy alloys was studied from absorption spectra. The amorphous and crystallized films were induced by pulse laser for 10 min. After laser irradiation on amorphous and crystalline films, optical band gap was measured. It has been found that the mechanism of the optical absorption follows the rule of indirect transition. The amorphous thin films show an increase in the optical band gap, while the crystallized (thermally annealed) thin films show a decrease in the optical band gap by inducing laser irradiation. Crystallization and amorphization of chalcogenide films were accompanied with the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase. These results are interpreted in terms of concentration of localized states due to shift in Fermi level.     

Structural,optical and electrical properties of laser irradiated Pb doped Ga–Se chalcogenide thin films

Using the Transverse Electrical Excitation at Atmospheric Pressure (TEA) nitrogen laser, we had irradiated the amorphous thin films of Ga₁₀Se₈₁Pb₉ chalcogenide glass and the results have been discussed in terms of the structural aspects of Ga₁₀Se₈₁Pb₉ glass. The observed changes are associated with the interaction of the incident photon and the lone-pairs electrons which affects the band gap. The X-ray structural characterization revealed the amorphous nature of as prepared films and polycrystalline nature of the laser irradiated films. The optical band gap of these thin films is measured by using the absorption spectra as a function of photon energy in the wavelength region 400–1200 nm. It is found that the optical band gap decreases while the absorption coefficient increases with increasing the irradiation time. The decrease in the optical band gap has been explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase in exposure time. The dc conductivities and activation energies of these thin films are measured in temperature range 303–403 K. It has been found that the activation energy in Ga₁₀Se₈₁Pb₉ chalcogenide thin films decreases whereas the dc conductivity increases at each temperature by increasing the irradiation time.     

Determination of optical properties in nanostructured thin films using the Swanepoel method

We present the methodological framework of the Swanepoel method for the spectrophotometric determination of optical properties in thin films using transmittance data. As an illustrative case study, we determined the refractive index, thickness, absorption index, and extinction coefficient of a nanostructured 3 mol% Y₂O₃-doped ZrO₂ (yttria stabilized zirconia, 3YSZ) thin film prepared by the sol-gel method and deposited by dipping onto a soda-lime glass substrate. In addition, using the absorption index obtained with the Swanepoel method, we calculated the optical band gap of the film. The refractive index was found to increase, then decrease, and finally stabilize with increasing wavelength of the radiation, while the absorption index and extinction coefficient decreased monotonically to zero. These trends are explained in terms of the location of the absorption bands. We also deduced that this 3YSZ thin film has a direct optical band gap of 4.6 eV. All these results compared well with those given in the literature for similar thin films. This suggests that the Swanepoel method has an important role to play in the optical characterization of ceramic thin films.