A study of the optical properties of titanium oxide films prepared by dc reactive magnetron sputtering

TiO₂ thin films were deposited on the glass substrates by dc reactive magnetron sputtering technique at different sputtering pressures (2 × 10⁻³ to 2 × 10⁻² mbar). The films prepared at low pressures have an anatase phase, and the films prepared at high pressures have an amorphous phase. The optical properties were studied by measuring the transmittance and the ellipsometric spectra. The optical constants of the films in the visible range were obtained by fitting the transmittance combined with the ellipsometry measurements using the classical model with one oscillator. The refractive index of the films decreases from 2.5 until 2.1 as the sputtering pressure increases from 2 × 10⁻³ to 2 × 10⁻² mbar. The films prepared at the pressure higher than 6 × 10⁻³ mbar show a volume inhomogeneity. This volume inhomogeneity has been calculated by fitting the transmittance and the ellipsometric spectra. The volume inhomogeneity of the film prepared at the highest sputtering pressure is about 10%. Although the films prepared at high pressures show a large volume inhomogeneity, they have low extinction coefficients. It is suggested that the anatase phase results in more light scattering than amorphous phase does, and then a high extinction coefficient.     

Albumin adsorption on oxide thin films studied by spectroscopic ellipsometry

Thin films of tantalum, niobium, zirconium and titanium oxides were deposited by reactive magnetron sputtering and their wettability and surface energy, optical properties, roughness, chemical composition and microstructure were characterized using contact angle measurements, spectroscopic ellipsometry, profilometry, X-ray photoelectron spectroscopy and X-ray diffraction, respectively. The purpose of the work was to correlate the surface properties of the films to the Bovine Serum Albumin (BSA) adsorption, as a first step into the development of an initial in vitro test of the films biocompatibility, based on standardized protein adsorption essays. The films were immersed into BSA solutions with different protein concentrations and protein adsorption was monitored in situ by dynamic ellipsometry; the adsorption-rate was dependent on the solution concentration and the immersion time. The overall BSA adsorption was studied in situ using spectroscopic ellipsometry and it was found to be influenced by the wettability of the films; larger BSA adsorption occurred on the more hydrophobic surface, the ZrO₂ film. On the Ta₂O₅, Nb₂O₅ and TiO₂ films, hydrophilic surfaces, the overall BSA adsorption increased with the surface roughness or the polar component of the surface energy.     

Effect of the heat treatment on the infrared emissivity of indium tin oxide (ITO) films

Indium tin oxide (ITO) films were deposited on glass substrates at temperatures ranging from 100 °C to 400 °C by direct current magnetron sputtering. The mean infrared emissivities at the waveband of 8-14 μm were measured in process of heating and cooling between room temperature and 350 °C. Microstructure and phases of ITO films before (Group A) and after (Group B) heat treatment were characterized by SEM and XRD, respectively. Electrical properties were characterized with a four-point probe method and by Hall measurement system. During heat treatment, the infrared emissivity of the film increases with the increase of temperature, and decreases with the decrease of temperature. While, the infrared emissivity of the films decreases slightly around 250 °C in heating process. On the other hand, after heat treatment, the crystalline phases of the films have no obvious change. However, both the resistivity and the infrared emissivity of all films decrease.     

Effect of some preparative parameters on optical properties of spray deposited iridium oxide thin films

Optical properties of iridium oxide films fabricated by the spray pyrolysis technique (SPT) have been investigated. The transmission and reflection spectra of the sprayed films were measured by using a double-beam spectrophotometer in the wavelength range from 200 to 2500 nm. Influences of the preparative parameters; namely, substrate temperature (350-500 °C) and solution molarity (0.005-0.03 M), on the optical characteristics were examined. The solution molarity of the iridium chloride solution was varied so as to prepare iridium oxide thin films with thicknesses ranging from 160 to 325 nm. Some important characteristics of optical absorption, such as optical dispersion energies, the dielectric constant, the ratio of the number of charge carriers to the effective mass, the single oscillator wavelength, and the average value of the oscillator strength, were evaluated. The value of the refractive index was found to depend on the chemical composition as well as the degree of stoichiometry of IrO₂. The values obtained for the high frequency dielectric constant through two procedures are in the range of 2.8-3.9 and 3.3-4.6 over the relevant ranges of the substrate temperature and solution molarity, respectively. Analysis of the energy dispersion curve of the absorption coefficient indicated a direct optical transition with the bandgap energy ranging between 2.61 and 2.51 eV when the substrate temperature increases from 350 to 500 °C.     

