Optical constants of vacuum-evaporated Cd0.96Zn0.04Te thin films measured by spectroscopic ellipsometry

The pseudodielectric-function spectra, (E)=₁(E)+i₂(E), of polycrystalline Cd_0.96Zn_0.04Te thin films in the 1.3–5.5 eV photon energy range at room temperature were obtained using spectroscopic ellipsometry. The measured dielectric-function spectra reveal that distinct structures at energies of the E₁, E₁+₁, and E₂ critical points are due to interband transitions. The Cd_0.96Zn_0.04Te thin films investigated were deposited by vacuum evaporation under a pressure better than 1.3×10^–3 Pa onto well-cleaned glass substrates kept at 300 K. The films exhibited zinc blende structure with predominant (1 1 1) orientation. The root mean square (r.m.s.) roughness of the vacuum-evaporated Cd_0.96Zn_0.04Te thin films evaluated by ex situ atomic force microscopy is 3.7 nm. Dielectric-related optical constants, such as the refractive index (n), extinction coefficient (k), absorption coefficient () and normal incidence of reflectivity (R) determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the films were also determined using optical transmittance measurements and the results are discussed.     

Effect of cerium oxide addition on the optical absorption edge of tungsten trioxide

Thin films of WO₃/CeO₂ deposited by vacuum co-evaporation (10^–6 torr) on to Corning 7059 glass slides at room temperature have been shown to be amorphous in structure. The optical absorption spectra of amorphous WO₃/CeO₂ thin films have been measured at room temperature. The optical absorption edge of WO₃/CeO₂ and WO₃ thin films above 10⁵ cm^–1 follows the Davis and Mott equation for the non-direct optical transitions. The optical absorption coefficient, , below this obeys the Urbach exponential relation. The value of the optical band gap of amorphous WO₃ thin films is in good agreement with that found by Deb. The shift of the optical gap to lower energies and that of the Urbach energies to higher energies as the content of CeO₂ is increased, indicate that both parameters are strongly dependent on the concentration of Ce⁴⁺ present in the structure. Thus, the increase of Ce⁴⁺ is thought to lead to strong potential fluctuations which cause an increase in the density of localized states in the gap.     

Characterization of thermally evaporated AgGaTe<Subscript>2</Subscript> films grown on KCl substrates

Silver gallium telluride (AgGaTe₂) films have been grown by thermal evaporation technique onto the KCl substrates kept at different temperatures (483–563 K) in a vacuum of 1.3 × 10^–3Pa. The experimental conditions were optimised to obtain better crystallinity of the films. The films so prepared have been studied for their structural, optical and electrical properties. Observations reveal that the crystallinity of the films increases with increase in substrate temperature. Average crystallite size of 0.2–0.5 m has been observed in case of films deposited at 563 K. Analysis of optical spectra of the films in the range 300–1100 nm show an allowed direct transition near the fundamental absorption edge (E_g1) in addition to a transition originating from crystal field split levels (E_g2). It has been observed that the carrier concentration and Hall mobility of films increases with increase in substrate temperature.     

Structural and optical properties of Bi x Se1−x films

Bi _x Se_1–x thin films have been studied because of their structural and optical properties with a view to judging their suitability as the recording medium in phase-change type optical recording. Amorphous films deposited at room temperature were crystallized by thermal annealing. X-ray diffraction analysis and surface morphological studies are reported. A maximum reflectivity difference of 25% at =830 nm was obtained upon amorphous-to-crystalline transition. The optical constants calculated by the Newton-Raphson method using the experimental transmittance, reflectance and thickness data are reported.     

Structural and optical studies on ortho-hydroxy acetophenone azine thin films

The bulk material and deposited thin films of ortho-hydroxy acetophenone azine (o-HAcPhAz) were identified by X-ray diffraction (XRD) to be single phase polycrystalline of the monoclinic structure. The unit cell lattice constants were determined to be a = 1.578 nm, b = 1.394 nm and c = 0.64 nm, as well as its plane angles as = 90°, = 123.8° and = 90°. FTIR spectra of the bulk and thin films of the compound under investigation were assigned in the wave number range 4000–500 cm^–1. It was revealed that they were similar to each other. The optical constants (the refractive index, n, and the extinction coefficient, k) of the compound thin films were determined from the measured transmittance, T, and reflectance, R, at normal incidence of light in the spectral range 200–2100 nm. The plot of the absorption coefficient () versus h, gave three intense bands and a shoulder which were designated as A (232.3 nm), B (299.5 nm) and C (440.6 nm, 404.8 nm, sh). The observed A and B bands were attributed to * transitions, while the C-bands were attributed to * (n). The optical high frequency dielectric constant (₁, ₂) as well as the real and imaginary parts of the optical conductivity (₁, ₂) were determined. The plots of both ₁, ₂, ₁ and ₂ versus h reveal the same obtained optical transitions. Also, the relationships between both of surface, volume and surface/volume energy losses against h gave the same optical transtions.     

