Structural and optical properties of TiO2 thin films grown by sol-gel dip coating process

The mono and bi-layer TiO₂ thin films have been prepared by sol-gel method on glass. X-Ray diffraction, Raman spectroscopy, atomic force microscopy, spectroscopic ellipsometry and m-lines spectroscopy techniques have been used to characterize the TiO₂ films. The mono-layer film is found to be amorphous, while the bi-layer film shows the presence of anatase phase. The bi-layer film exhibits more homogeneous surface with less roughness. The thickness effect on the refractive index, extinction ceofficient, packing density and optical band gap is analysed. The waveguiding measurements of the bi-layer film exhibit single-guided TE₀ and TM₀ polarized modes from which we can measure the refractive index and the film thickness.     

Optical characterization of HfO2 thin films

Hafnia films prepared onto silicon wafers at three substrate temperatures of 40, 160 and 280 °C are optically characterized utilizing the multi-sample method. The characterization uses the combination of variable angle spectroscopic ellipsometry and spectroscopic reflectometry within the spectral region 1.24-6.5 eV (190-1000 nm). The structural model of the HfO₂ films includes boundary nanometric roughness, thickness non-uniformity and refractive index profile. Spectral dependences of the film optical constants are expressed using a recently developed parametrized joint density of states model describing the dielectric response of both interband transitions and excitations of localized states below the band gap. It is shown that the observed weak absorption below the band gap does not correspond to the Urbach tail.     

Comparative study of physical properties of vapor chopped and nonchopped Al2O3 thin films

Aluminium oxide being environmentally stable and having high transmittance is an interesting material for optoelectronics devices. Aluminium oxide thin films have been successfully deposited by hot water oxidation of vacuum evaporated aluminium thin films. The surface morphology, surface roughness, optical transmission, band gap, refractive index and intrinsic stress of Al₂O₃ thin films were studied. The cost effective vapor chopping technique was used. It was observed that, optical transmittance of vapor chopped Al₂O₃ thin film showed higher transmittance than the nonchopped film. The optical band gap of vapor chopped thin film was higher than the nonchopped Al₂O₃, whereas surface roughness and refractive index were lower due to vapor chopping.     

Thickness dependence of structural and optical properties of indium tin oxide nanofiber thin films prepared by electron beam evaporation onto quartz substrates

Indium tin oxide (ITO) thin films, produced by electron beam evaporation technique onto quartz substrates maintained at room temperature, are grown as nanofibers. The dependence of structural and optical properties of ITO thin films on the film thickness (99-662 nm) has been reported. The crystal structure and morphology of the films are investigated by X-ray diffraction and scanning electron microscope techniques, respectively. The particle size is found to increase with increasing film thickness without changing the preferred orientation along (2 2 2) direction. The optical properties of the films are investigated in terms of the measurements of the transmittance and reflectance determined at the normal incidence of the light in the wavelength range (250-2500 nm). The absorption coefficient and refractive index are calculated and the related optical parameters are evaluated. The optical band gap is found to decrease with the increase of the film thickness, whereas the refractive index is found to increase. The optical dielectric constant and the ratio of the free carrier concentration to its effective mass are estimated for the films.     

Optical and microstructural characterization of sol/gel derived cerium-doped PZT thin films

Optical properties of cerium-doped PZT thin films on sapphire prepared by a sol-gel technique are investigated using both transmission and reflection spectra in the wavelength range 200 to 900 nm. The refractive index, extinction coefficient and thickness of the film are determined from the measured transmission spectra. The packing density of the film is calculated from its refractive index using the effective medium approximation (EMA), and average oscillator strength and wavelength are estimated using a Sellmeir-type dispersion equation. Absorption coefficient (α) and the band gap energy (E_g) of each film composition are also calculated. Possible correlations of microstructure and phase formation behaviour with changes in band gap energy and other optical properties are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of calcination ambient and film thickness on the optical and structural properties of sol-gel TiO2 thin films