Enhancing indium tin oxide (ITO) thin film adhesiveness using the coupling agent silane

The coupling agent γ-mercapto propyl trimethoxy silane (WD-80) was used to enhance the adhesiveness of the indium tin oxide (ITO) thin film, which was prepared on a glass substrate using the sol-gel method. The nano-scratching test, XRD, TEM, SEM, and UV-vis spectrophotometer were employed to examine film adhesion, crystal structure, surface morphology, and transmittance. The results indicated that silane coupling agent, used in low concentration, did not change the film structure but increased the critical load of the film by 49%, from 4.16 mN to 6.20 mN, when the film was peeled off from the substrate. The principle by which the coupling agent works is discussed. In addition to increasing with the light wavelength, the average transmittance of the film in the visible range varied from 78.9% to 83.6%. Moreover, as a function of the WD-80 silane coupling agent, the film exhibited a high smoothness and density due to the orderly arrangement of particles.     

Synthesis and optical properties of ZnO nanorods on indium tin oxide substrate

ZnO nanorods with uniform diameter and length have been synthesized on an indium-tin oxide (ITO) substrate by using a simple thermal evaporation method which is suitable to larger scale production and without any catalyst or additives. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-vis (UV-vis) absorption spectrum, photoluminescence (PL) spectrum and Raman spectrum. The single-phase ZnO nanorods grow well-oriented along the c-axis of its wurtzite structure on ITO substrate. The ZnO nanorods shows sharp and strong UV emission located at 380 nm without notable visible light emission in the PL spectrum, which suggests the good crystallinity of the nanorods, which was also testified by their Raman spectrum. The photodegradation of methylene orange (MO) in aqueous solution reveals that the well-arranged c-axis growth of ZnO nanorods possess evidently improved photocatalytic performance and these properties enable the ZnO nanorods potential application in UV laser.     

Vanadium oxide thin films deposited on indium tin oxide glass by radio—frequency magnetron sputtering

Highly oriented VO₂(B), VO₂(B) + V₆O₁₃ films were grown on indium tin oxide glass by radio-frequency magnetron sputtering. Single phase V₆O₁₃ films were obtained from VO₂(B) +V₆O₁₃ films by annealing at 480℃ in vacuum. The vanadium oxide films were characterized by x-ray diffraction and x-ray photoelectron spectra (XPS). It was found that the formation of vanadium oxide films was affected by substrate temperature and annealing time, because high substrate temperature and annealing were favourable to further oxidation. Therefore, the formation of high valance vanadium oxide films was realized. The V₆O₁₃ crystalline sizes become smaller with the increase of annealing time. XPS analysis revealed that the energy position for all the samples was almost constant, but the broadening of the V_2p3/2 line of the annealed sample was due to the smaller crystal size of V₆O₁₃.     

The effects of post-thermal annealing on the optical parameters of indium-doped ZnO thin films

Indium-doped ZnO thin films are deposited on quartz glass slides by RF magnetron sputtering at ambient temper- ature. The as-deposited films are annealed at different temperatures from 400 C to 800 C in air for 1 h. Transmittance spectra are used to determine the optical parameters and the thicknesses of the films before and after annealing using a nonlinear programming method, and the effects of the annealing temperatures on the optical parameters and the thickness are investigated. The optical band gap is determined from the absorption coefficient. The calculated results show that the film thickness and optical parameters both increase first and then decrease with increasing annealing temperature from 400 C to 800 C. The band gap of the as-deposited ZnO:In thin film is 3.28 eV, and it decreases to 3.17 eV after annealing at 400 C. Then the band gap increases from 3.17 eV to 3.23 eV with increasing annealing temperature from 400 C to 800 C.     

