Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Effect of indium incorporation on the optical properties of spray pyrolyzed Cd0.22Zn0.78S thin films

In this study, effect of indium incorporation on the optical properties is investigated for the spray pyrolyzed onto glass substrates at 275°C substrate temperature undoped and indium doped Cd_0.22Zn_0.78S thin films. The average optical transmittance of all the films was over 77% in the wavelength range between 450 and 800 nm. The optical band gap energies of the thin films have been investigated by the measurement of the optical absorbance as a function of wavelength. The optical absorption studies reveal that the transitions are direct band gaps of 3.02 and 3.05 eV for undoped and doped indium Cd_0.22Zn_0.78S thin films, respectively. The Urbach tail parameter and optical constants such as refractive index, extinction coefficient, and dielectric constants were calculated for these films. The dispersion parameters such as single-oscillator energy and dispersive energy were discussed in terms oft he single-oscillator Wemple—DiDomenico model.     

Effect of gamma doses on the optical parameters of Se76Te15Sb9 thin films

The effect of γ-radiation dose on the optical spectra and optical energy gap (E_opt.) of Se₇₆Te₁₅Sb₉ thin films was studied. The dependence of the absorption coefficient (α) on the photon energy (hν) was determined as a function of radiation dose. The films show indirect allowed interband transition that is influenced by the radiation dose. Both the optical energy gap and the absorption coefficient were found to be dose dependent. The indirect optical energy gap was found to decrease from 1.257 to 0.664 eV with increasing the radiation dose from 10 to 250 krad, respectively. The results can be discussed on the basis of γ-irradiation-induced defects in the film. The width of the tail of localized states in the band gap (E_e) was evaluated using the Urbach edge method. The refractive index (n) was determined from the analysis of the transmittance and reflectance data. Analysis of the refractive index yields the values of high frequency dielectric constant (ε_∞) and the carrier concentration (N/m^*). The dependence of refractive index on the radiation dose has also been discussed. Other optical parameters such as real and imaginary parts of the dielectric constant (ε₁, ε₂) and the extinction coefficient (k) have been evaluated. It was found that the spectral absorption coefficient is expected to a suitable control parameter of γ-irradiation-sensitive elements of dosimetric systems for high energy ionizing radiation (0.06-1.33 MeV).     

Optical properties of SrTiO<Subscript>3</Subscript> thin films by pulsed laser deposition

SrTiO₃ thin films were prepared on a fused-quartz substrate by pulsed laser deposition (PLD). Dense and homogeneous films with a thickness of 260 nm were prepared. Optical constants (refractive index n and extinction coefficient k) were determined from the transmittance spectra using the envelope method. The optical band gap energy of the films was found to be 3.58 eV, higher than the 3.22 eV for bulk SrTiO₃, attributable to the film stress exerted by the substrate. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The refractive index and the packing density for the PLD-prepared SrTiO₃ thin films are higher than those for the SrTiO₃ films prepared by physical vapor deposition, sol–gel and RF sputtering. Received: 18 March 2002 / Accepted: 7 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite BaHfO3

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BaHfO₃ has been reported using the pseudo-potential plane wave method within the local density approximation. The calculated equilibrium lattice is in a reasonable agreement with the available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffnesses is found. Band structures show that BaHfO₃ is a direct band gap between the occupied O 2p and unoccupied Hf d states. The variation of the gap versus pressure is well fitted to a quadratic function. Furthermore, in order to understand the optical properties of BaHfO₃, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. We have found that O 2p states and Hf 5d states play a major role in the optical transitions as initial and final states, respectively. This is the first quantitative theoretical prediction of the elastic, electronic and optical properties of BaHfO₃ compound, and it still awaits experimental confirmation.     

Electrical conductivity and optical properties of ZnO nanostructured thin film

The electrical conductivity, structural and optical properties of ZnO nanostructured semiconductor thin film prepared by sol-gel spin coating method have been investigated. The X-ray diffraction result indicates that the ZnO film has the polycrystalline nature with average grain size of 28 nm. The optical transmittance spectrum indicates the average transmittance higher than 90% in visible region. The optical band gap, Urbach energy and optical constants (refractive index, extinction coefficient, real and imaginary parts of the dielectric constant) of the film were determined. The electrical conductivity of the film dependence of temperature was measured to identify the dominant conductivity mechanism. The conductivity mechanism of the film is the thermally activated band conduction. The electrical conductivity and optical results revealed that the ZnO film is an n-type nanostructured semiconductor with a direct band gap of about 3.30 eV at room temperature.     