On the optical properties of Mn/SiO and SiO thin films

The ultraviolet, visible and infrared properties of unannealed and annealed amorphous Mn/SiO cermet thin films (300 to 1000 nm thick) prepared by vacuum evaporation at 5.0×10^–4 Pa are investigated. The ultraviolet and visible results are analysed assuming optical absorption by indirect transitions. A systematic reduction of the optical energy gap and an increase in the width of the band-tail region is observed with increasing metallic content. The effects of the ratio deposition rate/residual pressure and substrate temperature on the optical properties of SiO _x (1<x<2) thin films are also investigated.     

Structural,electrical and optical properties of copper selenide thin films deposited by chemical bath deposition technique

A low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu_2–xSe thin films on glass substrates. Structural, electrical and optical properties of these films were investigated. X-ray diffraction (XRD) study of the Cu_2–xSe films annealed at 523 K suggests a cubic structure with a lattice constant of 5.697 Å. Chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). It reveals that absorbed oxygen in the film decreases remarkably on annealing above 423 K. The Cu/Se ratio was observed to be the same in as-deposited and annealed films. Both as-deposited and annealed films show very low resistivity in the range of (0.04–0.15) × 10^–5 -m. Transmittance and Reflectance were found in the range of 5–50% and 2–20% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of 10⁸ m^–1. The band gap for direct transition, E_g.dir varies in the range of 2.0–2.3 eV and that for indirect transition E_g.indir is in the range of 1.25–1.5 eV.     

Optical constants of the nano-crystal and polymer composite thin film PbTiO3/PEK-c

Composite thin films of PbTiO₃ nano-crystals and high transparency PEK-c polymer were prepared by spin coating. The size of PbTiO₃ nanocrystals was evaluated to be 30–40 nm. The transparency spectra of PEK-c composite thin film in 360–800 nm were measured. The optical constants n, k of the film in the wavelength range 400–620 nm were investigated by the transmission spectrum. The dispersion of refractive index fits well to a three-term Sellmeier relation. At 633 nm wavelength the refractive index of PbTiO₃/PEK-c film was measured to be 1.6628 by a prism coupling method and showed a good agreement with the calculated value using the Sellmeier relation.     

Chemical solution method for fabrication of nanocrystalline iron(III) oxide thin films

A chemical solution technique for preparation of nanocrystalline iron(III) oxide thin films is developed. The deposition process is essentially based on the thermal decomposition of urea. The as-deposited and post-deposition heat-treated materials were characterized by X-ray analysis and Fourier transform infrared (FTIR) spectroscopy. Basic optical and electrical investigations were also performed. X-ray analysis confirmed that post-deposition heat-treated material is nontextured -iron(III) oxide, with an average crystal size of 22 nm. The optical investigations show that the absorption of films (as-deposited and post-deposition treated) gradually decreases with an increase of the wavelength in the 390–820 nm region. The optical band gap for the as-deposited and post-deposition heat-treated films was determined to be 3.2 eV and 2.0 eV, respectively. The obtained -Fe₂O₃ thin films exhibit a rather high resistivity at room temperature. However, our preliminary qualitative investigations have shown that the room temperature resistivity of -Fe₂O₃ thin films is highly sensitive to moisture, indicating their potential applicability in moisture sensing systems.     

Study of electrical and optical properties of Cd1 − x Zn x S thin films

Thin films of Cd_0.9Zn_0.1S and CdS were prepared by thermal evaporation under vacuum of 10^–6 Torr and with deposition rate of 60 nm/min. X ray diffraction studies confirm the hexagonal structure of both CdS and Cd_0.9Zn_0.1S films. The effect of heat treatments with or without CdCl₂ enhances the grain size growth and improves the crystalline of the films. Moreover, the activation energy is decreased by heat treatment with or without CdCl₂ for all thin films. The optical absorption coefficient of Cd_0.9Zn_0.1S thin films were determined from measured transmittance and reflectance in the wavelength range of 300 to 2500 nm. The optical absorption spectra reveal the existence of direct energy gap for these films. It was found that the optical energy gap decreases upon annealing or CdCl₂ treatments.     

