Description of TiO2 thin films treated in NH3 atmosphere by optical dispersion models

The influence of a reductive ammonia atmosphere on TiO₂ sol-gel film structural and optical characteristics was investigated. X-ray Diffraction technique, X-ray photoelectron spectroscopy and Rutherford Back Scattering analysis were applied to study the crystallinity, oxidation state and element concentration profile of the modified films. Their optical properties were investigated by UV-Vis spectroscopy. The refractive index and extinction coefficient were obtained by fitting theoretical transmittance curves to experimental ones using Forouhi-Bloomer (FB) and Tauc-Lorentz (TL) physical models. Both models revealed slight decrease (up to 2.6-2.7 eV) of the FB and TL band gaps with increase of the treatment temperature. These results were discussed in terms of the additional energy levels creation due to the defect TiO₂ structure formation during thermal treatment in reductive atmosphere.     

Comparison of different dispersion models for single layer optical thin film index determination

We here determine the optical properties of different single-layer thin films containing Ta₂O₅, Si, Indium Tin Oxide and Au in the ultraviolet-visible and near infrared ranges. More specifically, we deduce the complex refractive index and thickness from the reflectance and transmittance measured using a spectrophotometer at normal incidence. One major difficulty is to find an appropriate selection of dispersion laws for various types of material (dielectric, semiconductors, and metals). For this purpose, a number of models have been investigated from a theoretical point of view in consideration of the Kramers-Kronig relation. These include the Forouhi-Bloomer model, combined with the modified Drude, Tauc-Lorentz and multiple-oscillator Tauc-Lorentz models. A global optimization procedure had to be employed because of the large number of parameters (from 3 to 15) required to describe the optical dispersion laws. The calculated reflectance and transmittance are in good agreement with experimental data and the complex refractive index is consistent with our knowledge and that already reported.     

Influence of Relevant Gas Pressure on the Properties of Ionplated Ta2O5 Films. Einfluss relevanter Gasdrücke auf die Eigenschaften ionenplattierter Ta2O5 Schichten

Ta₂O₅ films were deposited onto unheated fused silica substrates (Suprasil®) by reactive low voltage ion plating (RLVIP). From these films of about 200 nm thickness the optical properties (refractive index n and the absorption coefficient k) and also the mechanical properties (density ρ and intrinsic stress σ) were investigated in dependence of the working gas pressure (Ar) and the reactive gas pressure (O₂). The experiments show a reasonable correlation between refractive index, density and intrinsic stress of the films. With low total pressure high refractive indices (up to n₅₅₀=2.25), high compressive film stress and high relative film density were found. However the film density, the refractive index and also the intrinsic stress decreased with films prepared under raising total gas pressure. The optical absorption depends on the amount of oxygen in the gas phase during deposition. By adding more oxygen to the Ar/O₂ gas mixture primarily the absorption could clearly be decreased.     

Thermal treatment effect on the optical properties of ZrO2 thin films deposited by thermionic vacuum arc

This paper shows the ex situ thermal treatment effects of the ZrO₂ thin films obtained by TVA (thermionic vacuum arc) technique on the optical properties (e.g., transmittance, refractive index and band-gap energy) of ZrO₂ thin films. The crystal structure, surface and optical properties were investigated for ZrO₂ thin films deposited on glass substrates by thermionic vacuum arc (TVA) method. The thermal treatment effect on the optical properties of the thin films was determined. The XRD analysis showed that the deposited ZrO₂ thin films have baddeleyite (monoclinic) and zirconium (hexagonal) structures. The thicknesses and refractive index were determined by interferometric measurements. The thin films were thermal treated at different temperatures (350 °C, 450 °C and 550 °C), and the analysis showed that the optical properties of ZrO₂ deposited thin films were improved by thermal treatment at 450 °C.     

