Au thin films deposited on SnO2:In and glass: Substrate effects on the optical and electrical properties

We report on a detailed study on the optical and electrical properties of Au films made by sputter deposition onto glass substrates with and without transparent and electrically conducting layers of SnO₂:In. The Au films had thicknesses up to 10.7 nm and hence spanned the range for thin film growth from discrete islands, via large scale coalescence and formation of a meandering conducting network, to the formation of a more or less “holey” film. Scanning electron microscopy and atomic force microscopy demonstrated that the SnO₂:In films were considerably rougher than the glass itself, and this roughness influenced the Au film formation so that large scale coalescence set in at a somewhat larger thickness for films on SnO₂:In than on glass. Measurements of spectral optical transmittance and reflectance and of electrical resistance gave a fully consistent picture that could be reconciled with impeded Au film formation on the SnO₂:In layer; this led to pronounced “plateaus” in the near infrared optical spectra for Au films on SnO₂:In and a concomitant change from such two-layer films having a lower resistance than the single gold film at thicknesses below large scale coalescence to the opposite behavior for larger film thicknesses. Our work highlights the importance of the substrate roughness for transparent conductors comprising coinage metal films backed by wide band gap transparent conducting oxides.     

First principles study on the properties of p-type conducting In:SnO2

Detailed theoretical investigations on the structural, electronic and optical properties of p-type conducting In:SnO₂ have been conducted by first principle calculations. Analysis on the thermal stability via standard enthalpy of formation calculations shows that In:SnO₂ remains stable at very high In concentration, although the lattice constant expands in a distorted rutile structure with the increase of indium content. This can be attributed to the larger ionic radii and the one less 5p electron of In³⁺. Due to the differences in thermal stabilities of the structures with the same indium concentration, the preferred In³⁺ distribution is to occupy the Sn sites in different (110) slabs, followed by occupying the location in the same (110) slab with a maximized distance between indium ions. Indium element in SnO₂ introduces a band in the low energy region originated from the In 4d orbitals and an acceptor energy level slightly above the Fermi energy. While the large effective mass of the electron holes in the valence band results in the small p-type conductivity of In:SnO₂. The tiny changes in the conduction band and band gap lead to the invariability of the optical spectra in the ultraviolet-visible region. On the contrary, the dramatic enhancement of dielectric function, reflectivity and absorption in infrared region can be interpreted by the transition from the occupied states to the empty bands near E_f as well as the exciton effect. These features make In:SnO₂ a good candidate for applications such as transparent conducting materials, infrared reflecting materials and gas sensors.     

Structural and optical properties of Al-doped SnO2 nanowires

We report a facile thermal evaporation method for the syntheses of Al-doped SnO₂ nanowires using Al-doped SnO₂ nanoparticles as precursors. High-density, single-crystalline Al-doped SnO₂ nanowires were directly grown on the 6H-SiC substrates without any catalyst. X-ray diffraction patterns show that the Al dopants are incorporated into the rutile SnO₂ nanowires. The X-ray photoelectron spectra confirm the SnO₂ nanowires doped with 5 at.% Al. The photoluminescence spectra of the Al-doped SnO₂ nanowires exhibit that the large blue shift of the emission band can be observed in the Al-doped SnO₂ nanowires compared with undoped nanowires. The distortion of the crystal lattices caused by incorporation of Al atoms at the interstitials should be responsible for the large blue shift of the emission band.     

Fabrication of novel SnO2 nanofibers bundle and their optical properties

Here we report on the synthesis of novel SnO₂ nanofibers bundle (NFB) by using ball milled Fe powders via chemical vapor deposition (CVD). The reaction was carried out in a horizontal tube furnace (HTF) at 1100 °C under Ar flow. The as prepared product was characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, high resolution transmission electron microscopy and selected area electron diffraction (SAED). The microscopy analysis reveals the existence of tubular structure that might be formed by the accumulation of nanofibers. The Raman spectrum reveals that the product is rutile SnO₂ with additional peaks ascribed to defects or oxygen vacancies. Room temperature Photoluminescence (PL) spectrum exhibits three emission bands at 369, 450 and 466.6 nm. Using optical absorbance data, a direct optical bandgap of 3.68 eV was calculated.     