Influence of both calcination ambient and film thickness on the optical and structural properties of sol-gel derived TiO₂ thin films have been studied. X-ray diffraction results show that prepared films are in an anatase form of TiO₂. Films calcined in argon or in low vacuum (∼2 × 10⁻¹ mbar) are found to be smaller in crystallite size, more transparent at low wavelength region of ∼300-450 nm, denser, have higher refractive index and band gap energy compared to air-calcined films. Scanning electron microscopic study reveals that surfaces of TiO₂ films calcined in argon or in low vacuum are formed by densely packed nano-sized particulates. Presence of voids and signs of agglomeration can be seen clearly in the surface microstructure of air-calcined films. In the thickness range ∼200-300 nm, band gap energy and crystallite size of TiO₂ films remain practically unaffected with film thickness but refractive index of thinner film is found to be marginally higher than that of thicker film. In this work, it has been shown that apart from temperature and soaking time, partial pressure of oxygen of the ambient is also an important parameter by which crystallite size, microstructure and optical properties of the TiO₂ films may be tailored during calcination period.     

Plasma influence on the properties and structure of indium tin oxide films produced by reactive middle frequency pulsed magnetron sputtering

Reactive pulsed magnetron sputtering was used to produce conductive and transparent tin-doped indium oxide (ITO) films with low thickness inhomogeneity. Due to the parallel operation of two magnetrons, the deposition system allows in situ investigations of the plasma influence on the film properties. The distribution of the film resistivity, refractive index, structure and stoichiometry along the substrate are presented and related to the spatial distribution of the plasma flow escaping the magnetrons, and the substrate temperature. A higher plasma flow likely causes a localized relaxation of the distorted In-O bonds in amorphous phase which prevails in ITO films prepared at unheated substrates. This leads to a decrease of the film resistivity due to free electrons density and mobility enhancement. The free electron density increase is caused likely by generation of oxygen vacancies. Deposition on a heated substrate (T_s / T_m = 0.3) leads to a change of the film growth mode due to enhanced surface diffusion of the adatoms which results in a textured low resistivity film. This also causes significant improvements of the homogeneity of the film properties that is important for ITO applications.     

Preparation of strain-included rutile titanium oxide thin films and influence of the strain upon optical properties

Rutile type titanium dioxide (TiO₂) thin films are prepared by pulse laser deposition (PLD) with controlled oxygen pressure. Transmission electron microscopy (TEM) analysis clarified that crystalline structures, surface structures and electric states of the TiO₂ films almost correspond to those of the bulk rutile. We observed crystalline domain including strain in the PLD film. The strain of the TiO₂ film was measured by nano-beam diffraction. The strain-included domains affected properties of the film as optical absorption. The obtained optical band gap energy value was 3.30 eV, which was larger than that of bulk. TEM results such as crystalline grain sizes and distribution were used to consider quantum size effect in order to explain the larger band gap value. Moreover the influence of strain in rutile crystalline grains upon optical properties was suggested in the present study. The difference of band gap energy between experimental and theoretically calculated ones was considered to come from the strain effects.     

Optical properties of SiOC film by PL spectrometer and reflectance

SiOC films made by the inductive coupled plasma chemical vapor deposition were analyzed by the photo luminance spectra and reflectance. Chemical shift obtained by PL spectra was observed and the reflectance was decreased at sample with low refractive index. The refractive index was in inverse proportion to the thickness of SiOC film. Lowering the reflectance caused to decrease the energy band gap and polarization, so increased an absorbance of the light. Low polarization of SiOC film was induced from the recombination and dissociation of alkyl sites by neighboring high electro negative oxygen. The electron affinity related to the energy band gap at surfaces increased at sample with low polarization, and finally, the reflectance decreased according to the reduction of polarity.     