Thermal annealing effect on the structure, bandgap, and optical constants of Er-Mn oxide thin films

Thin films of erbium-manganese oxide were grown on glass and p-type Si substrates. The films were thermally pre-annealed at different temperatures ranging from 400 to 1000 °C to produce different crystalline structures and agitate a solid-state reaction. The structural characterisation of the films was carried out by X-ray diffraction (XRD) and energy dispersion X-ray fluorescence (XRF). The XRD investigation shows that the films annealed at 400 °C were amorphous and nanocrystals of ErMnO₃ appear under pre-annealing at about 800 °C or more. Mn oxide and Er oxide prevent each other from crystallising alone. The optical properties of the films pre-annealed at different temperatures were studied in the fundamental absorption region of the spectrum in wavelength range 230-800 nm. The spectral complex refractive index, complex optical dielectric constant, and optical bandgap were determined. A modified single-oscillator Forouhi-Bloomer (FB) technique, Wemple-Didomenico (WD) equation, Urbach's relation, Tauc et al. relation, and pointwise unconditioned minimisation approach (PUMA) were used in the analysing of the obtained spectral data.     

The optical properties of amorphous V2O5 and SiO thin films and of the mixed dielectric system SiO/V2O5

The optical absorption of amorphous thin films of V₂O₅, SiO and of SiO/V₂O₅ is studied in the photon energy range 0.42 to 6.53 eV. The optical absorption edge of evaporated V₂O₅ films can be described by direct forbidden transitions while that of SiO films follows the non-direct transitions in k-space. The data of the SiO/V₂O₅ oxide mixtures are fitted to new values of the exponent in the well-known absorption equation and the corresponding optical band gaps are determined. Experimental data on the wavelength dependence of the refractive index of SiO films are presented. The dispersion of the refractive index follows a single oscillator model. The infrared spectra show that some bonding occurs between the two oxides so that the mixed dielectric system SiO/V₂O₅ cannot be considered as a simple physical mixture.     

Optical properties of TiO2 thin films deposited on polycarbonate by ion beam assisted evaporation

TiO₂ thin films were deposited on polycarbonate (PC) substrate by ion beam assisted evaporation. The grain size increased with the ion anode voltage and film thickness. The TiO₂ thin films had an amorphous structure. Moiré deflectometry was used to measure the nonlinear refractive indices of TiO₂ thin films on PC substrates. The nonlinear refractive index was measured to be of the order of 10^− 8 cm² W^− 1 and a change in refractive index was of the order of 10^− 5. Dense TiO₂ films exhibited high linear refractive indices, red-shift of the optical absorbance, and absorbance in the near-IR region.     

Comparison of the characteristics of TiO2 films prepared by low-pressure and plasma-enhanced chemical vapor deposition

Thin films of titanium dioxide (TiO₂) were deposited on silicon wafers at 450 °C by low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD), in the same reactor, using Ti(O-i-C₃H₇)₄ and oxygen as the reactants. The crystal structure, surface morphology, composition, chemical binding state and refractive index of the TiO₂ films, deposited with various r.f. powers, were characterized. Without plasma, LPCVD TiO₂ films show polycrystalline anatase structure, with a rough, granular surface and a typical refractive index of 2.43. On the contrary, the PECVD TiO₂ films are amorphous, with a smooth surface. By varying the r.f. power from 50 to 150 W, the refractive index of PECVD TiO₂ film increases with increasing power and becomes higher than 2.8 at 125 and 150 W. Despite the very different crystal structure, surface morphology and optical properties, the composition and chemical binding states of LPCVD and PECVD TiO₂ films are similar. The effect of plasma on the characteristics of CVD TiO₂ films is discussed.     

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.     

Optical dispersion of homogeneously mixed ZnSMgF2 films

Mixed films of ZnSMgF₂ were prepared by co-evaporation from a single source. The dispersion of the optical constants was obtained from the reflectance and transmittance data of the films for various compositions in the spectral range 2000–7000 Å. The values of the optical constants of the mixed films at any wavelength lie between the values of the optical constants of pure ZnS and MgF₂ films. The refractive index of the mixed films closely obeys the Lorentz-Lorenz relation at wavelengths at which the absorption is small. The fundamental absorption edge of the mixed films shifts from that of MgF₂ to that of ZnS continuously but non-linearly on increasing the concentration of ZnS in MgF₂. Thus it is possible to use these films as variable frequency cut-off filters. The fundamental absorption edge of the mixed films follows the (hv-E_g)² relation, indicating an indirect optical transition, in sharp contrast to the direct optical transition observed in bulk ZnS.     