Electrical conductivity and optical absorption in flash-evaporated CuInTe2 thin films

The flash evaporation technique produces thin films of stoichiometric CuInTe₂. From an analysis of the optical absorption spectra of the thin films, the band gap of CuInTe₂ is found to be 1.03 eV. The effect of the background absorption on the absorption spectrum is found to be negligible. The electrical conductivity of these films is limited by hopping conduction in the temperature range 77–200 K. Acceptor levels at 100 meV above the valence band maximum were observed and attributed to copper vacancies.     

Changes in the physicochemical and optical properties of chalcogenide thin films from the systems As—S and As—S—TI

Changes in some physical and optical properties of thermally evaporated thin films from the systems As—S and As—S—Tl over a wide range of concentrations have been investigated. The influence of the conditions of vacuum deposition and light exposure has been demonstrated. The optical transmission and reflection of thin layers deposited on BK-7 optical substrates have been measured in the spectral region of 350–2000 nm and the linear (n) and nonlinear (n ₂) refractive indices and optical band-gap, E _g, as well as the oscillator fitting constants, were calculated. Boling's formula is used to predict n ₂ from the dispersion and the magnitude of n. Data for changes in the glass-transition temperature, T _g, microhardness, and rate of dry etching of thin chalcogenide films before and after exposure to light are presented. The addition of Tl in As₂S₃ leads to an increase in the refractive index and decrease in the optical band-gap. After illumination a photodarkening or photobleaching effect was observed depending on the evaporation conditions. Some of the layers change their etching rate in a plasma which make them suitable for practical applications. Conclusions on the homogeneity of the layers and the origin of photostructural changes in them are drawn.     

A.c. photoconductivity and optical properties of bulk polycrystalline and amorphous In x Se1−x thin films

The a.c. photoconductivity of bulk polycrystalline In _x Se_1–x and the optical properties of amorphous In _x Se_1–x thin films were investigated. The effect of heat treatment on the absorption coefficient of InSe thin films was also studied. The spectral response of the photoconductivity of polycrystalline In _x Se_1–x related to the near-edge absorption spectrum, shows a well-defined maximum corresponding to the width of the forbidden gap. Polycrystalline In _x Se_1–x realizes the behaviour of photoconductors which are sensitive in a spectral range between 400 and 1700 nm. Optical absorption of amorphous In _x Se_1–x thin films was recorded as a function of photon energy and the data were used to deduce the value of optical energy gap of the films. It was found that the optical energy gap decreased with increasing indium content in both bulk polycrystalline and amorphous thin films. The effect of heat treatment on the optical gap of the film has been interpreted in terms of the density of states model of Mott and Davis.     

Composition and electronic properties of a-SiGe:H alloys produced from ultrathin layers of a-Si:H/a-Ge:H

Hydrogenated amorphous silicon-germanium (a-SiGe:H) alloys were prepared from a-Si:H (0.5 nm)/a-Ge:H(0.4–1.26 nm) multilayers at 200 °C by the r.f. glow discharge technique. The optical bandgap was controlled by changing the thickness of the a-Ge:H layers as well as the hydrogen dilution during its deposition. The configuration of bonded hydrogen was investigated by infrared absorption measurements of Si–H and Ge–H vibrational modes. The structure and photoconductivity of the prepared films were systematically investigated as functions of their optical gap. It is found that the optical and electrical properties of multilayer alloys are improved compared to a-SiGe:H films produced from a mixture of hydrides SiH₄+GeH₄, even when diluted with hydrogen. Films of optical gap 1.3E_3.51.5eV show high photosensitivity and low void fraction.     

Effect of composition,film thickness and annealing on the optical properties of Bi-Sb-Se thin films

Thin films of Bi₁₀Sb _x Se_90–x (x35, 40, 45) of different composition and thickness, were deposited on glass substrates by vacuum evaporation. Optical absorption measurements show that the fundamental absorption edge is a function of glass composition, film thickness and annealing temperature. The optical absorption is due to indirect electronic transitions. The value of the optical band gap was found to increase with thickness and decreasing the antimony content and with increasing temperature of heat treatment. The validity of the Urbach rule was investigated and the respective parameters estimated. X-ray diffraction was used to obtain an insight into the structural information.     