Preparation of SnO2 thin films at low temperatures with H2 gas by the hot-wire CVD method

H₂ additional effect for crystallization of SnO₂ films prepared by the hot-wire CVD method was investigated. The crystallization of SnO₂ films starts at 170 °C. The selectivity enhancement of the solar cell substrate will contribute to reduce the cost of silicon thin film solar cells. The atomic hydrogen assisted nano-crystallization exists for the depositions of SnO₂ films by the hot-wire CVD method. Furthermore, the addition of H₂ gas improved the electrical conductivity up to 5.3 × 10⁰ S/cm. However, these effects are limited in the deposition condition of a small amount of hydrogen. Addition of much higher hydrogen concentration starts an etching effect of oxygen atoms.     

Gamma-irradiation-induced Ag/SiO2 composite films and their optical absorption properties

Small Ag particles or clusters dispersed mesoporous SiO₂ composite films were prepared by a new method: First the matrix SiO₂ films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO₃ solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO₂ films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements.     

Structural, electrical and optical characteristics of SnO2:Sb thin films by ultrasonic spray pyrolysis

Antimony-doped tin oxide (SnO₂:Sb) thin films were fabricated by an ultrasonic spray pyrolysis method. The effect of antimony doping on the structural, electrical and optical properties of tin oxide thin films were investigated. Tin(II) chloride dehydrate (SnCl₂·2H₂O) and antimony(III) chloride (SbCl₃) were used as a host and a dopant precursor. X-ray diffraction analysis showed that the non-doped SnO₂ thin film had a preferred (211) orientation, but as the Sb-doping concentration increased, a preferred (200) orientation was observed. Scanning electron microscopy studies indicated that the polyhedron-like grains observed for the non-doped SnO₂ thin films became rounder and decreased in size with the Sb-doping concentration. The lowest resistivity (about 8.4 × 10^− 4 Ω·cm) was obtained for the 3 at.% Sb-doped films. Antimony-doping led to an increase in the carrier concentration and a decrease in Hall mobility. The transmittance level in the near infrared region was lowered with the Sb-doping concentration.     

Structure and thermal properties of transparent conductive nanoporous F:SnO2 films

The nanoporous F:SnO₂ materials prepared for the purpose of dye-sensitized solar cells (DSCs) application are composed of SnCl₄ (98.0%) and HF (48-51%), produce with a NH₄OH aqueous solution with sol-gel method as a catalyst. Acetylene black is needed to make nano-porous from FTO heated until 120 °C that will change it from sol to gel and with that 2 phase sintering with 500 °C and 550 °C can be predicted to produce nano-materials. The preferred orientation indicates (110), (101), (211) for SnO₂ and (200) for fluorine, respectively. The main IR features include resonances at 660, 620 and 540 cm^− 1. From the FE-SEM results, the mean pore size of the sample is range of 16-38 nm. Finally, the nanoporous F:SnO₂ film used for TCO layer of dye-sensitized solar cells (DSCs) exhibited an energy conversion efficiency of about 1.83% at light intensity of 100 mW/cm².     

Thin films of In2O3 by atomic layer deposition using In(acac)3

Thin films of indium oxide have been deposited using the atomic layer deposition (ALD) technique using In(acac)₃ (acac = acetylacetonate, pentane-2,4-dione) and either H₂O or O₃ as precursors. Successful growth using In(acac)₃ is contradictory to what has been reported previously in the literature [J.W. Elam, A.B.F. Martinson, M.J. Pellin, J.T. Hupp, Chem. Mater. 18 (2006) 3571.]. Investigation of the dependence of temperature on the deposition shows windows where the growth rates are relatively unaffected by temperature in the ranges 165–200 °C for In(acac)₃ and H₂O, 165–225 °C for In(acac)₃ and O₃. The growth rates obtained are of the order 20 pm/cycle for In(acac)₃ and H₂O, 12 pm/cycle for In(acac)₃.     

