Preparation and optical characterization of e-beam deposited cerium oxide films

Cerium oxide films, of 0.3–1 μm thickness, were reactively deposited in the oxygen atmosphere onto quartz plates by the PVD method. An electron gun was used as an evaporation source. Films were characterized with the AFM method, Raman spectroscopy and spectrophotometrically. Optical properties of these films were examined for the wavelength range 0.2–2.5 μm. Films were characterized by high transparency, between 0.38 and 2.5 μm. The complex refractive index, n^*=n − jk, was evaluated. The dispersion characteristics for n(λ) and k(λ) were presented. We found that the refractive index strongly depends on the temperature of substrates (300 K ⩽ T_s ⩽ 673 K) during film deposition. Estimated values of the refractive index (at λ = 0.55 μm) were in the range 1.91–2.34.     

Optical constants of thermally evaporated Se–Sb–Te films using only their transmission spectra

Amorphous Se_82 ? xTe₁₈Sb_x thin films with different compositions (x = 0, 3, 6 and 9 at.%) were deposited onto glass substrates by thermal evaporation. The transmission spectra, T(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Swanepoel has been applied to derive the optical constants and the film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing antimony content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 1.62 to 1.26 eV with increasing antimony content from 0 to 9 at.%. The chemical-bond approach has been applied successfully to interpret the decrease of the optical gap with increasing antimony content.     

Growth and characterization of indium oxide thin films prepared by spray pyrolysis

Highly transparent and conducting indium oxide thin films are prepared on glass substrates from precursor solution of indium chloride. These films are characterized by X-ray diffraction, scanning electron microscopy and optical transmission. The preferential orientation of these films is found to be sensitive to deposition parameters. A comparative study has been made on the dependence on the thickness of the film on substrate temperatures with aqueous solution and 1:1 C₂H₅OH and H₂O as precursors. Films deposited at optimum conditions have 167 nm thickness and exhibited a resistivity of 2.94 × 10⁻⁴ Ω m along with transmittance better than 82% at 550 nm. The analytical expressions enabling the derivation of the optical constants of these films from their transmission spectrum only have successfully been applied. Finally, the refractive index dispersion is discussed in terms of the single-oscillator Wemple and Didomenico model.     

Optical and dielectric properties of double helix DNA thin films

In this work, the thin film of wheat DNA was deposited by spin-coating technique onto glass substrate, and the optical and dielectric properties of the double helix DNA thin film were investigated. The optical constants such as refractive index, extinction coefficient, dielectric constant, dissipation factor, relaxation time, and optical conductivity were determined from the measured transmittance spectra in the wavelength range 190–1100 nm. Meanwhile, the dispersion behavior of the refractive index was studied in terms of the single oscillator Wemple–DiDomenico (W–D) model, and the physical parameters of the average oscillator strength, average oscillator wavelength, average oscillator energy, the refractive index dispersion parameter and the dispersion energy were achieved. Furthermore, the optical band gap values were calculated by W–D model and Tauc model, respectively, and the values obtained from W–D model are in agreement with those determined from the Tauc model. The analysis of the optical absorption data indicates that the optical band gap E_g was indirect transitions. These results provide some useful references for the potential application of the DNA thin films in fiber optic, solar cell and optoelectronic devices.     

Optical constants and band gap determination of Pb0.95La0.05Zr0.54Ti0.46O3 thin films using spectroscopic ellipsometry and UV–visible spectroscopy

We report the structural evolution and optical properties of lanthanum doped lead zirconate titanate (PLZT) thin films prepared on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition. X-ray diffraction demonstrates the post-deposition annealing induced crystallization for PLZT films annealed in a temperature (T_a) range of 550–750 °C. PLZT films annealed at higher temperature exhibit polycrystalline structure along with larger grain size. Optical band gap (E_g) values determined from UV–visible spectroscopy and spectroscopic ellipsometry (SE) for PLZT films were found to be in the range of 3.5–3.8 eV. E_g decreases with increasing T_a. The optical constants and their dispersion profiles for PLZT films were also determined from SE analyses. PLZT films show an index of refraction in the range of 2.46–2.50 (λ = 632.8 nm) with increase in T_a. The increase in refractive index at higher T_a is attributed to the improved packing density and crystallinity with the temperature.     