Preparation and properties of ferroelectric Pb1 − xCaxTiO3 thin films produced by the polymeric precursor method

Pb_{1 – x} Ca _x TiO₃ thin films with x = 0.24 composition were prepared by the polymeric precursor method on Pt/Ti/SiO₂/Si substrates. The surface morphology and crystal structure, and the ferroelectric and dielectric properties of the films were investigated. X-ray diffraction patterns of the films revealed their polycrystalline nature. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness. The multilayer Pb_{1 – x} Ca _x TO₃ thin films were granular in structure with a grain size of approximately 60–70 nm. The dielectric constant and dissipation factor were, respectively, 174 and 0.04 at a 1 kHz frequency. The 600-nm thick film showed a current density leakage in the order of 10^–7 A/cm² in an electric field of about 51 kV/cm. The C-V characteristics of perovskite thin films showed normal ferroelectric behavior. The remanent polarization and coercive field for the deposited films were 15 C/cm² and 150 kV/cm, respectively.     

Heat treatment effect on the structural and optical properties of Ga2Se3-Ga2S3 thin films

The structural properties of the 50 mol% Ga₂Se₃-50 mol% Ga₂S₃ system in thin-film form were studied using an X-ray diffraction technique. As-deposited films had an amorphous nature, whereas films heat treated for 2 h at 400 °C in a vacuum of 10^–2 Pa had a single-phase (cubic) polycrystalline nature with lattice constanta=0.532 nm. The optical properties of Ga₂Se₃-Ga₂S₃ thin films as-deposited and as-heat treated were also studied. It was found that heat treatment strongly affects the optical constants as well as the energy gap, which can be attributed to compositional as well as structural changes.     

Structural and electrical properties of cadmium-sulpho-selenide solid solutions

Complete solid solutions of CdS _x Se_1–x (0x1) were synthesized by vacuum fusion of stoichiometric proportions of CdS and CdSe. X-ray diffraction data revealed that they possess the hexagonal wurtzite structure. The unit cell lattice constants vary linearly with the composition parameter,x, following Vegard's law.Thin films of CdS _x Se_1–x (0x1) solid solutions could be deposited onto glass substrates by thermal evaporation of the bulk material in 10^–4 Pa vacuum. Structural investigation showed that the films are polycrystalline with predominant appearance of the (002) reflecting plane. On annealing at 250°C in 10^–2 Pa vacuum atmosphere, aggregation and rearrangement of the as-deposited tiny crystallite occurred, preserving the same crystal structure.The dark electrical resistivity of the films is independent on the film thickness, but it varies appreciably with the composition parameter,x, showing a minimum resistivity of 0.02 cm atx=0.55. The temperature dependence of the resistivity follows the semiconducting behaviour with an extrinsic and an intrinsic conduction below and above 70°C, respectively. The determined activation energies 0.2 eV and 0.8 eV, respectively, correspond to shallow and deep trap levels, respectively.     

Transport properties of CdGa2Se4 thin films

The d.c. conductivity was determined for CdGa₂Se₄ thin films in the temperature range 300–625 K for as-deposited and heat-treated films. The conductivity at room temperature of films of thickness 326 nm prepared at a temperature of 573 K was found to be 10^–12 ( cm)^–1. The dependence of the electrical conductivity on the annealing temperature in a vacuum of 1 Pa for a thin film of thickness 140 nm, shows that the electrical conductivity increases with increasing annealing temperature. However, the activation energies E and E decrease with increasing annealing temperature. The data of these annealed films are in agreement with the Meyer–Neldel rule. The thermoelectric power measurements indicate p-type conduction in the as-deposited films as well as for the heat-treated films. The p-type conduction is due to the cadmium deficiency as indicated by EDX. The difference between the value of the activation energy calculated from the thermoelectric power E _S and that obtained from the conductivity E indicates the presence of long-range potential fluctuations.     

Infrared Refractive Index and Extinction Coefficient of Polyimide Films

We have measured the transmittance of several polyimide (C₂₂H₁₀N₂O₄) films at wave numbers from 6000 to 500 cm^–1 (wavelengths from 1.67 to 20 m) using a Fourier-transform infrared (FT-IR) spectrometer. The free-standing polyimide films are made by spin coating and thermal curing processes. The thickness of the films ranges from 0.1 to 4 m. In the nonabsorbing region from 6000 to 4000 cm^–1, the minimum transmittance caused by interference is used to obtain the refractive index for film thicknesses greater than 1 m. The film thicknesses are determined by fitting the spectral transmittance using the refractive index. Molecular absorption strongly reduces the transmittance at wave numbers from 2000 to 500 cm^–1. The optical constants, i.e., the refractive index and the extinction coefficient, are determined from the measured transmittance for several films of different thickness using a least-squares method. A Lorentzian oscillator model is also developed, which in general agrees well with the measured transmittance at wave numbers from 6000 to 500 cm^–1. This study will facilitate the application of polyimide films in the fabrication of infrared filters and other optoelectronic applications. The methods presented in this paper can be used to determine the optical constants of other types of thin-film materials.