Structural, morphological and optical properties of thermal annealed TiO thin films

Structural, morphological and optical properties of TiO thin films grown by single source thermal evaporation method were studied. The films were annealed from 300 to 520 °C in air after evaporation. Qualitative film analysis was performed with X-ray diffraction, atomic force microscopy and optical transmittance and reflectance spectra. A correlation was established between the optical properties, surface roughness and growth morphology of the evaporated TiO thin films. The X-ray diffraction spectra indicated the presence of the TiO₂ phase for the annealing temperature above 400 °C.     

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.     

VO2 thin films deposited on silicon substrates from V2O5 target: Limits in optical switching properties and modeling

Thermochromic VO₂ thin films presenting a phase change at T_c = 68 °C and having variable thickness were deposited on silicon substrates (Si-001) by radio-frequency sputtering. These thin films were obtained from optimized reduction of low cost V₂O₅ targets. Depending on deposition conditions, a non-thermochromic metastable VO₂ phase might also be obtained. The thermochromic thin films were characterized by X-ray diffraction, atomic force microscopy, ellipsometry techniques, Fourier transform infrared spectrometry and optical emissivity analyses. In the wavelength range 0.3 to 25 μm, the optical transmittance of the thermochromic films exhibited a large variation between 25 and 100 °C due to the phase transition at T_c: the contrast in transmittance (difference between the transmittance values to 25 °C and 100 °C) first increased with film thickness, then reached a maximum value. A model taking into account the optical properties of both types of VO₂ film fully justified such a maximum value. The n and k optical indexes were calculated from transmittance and reflectance spectra. A significant contrast in emissivity due to the phase transition was also observed between 25 and 100 °C.     

The correlation between mechanical stress, thermal shift and refractive index in HfO2, Nb2O5, Ta2O5 and SiO2 layers and its relation to the layer porosity

We present extended experimental material about optical and mechanical properties of oxide optical coating materials, deposited by electron beam evaporation, ion and plasma ion assisted evaporation, sputtering and ion plating. A clear correlation between these experimental data is established and understood as being caused by the different degree of the porosity of the films. This assumption has been verified by investigation of the layer structure and accompanying simulations of the effect of porosity on refractive index, layer stress and thermal shift. As a practical conclusion, we find that a certain pore fraction in the films is essential in order to get a valuable balance between optical and mechanical coating properties.     

Vapor phase growth and optical properties of single-crystalline SnO2 nanobelts

Mass production of single-crystalline SnO₂ nanobelts was successfully achieved through a thermal evaporation of metallic Sn powders at 900 °C. The as-prepared SnO₂ nanobelts were typically 30-200 nm in width, 10-50 nm in thickness, and about tens of micrometers in length. In addition to the classical Raman models, two new Raman bands at 498 and 698 cm⁻¹ are observed for rutile-phased SnO₂ nanobelts, which can be attributed to the IR-active A_2u TO and A_2u LO modes, respectively. Photoluminescence (PL) spectrum of SnO₂ nanobelts featured an emission band at 615 nm (with a small shoulder at 585 nm), which might correspond to the existence of oxygen deficiencies in the produced belts. The formation of SnO₂ nanobelts followed a vapor-solid (VS) growth mechanism.     

Rare-earth ions doping effects on the optical properties of sol-gel fabricated PbTiO3 thin films

Ce, Sm, Dy, Er and Yb doped PbTiO₃ thin films were deposited by sol-gel method on (111)Pt/Ti/SiO₂/Si substrates. The optical properties of the films were characterized by means of ellipsometry using a HeNe-laser source (λ=632.8 nm). Real (n) and imaginary (k) parts of the refractive index were obtained applying the Fresnel equation. It is shown that for lead titanate thin films doped with 2 mol.% of lanthanide (Ln) ions the real part of the refractive index decreases smoothly with increasing atomic number of the element, with the exception of the Ce doped film. The experimental results are explained taking into account the electronic structure of the dopants. Specific results of Ce are explained by its oxidation state, which has been shown by means of X-ray photoelectron spectroscopy to be +4. Additionally, noticeable correlation of the optical properties of the films with respect to doping level was observed.     

