As-deposited TiO2 Thin Films Research Articles

Effect of Crystal Structure and Grain Size on Photo-Catalytic Activities of Remote-Plasma Atomic Layer Deposited Titanium Oxide Thin Film

The photo-catalytic activities of TiO2 thin films grown by remote plasma atomic layer deposition (RPALD) were investigated as a function of grain size and crystal structure. As the deposition temperature increased, it was shown that grain size of the anatase phase in the as-deposited TiO2 thin films progressively decreased. When the TiO2 films grown by RPALD were annealed at 900°C, the anatase TiO2 phase with relatively small grains provided large number of nucleation sites for formation of more stable rutile phase. On the other hand, the phase transition to rutile was retarded due to the reduction in grain boundaries in the case of the anatase TiO2 films with large grains. Thus, the anatase TiO2 phase with large grains has excellent thermal stability against the phase transition to rutile. Because the large grain boundary volume is a primary cause of degradation of photo-catalytic activity, the photo-catalytic activity of the TiO2 thin film was maximized when the anatase phase with large grains was formed in the TiO2 thin film. Therefore, RPALD can be effectively utilized to control the grain size of the as-deposited anatase TiO2 thin film with high thermal stability for potential application of photo-catalysts.

Effect of annealing temperature on the structure and optical properties of sputtered TiO2 films

TiO2 thin films were deposited by DC reactive magnetron sputtering. Some TiO2 thin films samples were annealed for 5min at different temperatures from 300 to 900°C. The structure and optical properties of the films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM) and ultraviolet–visible (UV–vis) spectrophotometry, respectively. The influence of the annealing temperature on the structure and optical properties of the films was investigated. The results show that the as-deposited TiO2 thin films are mixtures of anatase and rutile phases, and possess the column-like crystallite texture. With the annealing temperature increasing, the refractive index and extinction coefficient increase. When the annealing temperature is lower than 900°C, the anatase phase is the dominant crystalline phase; the weight fraction of the rutile phase does not increase significantly during annealing process. As the annealing temperature rises to 900°C, the rutile phase with the large extinction coefficient becomes the dominant crystalline phase, and the columnar structure disappears. The films annealed at 300°C have the best optical properties for the antireflection coatings, whose refractive index and extinction coefficient are 2.42 and 8×10−4 (at 550nm), respectively.

Effect of thermal annealing on the structure and microstructure of TiO2 thin films

Nanostructured TiO2 thin films have been prepared through chemical route using sol-gel and spin coating techniques. The deposited films were annealed in the temperature range 400–1000°C for 1 h. The structure and microstructure of the annealed films were characterized by GAXRD, micro-Raman spectroscopy and AFM. The as-deposited TiO2 thin films are found to be amorphous. Micro-Raman and GAXRD results confirm the presence of the anatase phase and absence of the rutile phase for films annealed up to 700°C. The diffraction pattern of the film annealed at 800 to 1000°C contains peaks of both anatase and rutile reflections. The intensity of all peaks in micro-Raman and GAXRD patterns increased and their width (FWHM) decreased with increasing annealing temperature, demonstrating the improvement in the crystallinity of the annealed films. Phase transformation at higher annealing temperature involves a competition among three events such as: grain growth of anatase phase, conversion of anatase to rutile and grain growth of rutile phase. AFM image of the asdeposited films and annealed films indicated exponential grain growth at higher temperature.

Effect of Annealing on Structural and Optical Properties of Pulsed Laser Deposited Titanium Dioxide Thin Films

TiO2 is a widely studied material for many important applications in areas such as environmental purification, photocatalyst, gas sensors, cancer therapy and high effect solar cell. However, investigations demonstrated that the properties and applications of titanium oxide films depend upon the nature of the crystalline phases present in the films, i.e. anatase and rutile phases. We report on the pulsed laser deposition of high quality TiO2 thin films. Pulsed Laser deposition of TiO2 thin films were performed in different ambient viz. oxygen, argon and vacuum, using a second harmonic of Nd:YAG laser of 6 ns pulse width. These deposited films of TiO2 were further annealed for 5hrs in air at different temperatures. TiO2 thin films were characterized using x-ray diffraction, SEM, photoluminescence, transmittance and reflectance. We observed effect of annealing over structural, morphological and optical properties of TiO2 thin films. The anatase phase of as-deposited TiO2 thin films is found to change into rutile phase with increased annealing temperature. Increase in crystalline behaviour of thin films with post-annealing temperature is also observed. Surface morphology of TiO2 thin films is dependent upon ambient pressure and post- annealing temperature. TiO2 thin films are found to be optically transparent with very low reflectivity hence will be suitable for antireflection coating applications.