Optical properties of CeO<Subscript>2</Subscript> thin films

Cerium oxide (CeO₂) thin films have been prepared by electron beam evaporation technique onto glass substrate at a pressure of about 6 × 10⁻⁶ Torr. The thickness of CeO₂ films ranges from 140–180 nm. The optical properties of cerium oxide films are studied in the wavelength range of 200–850 nm. The film is highly transparent in the visible region. It is also observed that the film has low reflectance in the ultra-violet region. The optical band gap of the film is determined and is found to decrease with the increase of film thickness. The values of absorption coefficient, extinction coefficient, refractive index, dielectric constant, phase angle and loss angle have been calculated from the optical measurements. The X-ray diffraction of the film showed that the film is crystalline in nature. The crystallite size of CeO₂ films have been evaluated and found to be small. The experimental d-values of the film agreed closely with the standard values.     

Effect of cobalt doping on the structural and optical properties of TiO2 films prepared by sol-gel process

Pure and cobalt doped titanium dioxide thin films have been prepared on glass and Si (100) substrates by sol-gel spin coating method. The structural and optical properties of the films as a function of cobalt concentration (up to 15 wt.%) have been systematically studied by Rutherford backscattering spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, optical spectroscopy and spectroscopic ellipsometry methods. Rutherford backscattering studies show the presence of cobalt dopant in the films is almost equal to the precursor stoichiometry. Grazing incidence X-ray diffraction and Raman spectroscopy studies confirm the amorphous phase of the as-deposited films and crystalline anatase phase for the films annealed at 600 °C. The optical spectroscopy measurements show that the films are fully transparent in the visible region and there is a band gap narrowing upon increasing cobalt dopant concentration. The refractive index, band gap (E_g) and thickness of the films were determined from spectroscopic ellipsometry measurements. The refractive index of the films was found to increase from 2.2 to 2.7 with the increase in cobalt concentration with a simultaneous decrease of band gap from 3.1 to 2.8, which is favorable for photocatalytic applications. The packing density of the films was calculated by Clausius-Mossotti relation and is found to increase with cobalt concentration.     

Properties of cu-doped low resistive ZnSe films deposited by two-sourced evaporation

Two-sourced evaporation technique is used to prepare hard ZnSe films by controlling the evaporation rates of both Zn and Se at substrate temperature of 400 °C. The films are doped with Cu by immersion in the Cu(NO₃)₂-H₂O solution for different periods of time. The XRD has not shown a drastic change in the film structure while the electrical resistivity of the deposited film dropped from 10⁹ Ω-cm to about 1.6 Ω-cm for solution immersed films after heat treatment. Optical properties of deposited and doped films, such as film thickness, absorption coefficient and optical band gap have been calculated from the normal transmission spectra in the range of 300-2200 nm.The optical results show a decrease of the transmission and an increase of the refractive index and a slight shift in the optical band gap. Chemical composition of the Cu is determined by using absorption of immersed films. The composition of Cu is also compared with the composition detected by electron microprobe analyzer (EMPA).     

The optical properties of Bi3.25La0.75Ti3O12 thin films with different thickness prepared by chemical solution deposition

Bi_3.25La_0.75Ti₃O₁₂(BiLT) thin films with different thickness were successfully deposited onto fused quartz by chemical solution deposition. X-ray diffraction analysis shows that BiLT thin films are polycrystalline with (0 0 2)-preferred orientation. The dispersion of refractive indices of the BiLT thin films was investigated by the optical transmittance spectrum. The optical band gap energy was estimated from the graph of (hνα)² versus hν. The results show that the refractive index and band-gap energy of the BiLT thin films decrease with the films thickness.     