Substrate bias voltage influenced structural, electrical and optical properties of dc magnetron sputtered Ta2O5 films

Tantalum oxide (Ta₂O₅) films were formed on silicon (111) and quartz substrates by dc reactive magnetron sputtering of tantalum target in the presence of oxygen and argon gases mixture. The influence of substrate bias voltage on the chemical binding configuration, structural, electrical and optical properties was investigated. The unbiased films were amorphous in nature. As the substrate bias voltage increased to −50 V the films were transformed into polycrystalline. Further increase of substrate bias voltage to −200 V the crystallinity of the films increased. Electrical characteristics of Al/Ta₂O₅/Si structured films deposited at different substrate bias voltages in the range from 0 to −200 V were studied. The substrate bias voltage reduced the leakage current density and increased the dielectric constant. The optical transmittance of the films increased with the increase of substrate bias voltage. The unbiased films showed an optical band gap of 4.44 eV and the refractive index of 1.89. When the substrate bias voltage increased to −200 V the optical band gap and refractive index increased to 4.50 eV and 2.14, respectively due to the improvement in the crystallinity and packing density of the films. The crystallization due to the applied voltage was attributed to the interaction of the positive ions in plasma with the growing film.     

Preparation, Phase Composition, and Optical Properties of Thin ZrO2–Y2O3 Films

ZrO₂–Y₂O₃ films were prepared over the entire composition range. Their phase composition was determined, and the conditions for the formation of cubic ZrO₂-based solid solutions were established. The optical properties of the films were found to depend on their thickness and phase composition and the nature of the substrate. The composition dependence of the refractive index was shown to correlate with the phase composition of the films.     

Third-order optical nonlinearities in anatase and rutile TiO2 thin films

Titanium dioxide (TiO₂) films have been fabricated on fused quartz and Si(001) substrates by pulsed laser deposition technique and the single-phase anatase and rutile films were obtained under the optimal conditions. The surface images and optical transmission spectra were investigated by scanning electron microscopy and double beam spectrophotometer, respectively. The values of optical band-gap and linear refractive index of the anatase and rutile films were determined. The optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. Through the open-aperture and closed-aperture Z-scan measurements, the real and imaginary parts of the third-order nonlinear optical susceptibility were calculated and the results show that the anatase phase TiO₂ films exhibit larger nonlinear refractive effects compared with rutile phase. The figure of merit, T, defined by T = βλ/n₂, was calculated to be 0.8 for anatase films, meeting the requirement of T < 1 and showing potential applications in all-optical switching devices.     

Sorption and optical properties of sol-gel thin films measured by X-Ray Reflectometry and Ellipsometric Porosimetry

Oxide thin films synthesized using the sol-gel technique have the advantages of low cost, high thickness control, tunable refractive index and silicon technology compatibility, properties that make them potential materials for optoelectronic applications. For very thin films with low porosity, the determination of sorption and optical properties is quite complex because of the small sample size. Thus, there is a need to use especially designed techniques to obtain reliable results. In this work, a comprehensive study on the porosity evolution of SiO₂ and TiO₂ thin films using X-Ray Reflectometry and Environmental Ellipsometric Porosimetry is presented. For sol-gel SiO₂ thin films, it was found that the effective refractive index increases with thermal treatment as the porosity decreases. However, the refractive index of the walls was found constant. For sol-gel TiO₂ films, crystallized in anatase phase, both the effective refractive index and the wall refractive index increase with thermal treatment. SiO₂ and TiO₂ thermal oxides were also characterized for comparison.     

