As-deposited TiO2 Films Research Articles

Fabrication and photocatalytic characteristics of TiO2 films on silicon substrates

Silicon (111) and Silicon (100) have been employed for fabrication of TiO2 films by metal organic chemical vapor deposition (MOCVD). Titanium (IV) isopropoxide (Ti[O(C3H7)4]) was used as a precursor. The as-deposited TiO2 films have been characterized with Field emission scanning electron microscopy (FE-SEM), X ray diffraction (XRD) and atomic force microscopy (AFM). The photocatalytic properties were investigated by decomposition of aqueous orange II. The crystalline and structural properties of TiO2 film had crucial influences on the photodegradation efficiency. For MOCVDin-situ deposited films on Si substrates, the photoactivities varied following a shape of “M”: At lower (350°C) middle (500 °C) and higher (800 °C) temperature of deposition, relative lower photodegradation activities have been observed. At 400 °C and 700 °C of deposition, relative higher efficiencies of degradation have been obtained, because one predominant crystallite orientation could be obtained as deposition at those two temperatures, especially a single anatase crystalline TiO2 film could be obtained at 700 °C growth.

In-Plane Orientation and Annealing Behavior of Rutile TiO2 Films on MgO Substrate Prepared by Inductively Coupled Plasma-Assisted Sputtering

In-plane orientation and annealing behavior of Rutile TiO2 films grown on MgO (100) substrates by means of inductively coupled plasma-assisted sputtering were investigated. We deposited TiO2 films on relatively low temperature (≦400°C) MgO substrates in Ar–O2 mixture gases. Several films were annealed in atmospheric O2 at 800°C for 60 min. X-ray φ scan measurements and AFM images revealed that rutile TiO2 film grows on MgO (100) substrate with two orientations of TiO2 (110)[001]∥MgO (100)[011] and TiO2 (110)[001]∥MgO (100)[011]. Post annealing in atmospheric O2 at 800°C improved the crystalline preference of TiO2 film, however, formation of MgTiO3 interface layer was clearly shown by cross sectional TEM image and XPS depth analysis. Values of refractive index and band gap energy were evaluated from optical characteristics of the films. High refractive index of 2.65 at wavelength of 500 nm in as-deposited TiO2 film degraded to 2.0 in annealed film due to the formation of MgTiO3 interface layer. Through this study, we offer stable rutile (110) surface and a formation route of MgTiO3 dielectric layer.

Photocatalytic Properties of TiO2 Films Deposited by Reactive Sputtering in Mid-Frequency Mode with Dual Cathodes

Titanium dioxide (TiO2) films were deposited on unheated nonalkali glass substrates by reactive midfrequency (mf) magnetron sputtering using dual cathodes with two Ti metal targets. In order to maintain the very high deposition rate, the depositions were successfully carried out stably in the “transition region” between the metallic and reactive (oxide) sputter modes using plasma control units (PCU). Very high-rate depositions of 12–70 nm/min were successfully achieved in the deposition of TiO2 films throughout the whole “transition region”. The as-deposited films deposited in the “oxide mode” had a polycrystalline anatase structure and exhibited both photoinduced hydrophilicity and photodecomposition of acetaldehyde. Whereas, all the as-deposited TiO2 films deposited in the “transition region” had amorphous structure, which did not exhibit photocatalytic activity. Such amorphous films deposited in the transition region were crystallized by postannealing in air at 200°C or 300°C and were then shown to have photocatalytic activity. Very thin TiO2 films with a thickness of 25 nm deposited in the transition region and postannealed at 300°C exhibited photoinduced hydrophilicity, whereas there was a clear thickness dependence of photodecomposition and the rather thick films of 300 nm exhibited pronounced photodecomposition.

Characterization of nonstoichiometric TiO2 and ZrO2 thin films stabilized by Al2O3 and SiO2 additions

Thin films of nonstoichiometric titanium dioxide (TiO2) and zirconium oxide (ZrO2) chemically stabilized by incorporating stoichiometric oxides of Al2O3 and SiO2, respectively, were fabricated by rf magnetron sputtering. These films deposited from ceramic targets of TiO2, TiO2–10 vol %Al2O3, ZrO2, and ZrO2–10 vol %SiO2 were designated as TiO2, TiO2–10Al2O3, ZrO2, and ZrO2–10SiO2. The dielectric and electrical properties of as-deposited and annealed TiO2 and ZrO2 films varied with the processing conditions. The addition of Al2O3 to TiO2 and SiO2 to ZrO2 films had not only reduced the dielectric loss tangent (tan δ) and increased resistivity, but also retained the competitive dielectric constant (k). The dielectric properties of the TiO2–10Al2O3 and ZrO2–10SiO2 films became stable under different deposition temperatures and annealing temperatures. The TiO2–10Al2O3 films with k=62, tan δ=0.012, resistivity of 2.5×108 Ω cm, and breakdown field of 2.2 MV/cm were obtained. ZrO2–10SiO2 films with good k values of 20–25 and low tan δ values of 0.011–0.014 can be obtained under rf power of 150 W, while k=14.5–16.4 and tan δ=0.012–0.019 under 100 W can be obtained. The advantage of adding the stoichiometric oxides of Al2O3 and SiO2 is the reduction of oxygen vacancies through the strong affinity of Al3+ and Si4+ to capture more oxygen for nonstoichiometric oxides of TiO2 and ZrO2.