Optical and electrical characterization of CdS thin films

Thin CdS films have been grown by chemical bath (CdCl₂, thiourea, ammonia) deposition (CBD) on SnO₂ (TO)-coated glass substrate for use as window materials in CdS/CdTe solar cells. High-resolution transmission electron microscopy revealed grains with an average size of 10 nm. The structure was predominantly hexagonal with a high density of stacking faults. The film crystallinity improved with annealing in air. Annealing in a CdCl₂ flux increased the grain size considerably and reduced the density of stacking faults. The optical transmission of the as-deposited films indicated a band gap energy of 2.41 eV. Annealing in air reduced the band gap by 0.1 eV. Annealing in CdCl₂ led to a sharper optical absorption edge that remained at 2.41 eV. Similar band gap values were obtained by photocurrent spectroscopy and electroabsorption spectroscopy (EEA) using an electrolyte contact. EEA spectra were broad for the as-deposited and air-annealed samples, but narrower for the CdCl₂-annealed films, reflecting the reduction in stacking fault density. Donor densities of ca. 10¹⁷ cm ^–3 were derived from the film/electrolyte junction capacitance.     

Investigations on the optical properties of sol–gel derived lanthanum doped lead titanate thin films

In the present work we studied the optical properties of undoped and La doped lead titanate thin films, and also demonstrated that the optical characterization of thin films can be used as an effective diagnostic tool to assess film quality. The optical properties of Pb_1−xLa_xTi_1−x/4O₃ [where x=0 (undoped), 10, 15, 20, 25 and 30 at.%] thin films were investigated using both transmission and reflection spectra in the 200–900-nm wavelength range. The refractive index (n), extinction coefficient (k) and the thickness of the film (d_f) were determined from the measured transmission spectra. The thickness of the film obtained from the interference fringes in transmission or reflection spectra matched well with those obtained from other methods. The appearance of interference fringes is an indication of the thickness uniformity of the film. The low value of extinction coefficient (in the order of 10⁻²) as observed in our films is a qualitative indication of excellent surface smoothness of the films. The densities of the films were estimated from their refractive indices using effective medium approximation. The average oscillator strength and its associated wavelength were estimated using a Sellmeier-type dispersion equation. Absorption coefficient (α) and the band-gap energy (E_g) were obtained for undoped and La doped films with varying La concentration. It was found that the refractive index and packing fraction values decrease with La doping. La doping was found to decrease the grain size of the films and increase the density of individual grains. Increased La content led to clustering of smaller grains. The observed variation of band-gap energy with La doping has been correlated to the observed microstructure of these films.     

Oxidation of thin titamium films in ambient conditions

The room temperature oxidation of vapor-deposited titanium films was investigated as a function of film thickness by resistivity and ellipsometric measurements. The thickness of the films ranged from 3.0 to 120 nm. The electron mean free path in the films varied from 11.3 to 26.0 nm, and the product of the bulk resistivity and the electron mean free path was 1.5 × 10^-10Ω cm². The values of the surface electron scattering parameter in the films were between 0.2 and 0.6 and decreased to 0.18 at the onset of oxidation. The best value for the complex refractive index of titanium was found to be 3.61-i4.06. The refractive index of the oxide film was 2.75. After a 1 d exposure of the films to room air at atmospheric pressure both the resistance and ellipsometric measurements indicated a residual oxide thickness of 2.6±0.3 nm, regardless of the original metal film thickness. These results disagree with the theoretical and experimental results of Mindel and Pollack, which implied a rapid decrease in oxide thickness with decreasing titanium film thickness.     