Growth and properties of the CuInS2 thin films produced by glancing angle deposition

We use the glancing angle deposition technique (GLAD) to grow CuInS₂ thin films by a vacuum thermal method onto glass substrates. During deposition, the substrate temperature was maintained at 200 °C. Due to shadowing effect the oblique angle deposition technique can produce nanorods tilted toward the incident deposition flux. The evaporated atoms arrive at the growing interface at a fixed angle θ measured from the substrate normal. The substrate is rotated with rotational speed ω fixed at 0.033 rev s^− 1. We show that the use of this growth technique leads to an improvement in the optical properties of the films. Indeed high absorption coefficients (10⁵–3.10⁵ cm^− 1) in the visible range and near-IR spectral range are reached. In the case of the absence of the substrate rotation, scanning electron microscopy pictures show that the structure of the resulting film consists of nanocolumns that are progressively inclined towards the evaporation source as the incident angle was increased. If a rapid azimuthal rotation accompanies the substrate tilt, the resulting nanostructure is composed of an array of pillars normal to the substrate. The surface morphology show an improvement without presence of secondary phases for higher incident angles (θ > 60°).     

Annealing effects on the structural properties of IrO2 thin films

We have demonstrated the structural and morphological changes of iridium oxide (IrO₂) films by the thermal annealing process. We have characterized the samples by using the X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The Ir-related XRD peaks predominantly appeared after the thermal annealing at 750-1000 °C. SEM images revealed that the films became quite uneven in thickness by annealing at 750 °C, whereas island-like structures were found to agglomerate on substrate surfaces by annealing at 1000 °C. From EDX and XRD analysis, we suggested that the agglomerated structures mainly consisted of Ir phase.     

Formation and properties of AgInSe2 thin films by co-evaporation

The electrical and optical properties of silver indium selenide thin films prepared by co-evaporation have been studied. X-ray diffraction indicates that the as prepared films were polycrystalline in nature. The lattice parameters were calculated to be a=0.6137 and b=1.1816 nm. Composition was determined from energy dispersive analysis of X-ray. Silver indium selenide thin films were also prepared by bulk evaporation of powdered sample for comparative study. They have an optical band gap (E_g) of 1.25 eV and it is a direct allowed transition. Refractive index (n) and extinction coefficient (k) were calculated from absorption and reflection spectra. Steady-state photoconductivity was measured from 300 to 400 K. Carrier lifetime was calculated from transient photoconductivity measurements at room temperature at different intensities of illumination.     

Polarization dependence of the optical response in SnO2 and the effects from heavily F doping

The optical properties of intrinsic SnO₂ (TO) and fluorine doped (FTO) are characterized in terms of the dielectric function ε(ħω) = ε₁(ħω) + iε₂(ħω) by electronic structure calculations. The intrinsic TO shows intriguing absorption characteristics in the 3.0–8.0 eV region: (i) the low energy region of the fundamental band gap (3.2 < ħω < 3.9 eV), the optical transitions Г₃⁺ → Г₁⁺ (valence-band maximum to conduction-band minimum) is symmetry forbidden, and the band-edge absorption is therefore extremely weak. (ii) In the higher energy region (3.9 < ħω < 5.1 eV) the Г₅⁻ → Г₁⁺ transitions (from the second uppermost valence band) is strongly polarized perpendicular to the main c axis. (iii) Transitions with polarization axis parallel to c axis are generated from Г₂⁻ → Г₁⁺ transitions (from the third uppermost valence bands), and dominates at high energies (5.1 < ħω eV). Heavily F doped TO (FTO) with doping concentrations n_F = 4 × 10²⁰ cm^− 3 changes the absorption significantly: (iv) Substitutional F_O generates strong inter-conduction band absorption at 0.8, 2.2, and 3.8 eV which affects also the high frequency dielectric constant ε_∞. (v) Interstitial F_i is inactive as a single dopant, but act as a compensating acceptor in highly n-type FTO. This explains the measured non-linear dependence of the resistivity with respect to F concentration.