Structural, optical and electrochromic properties of nickel oxide thin films grown from electrodeposited nickel sulphide

Nickel oxide thin films were grown onto FTO-coated glass substrates by a two-step process: electrodeposition of nickel sulphide and their thermal oxidation at 425, 475 and 525 °C. The influence of thermal oxidation temperature on structural, optical, morphological and electrochromic properties was studied. The structural properties undoubtedly revealed NiO formation. The electrochromic properties were studied by means of cyclic voltammetry. The films exhibited anodic electrochromism, changing from a transparent state to a coloured state at +0.75 V versus SCE, i.e. by simultaneous ion and electron ejection. The transmittance in the coloured and bleached states was recorded to access electrochromic quality of the films. Colouration efficiency and electrochromic reversibility were found to be maximum (21 mC/cm² and 89%, respectively) for the films oxidized at 425 °C. The optical band gap energy of nickel oxide slightly varies with increase in annealing temperature.     

Oxidation-induced changes in the electro-optical properties of thin copper films

Summary The room temperature oxidation of vapour deposited copper films has been investigated as a function of film thickness and time by the sheet resistance and optical transmittance measurements. An increase of both sheet resistance and transmittance with a tendency to saturation has been observed. Time variation of the sheet resistance shows that the kinetics of oxidation could be described by a model whereby an initial logarithmic oxide growth changes to an inverse logarithmic one as time progresses; the thicker the film, the longer the change-over time. Absorption coefficients of oxidized films show that the resulting oxide is most probably Cu₂O. Evaluation of the oxidized films for possible use as transparent electrode material shows the existence of an optimum thickness value.

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Riassunto Si è studiata l’ossidazione a temperatura ambiente di pellicole di rame depositate per vaporizzazione in funzione dello spessore della pellicola e del tempo mediante la resistenza del foglio e misurazioni di trasmittenza ottica. Si è osservato un aumento sia della resistenza del foglio, sia della trasmittenza con una tendenza alla saturazione. La variazione temporale della resistenza del foglio mostra che la cinetica di ossidazione potrebbe essere descritta da un modello con cui un’iniziale crescita logaritmica dell’ossido cambia in una logaritmica inversa al crescere del tempo; più spessa è la pellicola più lungo è il tempo del cambiamento. I coefficienti di assorbimento delle pellicole ossidate mostrano che l’ossido risultante è molto probabilmente il Cu₂O. La valutazione delle pellicole ossidate per un possibile uso come materiale di elettrodo trasparente mostra l’esistenza di un valore ottimo dello spessore.

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Резюме Исследуется окисление при комнатной температуре пленок меди в зависимости от толщины пленки и времени, используя измерения сопротивления слоя и оптической прозрачности. Наблюдается увеличение сопротивления слоя и прозрачности с тенденцией к насыщению. Временное изменение сопротивления слоя показывает, что кинетика окисления может быть описана с помощью модели, согласно которой начальный логарифмический рост окисла изменяется на обратно логарифмический, когда время увеличивается; для более толстой пленки изменение происходит при больщих временах. Коэффициенты поглощения окисленных пленок показывают, что результирующий окисел представляет Cu₂O. Оценка возможности использования окисленных пленок в качестве прозрачного материала электродов указывает на существование оптимальной толщины пленки.

     

Optical and electrical properties of thermally oxidized bismuth thin films

Bismuth trioxide (Bi₂O₃) thin films were prepared by dry thermal oxidation of metallic bismuth films deposited by vacuum evaporation. The oxidation process of Bi films consists of a heating from the room temperature to an oxidation temperature (T_o = 673 K), with a temperature rate of 8 K/min; an annealing for 1 h at oxidation temperature and, finally, a cooling to room temperature. The optical transmission and reflection spectra of the films were studied in spectral domains ranged between 300 nm and 1700 nm, for the transmission coefficient, and between 380 nm and 1050 nm for the reflection coefficient, respectively. The thin-film surface structures of the metal/oxide/metal type were used for the study of the static current-voltage (I-U) characteristics. The temperature of the substrate during bismuth deposition strongly influences both the optical and the electrical properties of the oxidized films. For lower values of intensity of electric field (ξ < 5 × 10⁴V/cm), I-U characteristics are ohmic.     