Microstructural and optical investigations of sol-gel derived ferroelectric BaTiO3 nanocrystalline films determined by spectroscopic ellipsometry

Ferroelectric BaTiO₃ nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO₂/Si substrates by a modified sol-gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5-5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO₃/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi-Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. It was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n(0) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262±2 and 268±1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.     

First-principles calculations of structural, elastic, electronic and optical properties of the antiperovskite AsNMg3

The density functional theory (DFT) calculations of structural, elastic, electronic and optical properties of the cubic antiperovskite AsNMg₃ has been reported using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). The equilibrium lattice, bulk modulus and its pressure derivative have been determined. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus and Poisson's ratio for ideal polycrystalline AsNMg₃ aggregate. We estimated the Debye temperature of AsNMg₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of AsNMg₃ compound, and it still awaits experimental confirmation. Band structure, density of states and pressure coefficients of energy gaps are also given. The fundamental band gap (Γ-Γ) initially increases up to 4 GPa and then decreases as a function of pressure. Furthermore, the dielectric function, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. The all results are compared with the available theoretical and experimental data.     

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.     

Optical characterization of poly (ethylene oxide)/zinc oxide thin films

The optical characterization of poly (ethylene oxide)/zinc oxide thin films has been done by analyzing the absorption spectra in the spectral wavelength region 380–800 nm using a ultraviolet-spectrophotometer at room temperature. Thin film polymer composites made of poly (ethylene oxide) (PEO) containing zinc oxide (ZnO) filler concentrations (0%, 2%, 6%, 10%, and 14%) by weight were used in this study. The optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The optical energy gap and other basic optical constants such as dielectric constants and optical conductivity were investigated and showed a clear dependence on the ZnO filler concentration. It was found that the optical energy gap for the composite films is less than that for the neat PEO, and that it decreases as the ZnO concentration increases. Enhancement of the optical conductivity was observed with increase in the ZnO concentration. Dispersion of refractive index was analyzed using the Wemple–DiDomenico single oscillator model. The refractive index (n), extinction coefficient (k), and dispersion parameters (E_o, E_d) were calculated for the investigated films.     

Characteristics of filtered vacuum arc deposited ZnO-SnO2 thin films on room temperature substrates

ZnO, SnO₂ and zinc stannate thin films were deposited using filtered vacuum arc deposition (FVAD) system on commercial microscope glass and UV fused silica substrates (UVFS) at room temperature (RT). The structural and morphological analyses were performed using X-ray diffraction (XRD) and Atomic Force Microscopy (AFM), respectively. XRD patterns of ZnO films deposited at RT had strongly c-axis orientation, whereas SnO₂ and zinc stannate films had amorphous structure as they did not have any defined patterns. Average crystalline size and surface grain size of ZnO films were ∼16 nm, as determined from diffraction line broadening and AFM images, respectively. Optical constants in the 250-1100 nm wavelength range were determined by variable angle spectroscopic ellipsometry and transmission measurements. The transmission of the deposited films in the VIS was 80-90%, affected by interference. The refractive indices and the extinction coefficients of deposited ZnO, SnO₂ and zinc stannate films were in the range 1.87-2.15 and 0.02-0.04, depending on wavelengths and deposition parameters. The optical band gap (E_g) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. Its values for ZnO, SnO₂ and zinc stannate were in the range 3.25-3.30 eV, 3.60-3.98 eV and 3.43-3.52 eV, respectively, depending on the deposition pressure.     

Structural, morphological and optical properties of CuAlS2 films deposited by spray pyrolysis method

The structural, morphological and optical properties of CuAlS₂ films deposited by spray pyrolysis method have been investigated. CuAlS₂ in the form of films is prepared at different deposition conditions by a simple and economical spray pyrolysis method. The structural, surface morphology and optical properties of the films were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and absorbance spectra, respectively. The films were polycrystalline, crystallized in a tetragonal structure, and are preferentially orientated along the (1 1 2) direction. Grain size values, dislocation density, and d% error of CuAlS₂ films were calculated. The optical band gap of the CuAlS₂ film was found to be 3.45 eV. The optical constants such as refractive index, extinction coefficient and dielectric constants of the CuAlS₂ film were determined. The refractive index dispersion curve of the film obeys the single oscillator model. Optical dispersion parameters E_o and E_d developed by Wemple-DiDomenico were calculated and found to be 3.562 and 12.590 eV.     