Determination of optical constants and thicknesses of In2O3:Sn films from transmittance data

Indium tin oxide (ITO) thin films were deposited on quartz substrates by direct current magnetron sputtering and annealed in N₂ and air. The normal incidence transmittance of the films was measured by a spectrophotometer. The electrical parameters such as carrier concentration, mobility and resistivity were investigated by van der Pauw method. An optical model has been proposed to simulate the optical constants and thicknesses of the films from transmittance data, which combines the Forouhi-Bloomer model and modified Drude model. The relaxation energy in the Drude term is taken as energy-dependent for a better fitting in the visible spectral range. The simulated transmittance is in good agreement with the measured spectrum in the whole measurement wavelength range. The electrical parameters obtained from the optical simulation are well consistent with those measured electrically by van der Pauw method. The experimental results also indicate that the different post-deposition annealing treatments yield the distinct optical and electrical properties of ITO films.     

Photo-induced changes in optical properties of As2S3 and As2Se3 films deposited at normal and oblique incidence

Photostructural transformations in amorphous chalcogenide films have been a subject of intensive research so far. In this paper we discuss the changes in the optical properties of typical As-based chalcogenide glasses (As₂S₃ and As₂Se₃) on exposure to ultraviolet (UV) light. An attempt has been made to systematically investigate the optical parameters like extinction coefficient, refractive index and optical bandgap of the films by measuring the same for as-grown and UV-exposed amorphous films of As₂S₃ and As₂Se₃ prepared by vacuum evaporation technique.     

Optical and morphological properties of silicon dioxide thin films

The purpose of the present work is to compare the structural and optical properties of the Silicon dioxide films obtained from TMOS Si(OCH₃)₄), N₂O, and NH₃ as precursor gases, which with respect to their potential optical applications, have been deposited by using inductively coupled plasma-enhanced chemical-vapor deposition method. The optical property as well as the thickness of the films were analyzed by means of variable angle spectroscopic ellipsometry. Morphological studies were carried out by scanning electron microscopy, and chemical composition characterization was performed with the help of energy dispersive spectroscopy unit coupled with the electron microscope. The type of the substance and the precursor composition used for silicon dioxide synthesis are effective on the chemical composition of the films. The refractive index values of these films advocate their use as high refractive index materials while their low extinction coefficients assure the devices transparency. The work presents deposition rates as well as the films optical properties, chemical composition and morphology regarding the operational parameters of their synthesis. It also provides a comparison of the characteristics of the two competitive precursor compounds.     

Optical properties of In2O3 films prepared by spray pyrolysis

In₂O₃ thin films have been deposited on glass substrates by spray pyrolysis. InCl₄ was used as the solute to prepare the starting solution with a concentration of 0.1 M. The films were grown at different substrate temperatures ranging from 300 to 400 °C. The as-grown layers were optically characterized in order to evaluate the absorption coefficient, optical band gap, refractive index, extinction coefficient and other optical parameters. The influence of substrate temperature on these parameters was reported and discussed.     

Optical properties of LPCVD silicon oxynitride

Low pressure chemical vapour deposition (LPCVD) silicon oxynitride films of various compositions (from pure SiO₂ to pure Si₃N₄) were deposited by changing the relative gas flow ratio. The effects of oxygen on the physical properties of the films were studied by spectroellipsometry (using Bruggeman approximation and Wemple Di Domenico model) and infrared spectroscopy. Refractive index measured by spectroellipsometry method is studied as a function of some deposition parameters: temperature of deposition, gases fluxes ratio. The high value of deposition temperature means low values in refractive index. More oxygen into films decreases the refractive index. The refractive index dispersion is studied by single-oscillator Wemple Di Domenico model. The optical band gap varies monotonically from 5 eV for silicon nitride, to 9eV for HTO LPCVD silicon dioxide and for the studied silicon oxynitride was found to be between 5 and 6 eV.     

Effect of substrate temperatures on the optical properties of evaporated Sc2O3 thin films

Scandium oxide (Sc₂O₃) films were deposited by electron beam evaporation with substrate temperatures varying from 50 to 350 °C. X-ray diffraction, scanning electron microscopy, spectrometer, and optical profilograph were employed to investigate the structural and optical properties of the films. The refractive index and extinction coefficient were calculated from the transmittance and reflectance spectra, and then the energy band gaps were deduced and discussed. Laser induced damage threshold of the films were also characterized. Optical and structural properties of Sc₂O₃ films were found to be sensitive to substrate temperature.