Enhanced electrical stability of flexible indium tin oxide films prepared on stripe SiO2 buffer layer-coated polymer substrates by magnetron sputtering

The electrical stability of flexible indium tin oxide (ITO) films fabricated on stripe SiO₂ buffer layer-coated polyethylene terephthalate (PET) substrates by magnetron sputtering was investigated by the bending test. The ITO thin films with stripe SiO₂ buffer layer under bending have better electrical stability than those with flat SiO₂ buffer layer and without buffer layer. Especially in inward bending text, the ITO thin films with stripe SiO₂ buffer layer only have a slight resistance change when the bending radius r is not less than 8 mm, while the resistances of the films with flat SiO₂ buffer layer and without buffer layer increase significantly at r = 16 mm with decreasing bending radius. This improvement of electrical stability in bending test is due to the small mismatch factor α in ITO-SiO₂, the enhanced interface adhesion and the balance of residual stress. These results indicate that the stripe SiO₂ buffer layer is suited to enhance the electrical stability of flexible ITO film under bending.     

Surface modification of indium tin oxide films by amino ion implantation for the attachment of multi-wall carbon nanotubes

Amino ion implantation was carried out at the energy of 80 keV with fluence of 5 × 10¹⁵ ions cm⁻² for indium tin oxide film (ITO) coated glass, and the existence of amino group on the ITO surface was verified by X-ray photoelectron spectroscopy analysis and Fourier transform infrared spectra. Scanning electron microscopy images show that multi-wall carbon nanotubes (MWCNTs) directly attached to the amino ion implanted ITO (NH₂/ITO) surface homogeneously and stably. The resulting MWCNTs-attached NH₂/ITO (MWCNTs/NH₂/ITO) substrate can be used as electrode material. Cyclic voltammetry results indicate that the MWCNTs/NH₂/ITO electrode shows excellent electrochemical properties and obvious electrocatalytic activity towards uric acid, thus this material is expected to have potential in electrochemical analysis and biosensors.     

Nanocomposite oxide thin films grown by pulsed energy beam deposition

Highly non-stoichiometric indium tin oxide (ITO) thin films were grown by pulsed energy beam deposition (pulsed laser deposition-PLD and pulsed electron beam deposition-PED) under low oxygen pressure. The analysis of the structure and electrical transport properties showed that ITO films with a large oxygen deficiency (more than 20%) are nanocomposite films with metallic (In, Sn) clusters embedded in a stoichiometric and crystalline oxide matrix. The presence of the metallic clusters induces specific transport properties, i.e. a metallic conductivity via percolation with a superconducting transition at low temperature (about 6 K) and the melting and freezing of the In-Sn clusters in the room temperature to 450 K range evidenced by large changes in resistivity and a hysteresis cycle. By controlling the oxygen deficiency and temperature during the growth, the transport and optical properties of the nanocomposite oxide films could be tuned from metallic-like to insulating and from transparent to absorbing films.     

Synthesis of P-type transparent conducting silver:indium oxide (AIO) thin films by reactive electron beam evaporation technique

Present paper reports the synthesis, electrical and optical properties of p-type conducting and transparent silver indium oxide (AIO) thin films prepared on glass substrates by reactive electron beam evaporation technique at three substrate temperatures (50, 200 and 250 °C) and at five evaporation rates (0.05 to 16.0 nm/s). The source material is pure powders of Ag₂O:In₂O₃=50:50 mol%. The AIO films are amorphous. The films, though not corresponding to Delafossite crystal structure, exhibit p-type conductivity, when prepared at an evaporation rate of 0.05 nm/s at all the three substrate temperatures. With increasing filament current, it is observed that (i) the electrical resistivity decreases and (ii) the refractive index of the films (at 632.8 nm, and is in the range: 1.219-1.211) decreases. The work function (effective Fermi level) has been measured on these samples by Kelvin Probe method. The results are explained on the basis of partial ionic charge and localization of covalent bonds in the AIO thin films.     

Electrical, structural, and optical properties of ITO thin films prepared at room temperature by pulsed laser deposition

Indium tin oxide (ITO) thin films were prepared by pulsed laser deposition (PLD) on glass substrate at room temperature. Structural, optical, and electrical properties of these films were analyzed in order to investigate its dependence on oxygen pressure, and rapid thermal annealing (RTA) temperature. High quality ITO films with a low resistivity of 3.3 × 10⁻⁴ Ω cm and a transparency above 90% were able to be formed at an oxygen pressure of 2.0 Pa and an RTA temperature of 400 °C. A four-point probe method, X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-NIR grating spectrometer are used to investigate the properties of ITO films.     