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

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

Influence of annealing on the optical properties of 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride thin films

Optical constants (refractive index, n, and absorption index, k) of the as-deposited and annealed films of 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) have been obtained in the wavelength range 190–2500 nm by using spectrophotometric measurements. The obtained optical constants were used to estimate the type of transition for the as-deposited and annealed films. We present a single oscillator model that describes the dispersion of refractive index. Drude model of free carriers absorption have been described for the analysis the dispersion of refractive index dispersion before and after annealing.     

Reflectance spectra and refractive index of a Nd:YAG laser-oxidized Si surface

The reflectance spectra and refractive index of Nd:YAG laser-oxidized SiO₂ layers with thicknesses from 15 to 75 nm have been investigated with respect to the laser beam energy density and substrate temperature. Thickness and refractive index of films have been determined from reflectance measurements at normal light incidence in the spectral range 300–800 nm. It was found that the oxide-growth conditions at higher substrate temperatures and laser powers greater than 3.36 J cm⁻² provides a better film quality in terms of both optical thickness and refractive index. However, the refractive indices of the films are smaller in the whole spectral range studied as compared to that of conventional thermally grown SiO₂. This might be due to the porous structure formed during the laser-assisted oxidation. The results suggest the need of post-oxidation annealing to improve the refractive indices of the films, suitable for Si-device applications.     

Ultraviolet–green photoluminescence of β-Ga2O3 films deposited on MgAl6O10 (1 0 0) substrate

β-Ga₂O₃ films were grown on double-side polished MgAl₆O₁₀ (1 0 0) substrate by metal organic chemical vapor deposition (MOCVD) at 600, 650 and 700 °C. The refractive index dispersive behaviors of Ga₂O₃ films have the typical shape of the normal dispersion curve. Photoluminescence (PL) spectra measured at room temperature revealed that all the films exhibited intense ultraviolet (UV)–green emission from 300 to 650 nm. A minor deep UV emission around 275 nm (∼4.51 eV) was observed for the sample prepared at 700 °C. The intensity of the emission increased markedly when measured at low temperature. The corresponding PL mechanisms were discussed in detail and a schematic diagram was proposed.     

Optical properties of hafnium oxide thin films and their application in energy-efficient windows

Thin films of hafnium oxide were deposited by electron beam evaporation. The films were characterized using X-ray diffraction, X-ray photoelectron spectroscopy and normal incidence transmittance. The films were amorphous, stoichiometric, and transparent down to a wavelength of 300 nm. The optical properties of the films, including the refractive index, the absorption index and the bandgap, were determined. The refractive index, in the visible, was relatively high (1.89). The direct bandgap was found to be 5.41 eV. Absorption was insignificant for wavelengths above 250 nm. A heat mirror was built based on the hafnium oxide/silver/hafnium oxide/glass system. This heat mirror was found to be transparent in the visible with an average transmittance of 72.4%, and reflective in the near infrared (wavelength = 700–2000 nm) with an average reflectance of 67.0%. Such a heat mirror can be used in applications involving energy-efficient windows.     

Influence of thermal annealing on optical properties and structure of aluminium oxide thin films by filtered cathodic vacuum arc

Optical and structural properties of aluminium oxide thin films are investigated in the annealing temperature range of 200–900 °C. The changes in optical properties and film structure show the great dependence on the temperature. For the film annealed at low temperatures (from 200 °C to 600 °C), the film optical properties, such as transmittance and optical constants, could be improved by thermal annealing with amorphous structure and smooth surface. However, for the film annealed at higher temperature (e.g. 900 °C), the poor performance of optical properties indicates undesirable application for precise use in optics due to significant changes in both structure and surface roughness. At optimum annealing temperature of 600 °C, the transmittance could reach as high as that of substrate and the film possesses better optical constants (refractive index was 1.73 and extinction coefficient was ∼10⁻⁴ at 550 nm) with remaining amorphous structure and smooth surface.     

Structural,optical and electrical characterization of ZnO:Ga thin films for organic photovoltaic applications

Gallium-doped zinc oxide (ZnO:Ga) films were prepared on glass substrates by RF magnetron sputtering. The effect of growth temperature on microstructure, optical and electrical properties of the films was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible spectrophotometer and four-point probe. The results show that all the films are polycrystalline and (002) oriented, and that the growth temperature significantly affects the microstructure and optoelectrical properties of the films. The film deposited at 670 K has the largest grain size of 71.9 nm, the lowest resistivity of 8.3 × 10^? 4 Ω?cm and the highest figure of merit of 2.1 × 10^? 2 Ω^? 1. Furthermore, the optical energy gaps and optical constants were determined by optical characterization methods. The dispersion behavior of the refractive index was also studied using the Sellmeir's dispersion model and the oscillator parameters of the films were obtained.     