Correlation between crystallographic orientations and Raman spectra of TiO2 sputtered films with changing degrees of plasma exposure

Most of the titanium dioxide (TiO2) films deposited in this study were transparent and had anatase crystal structures, while some of the films were semitransparent. The anatase phase of the as-deposited films was revealed in the Raman spectra with a Raman shift of 145 cm−1 and x-ray diffraction patterns. When the films were deposited at the working gas pressure (PW) of 3 mTorr with increasing plasma exposure, the highest intensity of the peak which had a Raman shift of 145 cm−1 steeply increased with preferential enhancement of the crystallographic orientation of the A(220) plane, where A shows the anatase phase of TiO2. However, in the films deposited at the PW of 1 mTorr with decreasing plasma exposure, such Raman peak intensity gradually decreased at the condition that the preferential crystallographic orientation changed from A(220) to A(101). This implies that the change in the Raman peak intensity may be closely related to the change in the A(220) peak intensity. When the TiO2 crystallites became larger regardless of the PW, the A(220) peak was significantly higher than other peaks. It was found that the Raman peak intensity of the as-deposited TiO2 films was strongly affected by the degree of the plasma exposure, which was related to the substrate position during film deposition.

Effects of plasma exposure on structural and optical properties of TiO2 films deposited by off-axis target sputtering

TiO 2 films with thickness of about 800–2300 nm have been reactively deposited on glass-slide substrates at an atmosphere of Ar and O2 mixture, using the off-axis target sputtering method. The maximum deposition rate of the film was 12.8 nm/min. The optical properties and the crystal structure of the films were measured using spectrophotometry, Raman spectroscopy, and x-ray diffractometry, respectively. Most of the films deposited in this study were transparent and had anatase crystal structure, while some of them were semitransparent. The A(101), A(112), A(211) and A(220) peaks were observed in the x-ray diffraction patterns, where A shows the anatase phase of TiO2. With increasing plasma exposure, the x-ray peak intensities IP of A(101) and A(112) gradually decreased, while the IP of A(211) and A(220) gradually increased. The IP depended strongly on the substrate position in this study. Moreover, the A(220) peak was significantly higher than other peaks when the TiO2 crystallites became larger. The anatase phase of the TiO2 films was also revealed in the Raman spectra with a Raman shift of 145 cm−1. With decreasing plasma exposure, the changes in the optical transmittance spectra of TiO2 films were very small at the wavelength of 350–900 nm. However, the transmittance spectra significantly depended on the wavelength with increasing plasma exposure. Consequently, it was found that the crystal structure and the optical properties of as-deposited TiO2 films were strongly affected by plasma exposure during film deposition.

Preparation and characterization of TiO2 thin films by sol-gel method

Titanium dioxide (TiO2) thin films have been deposited on silicon and glass substrates by the sol-gel process using titanium iso-propoxide [Ti(O-i-C3H7)4]. The bond configuration of the TiO2 thin films was analyzed by using FTIR in the wavenumber range from 400 to 4000 cm−1. The spectral transmittance of as-deposited TiO2 films deposited on fused silica glass was measured in the wavelength range from 200 to 900 nm. X-ray diffraction measurements were performed to determine the crystallinity of the TiO2 films. As-deposited films were amorphous. As the film was annealed at higher temperature, the structure was transformed from amorphous to the anatase crystalline state. The chemical composition of the deposited film was investigated using X-ray photoelectron spectroscopy (XPS). The films are essentially stoichiometric with carbon as the dominant impurity on the surface. Raman spectra show the characteristic of TiO2 anatase phase. The electrical properties of the TiO2 films were measured using capacitance-voltage (C-V) and current-voltage techniques. From C-V measurements, the dielectric constants were calculated to be approximately 26 for the as-deposited films and 75–82 for films annealed at 700°C in different atmosphere. For the as-deposited samples, the breakdown voltage was 2.7 MV/cm, and for an electric field of 1 MV/cm, the leakage current was 5 × 10−5 A/cm2 and the resistivity was 2.2 × 1010 Ω-cm.

Preparation and Faraday effect of EuS microcrystal-embedded oxide thin films

EuS microcrystal-embedded SiO2, Al2O3, and TiO2 thin films have been prepared by using rf sputtering method. X-ray diffraction analysis indicates that EuS microcrystal is precipitated as a single phase in Al2O3 and TiO2 films while precipitation of EuS is not detected in SiO2 film. Faraday effect attributable to the 4f7–4f65d transition of Eu2+ in EuS microcrystalline phase is observed in Al2O3 and TiO2 films. In particular, Faraday rotation angle observed for EuS-embedded TiO2 film is large; for instance, the magnitude of Verdet constant for as-deposited TiO2 film prepared without heating of substrate during the sputtering is 0.15 deg/cm Oe at wavelength of 700 nm. This value is larger by two orders of magnitude than those of Eu2+- or Tb3+-containing oxide glasses which show the largest Faraday rotation angle among rare-earth-containing glasses, and is comparable to the value for EuSe single crystal.

Surface Sol–gel Synthesis of Ultrathin Titanium and Tantalum Oxide Films

TiO2, Ta2O5, and mixed TiO2/Ta2O5 ultrathin films were grown layer-by-layer using a surface sol–gel deposition technique. The technique allows one to prepare smooth films of mixed composition with angstrom-level control of thickness. The thickness increases per adsorption/hydrolysis cycle were 4.5, 2.1, and 2.5 A, respectively, for TiO2, Ta2O5, and mixed TiO2/Ta2O5 (1.6sol;1) films. By combining X-ray diffraction and ellipsometric analysis, it was determined that the as-deposited TiO2 films are less dense than bulk TiO2, and do not adhere persistently to Si/SiO2 or Au/2-mercaptoethanol substrates. These ‘green’ films were annealed at 400°C to produce denser and highly adherent films. Patterning the surface by microcontact printing of siloxane polymer thin films allows one to prepare patterned sol–gel oxide thin films.