Photoluminescence and photoelectrochemical properties of nanocrystalline ZnO thin films synthesized by spray pyrolysis technique

A simple and inexpensive spray pyrolysis technique (SPT) was employed for the synthesis of nanocrystalline zinc oxide (ZnO) thin films onto soda lime glass and tin doped indium oxide (ITO) coated glass substrates at different substrate temperatures ranging from 300 °C to 500 °C. The synthesized films were polycrystalline, with a (0 0 2) preferential growth along c-axis. SEM micrographs revealed the uniform distribution of spherical grains of about 80-90 nm size. The films were transparent with average visible transmittance of 85% having band gap energy 3.25 eV. All the samples exhibit room temperature photoluminescence (PL). A strong ultraviolet (UV) emission at 398 nm with weak green emission centered at 520 nm confirmed the less defect density in the samples. Moreover, the samples are photoelectrochemically active and exhibit the highest photocurrent of 60 μA, a photovoltage of 280 mV and 0.23 fill factor (FF) for the Zn₄₅₀ films in 0.5 M Na₂SO₄ electrolyte, when illuminated under UV light.     

Effect of precursors on structure, optical and electrical properties of chemically deposited nanocrystalline ZnO thin films

Nanocrystalline ZnO thin films were chemically deposited on glass substrates using two different precursors namely, zinc sulphate and zinc nitrate. XRD studies confirm that the films are polycrystalline zinc oxide having hexagonal wurtzite structure with crystallite size in the range 25-33 nm. The surface morphology of film prepared using zinc sulphate exhibits agglomeration of small grains throughout the surface with no visible holes or faulty zones, while the film prepared using zinc nitrate shows a porous structure consisting of grains with different sizes separated by empty spaces. The film prepared using zinc sulphate shows higher reflectance due to its larger refractive index which is related to the packing density of grains in the film. Further, the film prepared using zinc sulphate is found to have normal dispersion for the wavelength range 550-750 nm, whereas the film prepared using zinc nitrate has normal dispersion for the wavelength range 450-750 nm. The direct optical band gaps in the two films are estimated to be 3.01 eV and 3.00 eV, respectively. The change in film resistance with temperature has been explained on the basis of two competing processes, viz. thermal excitation of electrons and atmospheric oxygen adsorption, occurring simultaneously. The activation energies of the films in two different regions indicate the presence of two energy levels - one deep and one shallow near the bottom of the conduction band in the bandgap.     

Effects of electron-beam irradiation on structural,electrical, and optical properties of amorphous indium gallium zinc oxide thin films

High-energy electron-beam irradiation of indium gallium zinc oxide (IGZO) films improved the short-range arrangement. The increase in band gap was used as an indication of such improvement. X-ray diffraction confirmed that the films treated with a DC voltage of 2–4.5 kV for duration of up to 35 min are in the amorphous state or nanocrystalline phase. Higher energy electron-beam irradiation led to increased conductivity, which mainly comes from the drastic increase in electron concentration. Electron-beam treatment could be a viable route to improve the contact resistance between the source/drain and channel layer in thin-film transistor devices.     

Structural and optical properties of electrodeposited molybdenum oxide thin films

Electrosynthesis of Mo(IV) oxide thin films on F-doped SnO₂ conducting glass (10-20/Ω/□) substrates were carried from aqueous alkaline solution of ammonium molybdate at room temperature. The physical characterization of as-deposited films carried by thermogravimetric/differential thermogravimetric analysis (TGA/DTA), infrared spectroscopy and X-ray diffraction (XRD) showed the formation of hydrous and amorphous MoO₂. Scanning electron microscopy (SEM) revealed a smooth but cracked surface with multi-layered growth. Annealing of these films in dry argon at 450 °C for 1 h resulted into polycrystalline MoO₂ with crystallites aligned perpendicular to the substrate. Optical absorption study indicated a direct band gap of 2.83 eV. The band gap variation consistent with Moss rule and band gap narrowing upon crystallization was observed.Structure tailoring of as-deposited thin films by thermal oxidation in ambient air to obtain electrochromic Mo(VI) oxide thin films was exploited for the first time by this novel route. The results of this study will be reported elsewhere.