Spectroscopic ellipsometry study of thin film thermo-optical properties

Spectroscopic ellipsometry (SE) was employed to realize in-situ monitoring and the determination of thermo-optic coefficients (TOC) of thin films by integrating a temperature controlled hot stage to the ellipsometer and applying the empirical relationship of Cauchy between the refractive index and wavelength in the data analysis. Magnetron sputtered titanium oxide thin films of 350 nm thick both as-deposited and post-deposition annealed were prepared on silicon wafers for this investigation. Results of ellipsometric analysis show that as-deposited TiO₂ films have a negative TOC of ? 1.21 × 10^? 4 K^? 1 at 630 nm over the test temperature range 304–378 K. The post-deposition annealing at 923 K for 2 hours leads an increase in film refractive index to 2.29 from 2.17 for as-deposited TiO₂ films, and an enhancement in TOC up to ? 2.14 × 10^? 4 K^? 1. X-ray diffraction (XRD) and scanning electron microscopy (SEM) cross-sectional analysis were performed for film structure characterization.     

New natively textured surface Al-doped ZnO-TCOs for thin film solar cells via magnetron sputtering

Natively textured surface aluminum doped zinc oxide (ZnO:Al) thin films were directly deposited via pulsed direct current (DC) reactive magnetron sputtering on glass substrates. During the reactive sputtering process, the oxygen gas flow rate was varied from 8.5 sccm to 11.0 sccm. The influences of oxygen flow rate on the structural, electrical and optical properties of naturally textured ZnO:Al TCO thin films with milky surface were investigated in detail. Gradual oxygen growth (GOG) technique was developed in the reactive sputtering process for textured ZnO:Al thin films. The light-scattering ability and optical transmittance of the natively textured ZnO:Al TCO thin films can be improved through gradual oxygen growth method while maintaining a low sheet resistance. Typical natively textured ZnO:Al TCO thin film with crater-like surface exhibits low sheet resistance (R_s ～ 4 Ω), high transmittance (T_a > 85%) in visible optical region and high haze value (12.1%).     

Irreversible photobleaching,photorefraction and photoexpansion in GeS2 amorphous film

The photoinduced irreversible effects in thermally evaporated amorphous GeS₂ films were investigated. The magnitude of the photobleaching induced by gap photons was found at around ≈13%. The photoinduced increase in the optical band gap and the increase in the slope of the short wavelength absorption edge are discussed within the Davis–Mott model and within Tauc's model for optical transitions in amorphous solids. Based on the optical measurements and the Atomic Force Microscopy we also observed photoexpansion at around ≈4.8% accompanied by a decrease in the refractive index from n = 2.02 for the virgin film to n = 1.94 for a film illuminated until a nearly steady state.     

On the optical constants of amorphous GexSe1−x thin films of non-uniform thickness prepared by plasma-enhanced chemical vapour deposition

The preparation of layers of amorphous Ge_xSe_1−x (with Ge atomic concentrations x=0, 0.17, 0.25 and 0.34) by plasma-enhanced chemical vapour deposition (PECVD) using the hydrides, GeH₄ and H₂Se, as precursor gases is described in detail. Information concerning the structure of the films was obtained from Raman spectroscopy. The optical transmission was measured over the 300 to 2500 nm spectral region in order to derive the refractive index and extinction coefficient of these PECVD films. The expressions proposed by Swanepoel, enabling the calculation of the optical constants of a thin film with non-uniform thickness, have successfully been applied. The refractive-index dispersion data were analysed using the Wemple–DiDomenico single-oscillator fit. The optical-absorption edges have been all of them described using the `non-direct transition' model proposed by Tauc. The optical gaps were calculated using Tauc's extrapolation, resulting in values ranging from 1.93 eV for a-Se to 2.26 eV for a-GeSe₂.     

Porous titania films fabricated via sol gel rout – Optical and AFM characterization

Mesoporous titania films of low refractive index ∼1.72 and thickness within the range of 57–96 nm were fabricated via sol–gel rout and dip-coating technique on a soda–lime glass substrate. Tetrabutylorthotitanate Ti(OBu)₄ was used as a titania precursor. High porosity and consequently low refractive index were achieved using the polyethylene glycol (PEG 1100) as a template. Based on transmittance, using Tauc’s relations, the optical energy band gaps and the Urbach energy were determined. The research shows that in the fabricated titania films there are two types of optical energy band gaps, connected with direct and indirect electron transitions and brought about by the presence of amorphous and crystalline phase respectively. Based on the quantum size effect, the diameters of nanocrystals versus film thickness were determined. AFM studies of the titania films have demonstrated that there are changes of surface morphology taking place with the change of thickness. We have demonstrated that the surface morphology of titania films has influence on wettability.     