Spectroscopic study of nanocrystalline V2O5·nH2O films doped with Li ions

Optical properties of nanocrystalline, Li_xV₂O₅·nH₂O films (0<x<22 mol%), are explored in the present work. These films have been produced by the sol–gel technique (colloidal route), which was used for the preparation of high purity and homogeneity films. Optical measurements were carried out using a double-beam spectrophotometer. The optical constants such as refractive index n, the extinction coefficient k, absorption coefficient α, and optical band gap of the films material have been evaluated. The optical absorption coefficient was calculated from the measured normal reflectance, R, and transmittance, T, spectra. The optical spectra of all samples exhibited two distinct regions: at high energy, which suggests a direct forbidden transition with optical gap ranging from 1.75 to 2.0 eV and increases with increase in Li-content. On the other hand a second low-energy band suggests a direct allowed transition with optical gap ranging from 0.40 to 0.42 eV. The width of the localized states (band tail) E_e was also estimated for all samples. Additional calculations applying the real part of the optical dielectric function led to the evaluation of the charge carrier concentration and their effective mass.     

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.     

Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements

40 S₄₀Se₂₀, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc’s procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As₄₀S₄₀Se₂₀ thin films were also studied, using both transmission and reflection spectra. Received: 11 February 1998/Accepted: 16 February 1998     

Studies on the optical and dielectric properties of a zinc thiourea chloride NLO single crystal

Single crystals of a nonlinear optical material, zinc thiourea chloride were grown by the slow evaporation technique. The crystal structure and lattice parameters of the grown crystal were determined by the single crystal X-ray diffraction studies. The single crystal XRD revealed that the material crystallized in a orthorhombic crystal system. Optical studies were carried out and it was found that the tendency of transmission observed from the specimen, with respect to the wavelength of light, is practically more suitable for opto-electronic applications. The optical band gap is found to be 4.30 eV. Optical constants such as the band gap, refractive index, reflectance, extinction coefficient and real (?_r) and imaginary (?_i) components of the dielectric constant and electric susceptibility were determined from the UV–vis–NIR spectrum. The dielectric constant and dielectric loss of zinc thiourea chloride were measured in the different frequency range from 50 Hz to 5 MHz at different temperatures. Further, electronic properties, such as valence electron plasma energy, Penn gap, Fermi energy and electronic polarizability of the grown crystal have been estimated.     

Structural, electronic and optical properties of orthorhombic distorted perovskite TbMnO3

This paper calculates the structural parameters, electronic and optical properties of orthorhombic distorted perovskite-type TbMnO3 by first principles using density functional theory within the generalised gradient approximation. The calculated equilibrium lattice constants are in a reasonable agreement with theoretical and experimental data. The energy band structure, density of states and partial density of states of elements are obtained. Band structures show that TbMnO3 is an indirect band gap between the O 2p states and Mn 3d states, and the band gap is of 0.48 eV agreeing with experimental result. Furthermore, the optical properties, including the dielectric function, absorption coefficient, optical reflectivity, refractive index and energy loss spectrum are calculated and analysed, showing that the TbMnO3 is a promising dielectric material.     

Optical characterization of CuIn5S8 crystals by ellipsometry measurements

Optical properties of CuIn₅S₈ crystals grown by Bridgman method were investigated by ellipsometry measurements. Spectral dependence of optical parameters; real and imaginary parts of the pseudodielectric function, pseudorefractive index, pseudoextinction coefficient, reflectivity and absorption coefficients were obtained from the analysis of ellipsometry experiments performed in the 1.2–6.2 eV spectral region. Analysis of spectral dependence of the absorption coefficient revealed the existence of direct band gap transitions with energy 1.53 eV. Wemple–DiDomenico and Spitzer–Fan models were used to find the oscillator energy, dispersion energy, zero-frequency refractive index and high-frequency dielectric constant values. Structural properties of the CuIn₅S₈ crystals were investigated using X-ray diffraction and energy dispersive spectroscopy analysis.