Optical constants and Drude analysis of sputtered zirconium nitride films

Opaque and semitransparent dc magnetron-sputtered ZrN films on glass and silicon have been optically characterized with spectral reflectance measurements and ellipsometry. High rate sputtered ZrN has good optical selectivity, i.e., higher than 90% infrared reflectance and a pronounced reflectance step in the visible to a reflectance minimum of less than 10% at 350 nm. The results are comparable with those obtained for single crystalline samples and those prepared by chemical vapor deposition. The complex optical constant (N = n v ik) for opaque films has been determined in the 0.23-25-μm wavelength range with Kramers-Kronig integration of bulk reflectance combined with oblique incidence reflectance for p-polarized light. A variable angle of incidence spectroscopic ellipsometer has been used for determination of the optical constants in the 0.28-1.0-μm wavelength region. The results of the two methods show excellent agreement. The results indicate that ZrN is free electronlike and the Drude model can be applied. The best opaque films had Drude plasma energies (?ω(p) between 6.6 and 7.5 eV and relaxation energies (?/τ) between 0.29 and 0.36 eV. Ellipsometer data for the semitransparent films show that the refractive index (n) in the visible increases with decreasing film thickness whereas the extinction coefficient (k) is essentially unchanged. The optical properties are improved by deposition upon a heated substrate.     

Spectrographic ellipsometry study of a liquid crystal display substrate consisting of thin films of SiO2, polyimide and indium tin oxide on glass

Variable angle spectrometric ellipsometry at room temperature is used to determine thin film parameters of substrates used in liquid crystal displays. These substrates consist of sequential thin films of polyimide (PI), on indium tin oxide (ITO),on SiO₂ deposited on a glass backing approximately 1.1 mm thick. These films were studied by sequentially examining more complex systems of films (SiO₂, SiO₂-ITO, SiO₂-ITO-PI). The SiO₂ layer appears to be optically uniform and flat. The ITO film is difficult to characterize. When this surface film's lower surface is SiO₂ and upper surface is an air-ITO-interface it is found that including surface roughness and variation of the optical properties with ITO thickness in the model improved the fit; suggesting that both phenomena exist in the ITO films. However, the surface roughness and graded nature of optical properties could be not determinable by ellipsometry when the ITO is coated with a polyimide film. The PI films are ellipsometrically flat and over the wavelength range from 500 to 1400 nm the real refractive index of polyimide films varying in thickness between 25 and 80 nm is well modeled by a two-term Cauchy model with no absorption. The ellipsometric thickness of the ITO layer is the same as the profilometric thickness; however, the ellipsometric thickness of the polyimide layers is roughly 10 nm larger than that obtained from the profilometer. These final observations are consistent with the literature.     

Post-deposition treatment behaviour of zinc oxide thin films in hydrogen peroxide solution

Abstract

Zinc oxide thin films with c axis orientation were grown on Si (100) by radio frequency magnetron sputtering in mixture of argon and oxygen environment using Zn target. These films are treated in hydrogen peroxide (H₂O₂) solution at room temperature for one hour and two hours. During this study it has been found that post-deposition treatment of ZnO films improves the film quality. Structural, electrical and optical properties have been compared before and after H₂O₂ treatment by X-ray diffraction analysis, ac conductivity, band gap and refractive index. The full width at half maximum decreases after post-deposition process, which improves the crystal quality. The relief in stress has been observed but films are not fully stress free. The film after treatment becomes highly insulating having resistivity of the order of 10¹⁴ Ω cm^?1 and can be used for piezoelectric applications. The increase in the band gap and refractive index, near to bulk value, has been observed after post-deposition treatment indicating the increase in grain size and crystal quality.     

Spectroscopic ellipsometry studies of amorphous PZT thin films with various Zr/Ti stoichiometries

Amorphous stoichiometric Pb(Zr _x Ti_{1 – x)}O₃) (PZT) thin films with various values of x were deposited on Si(100) substrates by the sol-gel technique. The influence of Ti content on the optical properties was studied by spectroscopic ellipsometry (SE) in the UV-visible region. Using a four-phase fitting model, the refractive index n and extinction coefficient k was obtained by analyzing the SE spectra. The optical band gap energies E _g for these films were reported under the assumption of a direct band-to-band transition. It has been found that the refractive index, extinction coefficient and band gap energy of the films were functions of the film compositions. The refractive index of the PZT films increases linearly with increasing Ti content. On the other hand, the optical band gap energy of the PZT films decreases with increasing Ti content.