A promising birefringent crystal Ba2Na3(B3O6)2F

Bulk crystals of Ba₂Na₃(B₃O₆)₂F (BNBF) have been successfully grown by top-seeded solution growth (TSSG) technique. Its transmittance spectra show a wide transparency range from 186 nm to 3000 nm. The refractive indices in 13 wavelengths were measured with high accuracy and the Sellmeier equations were obtained, which demonstrated that the title crystal displayed a birefringence (Δn = 0.1030 at 588 nm) comparable to that of the commercial birefringent crystal α-BBO (the high temperature form of BaB₂O₄). A prototype Glan-Taylor polarizer made of BNBF prisms was fabricated, which showed high transparency and large optical extinction ratio similar to the commercial polarizer made of α-BBO. In addition, BNBF crystal is less moisture sensitive than that of α-BBO, thus BNBF can be a potential new birefringent crystal.     

Determination of the surface roughness and refractive index of amorphous As40S60 films deposited by spin coating

An envelope method, based on the optical reflection spectrum taken at normal incidence, has been successfully applied to the geometrical–optical characterization of thin dielectric films having significant surface roughness. Such a method allows the determination of the average thickness and the refractive index of the films with accuracies better than 1%, as well as the average amplitude of the surface roughness with an accuracy of about 2%. Amorphous As₄₀S₆₀ thin films have been deposited by spin coating, onto glass substrates, from a solution of the bulk material in n-propylamine. Indications of the surface roughness in these films were found from total (specular plus diffuse) reflectance measurements using an integrating sphere, and also from mechanical measurements using a stylus profiler. The latter technique provided a value for the average surface roughness of 20 ± 4 nm, which is in excellent agreement with the optically determined value of 17.4 ± 0.4 nm.     

Spectroscopic ellipsometry and magneto-transport investigations of Mn-doped ZnO nanocrystalline films deposited by a non-vacuum sol–gel spin-coating method

Nanocrystalline Zn_1?xMn_xO films (x = 0, 0.05, 0.1, 0.15, and 0.2) were deposited onto corning glass substrates by a non-vacuum sol–gel spin coating method. All of the films were annealed at 450 °C for 2 h. The structural, optical and magneto-transport properties were investigated by X-ray diffraction, spectroscopic ellipsometry and a system for the measurement of the physical properties. X-ray diffraction analysis of the films reveals that the Mn-doped ZnO films crystallize in the form of a hexagonal wurtzite-type structure with a crystallite size decreases with an increase of the Mn concentration. It was also found that the microstrain increases with the increase of the Mn content. Evidence of nanocrystalline nature of the films was observed from the investigation of surface morphology using transmission, scanning electron microscopy and atomic force microscopy. The optical constants and film thicknesses of nanocrystalline Zn_1?xMn_xO films were obtained by fitting the spectroscopic ellipsometric data (ψ and Δ) using a three-layer model system in the wavelength range from 300 to 1200 nm. The refractive index was observed to increase with increasing Mn concentration. This increase in the refractive index with increasing Mn content may be attributed to the increase in the polarizability due to the large ionic radius of Mn²⁺ compared to the ionic radius of Zn²⁺. The optical band gap of the nanocrystalline Mn–ZnO films was determined by an analysis of the absorption coefficient. The direct transition of the series of films was observed to have energies increasing linearly from 3.17 eV (x = 0) to 3.55 eV (x = 0.2). Magnetoresistance (MR) was measured from 5 K to 300 K in a magnetic field of up to 6 T. Low-field positive MR and high-field negative MR were detected in Mn-doped ZnO at 5 K. Only negative MR was observed for temperatures above 200 K. The positive MR in Mn-doped ZnO films was observed to decrease drastically when the temperature increased from 5 K to 100 K. The isothermal MR of Zn_1?xMn_xO films with different Mn concentrations at 5 K reveals that the increase of the Mn content induces a giant positive MR above x = 0.05 and reaches up to 55% at an applied field of 30 kOe for x = 0.2.