Effects of defects generated in ALD TiO2 films on electrical properties and interfacial reaction in TiO2/SiO2/Si system upon annealing in vacuum

Thin TiO2 layers grown at 130°C on SiO2-coated Si substrates by atomic layer deposition (ALD) using TTIP and H2O as precursors were annealed, and the effects of the annealing temperature on the resulting electrical properties of TiO2 and the interface properties between a Pt electrode and TiO2 were examined using transmission line model (TLM) structures. The as-deposited TiO2 thin film had an amorphous structure with OH groups and a high resistivity of 6×103Ω-cm. Vacuum annealing at 700 °C transformed the amorphous film into an anatase structure and reduced its resistivity to 0.04Ω-cm. In addition, the vacuum-annealing of the TiO2/SiO2 structure at 700°C produced free silicon at the TiO2-SiO2 interface as a result of the reaction between the Ti interstitials and SiO2. The SiO2 formed on the TiO2 surface caused a Schottky contact, which was characterized by the TLM method. The use of the TLM method enabled the accurate measurement of the resistivity of the vacuum-annealed TiO2 films and the characterization of the Schottky contacts of the metal electrode to the TiO2.

Fabrication of rutile TiO2 thin films by low-temperature, bias-assisted cathodic arc deposition and their dielectric properties

The use of rutile-type titanium dioxide (TiO2) thin films as advanced gate dielectrics has been hampered by thermodynamic instability during the high deposition or annealing temperature of 800 °C. In this work, we demonstrate that rutile-type TiO2 thin films can be produced on p-type Si (100) at lower substrate temperature by means of bias-assisted cathodic arc deposition. The influence of the substrate bias on the microstructural and dielectric characteristics of the TiO2 thin films is investigated in detail. Our results show that by applying a suitable bias to the Si substrate, as-deposited rutile-type TiO2 thin films can be obtained at 450 °C. The permittivity of the materials increases significantly from 21 up to 76. The interfacial and electrical properties of TiO2/Si (100) are also improved. The effects and mechanism of the bias on the microstructural and dielectric characteristics are described.

Spectroscopic Ellipsometry and Raman Studies on Sputtered TiO<sub>2</sub> Thin Films

Titanium dioxide films were deposited using DC magnetron sputtering technique onto silicon substrates at ambient temperature and at an oxygen partial pressure of 7×10 –5 mbar and sputtering pressure (Ar + O2) of 1×10–3 mbar. The composition of the films, analyzed by Auger Electron Spectroscopy (AES), revealed the stoichiometry with an O and Ti ratio of 2.08. The optical constants of the as-deposited TiO2 thin film were determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric-function spectra reveal distinct structures at energies of the E1, E1+D1 and E2 critical points due to interband transitions. The Dielectric constant values were found to be substantially lower than those for the bulk TiO2. The dielectric related optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity are presented and analyzed. The deposited films were calcinated at 673 and 773 K. The influence of post-deposition calcination on the Raman scattering of the films was studied. The existence of Raman active modes A1g, B1g and Eg corresponding to the Raman shifts are reported in this paper. The improvement of crystallinity of the TiO2 films as shown by the Raman scattering studies has also been reported.

Biomimetic Micropatterning of Titanium Dioxide Thin Films for Gate Dielectrics

We have succeeded in fabricating micropatterns of TiO2 thin films on self-assembled monolayers (SAMs). SAMs of OTS (octadecyltrichlorosilane) were formed on Si wafers and modified by UV irradiation using a photomask to generate octadecyl/silanol-pattern. They were used as templates to deposit TiO2 thin films by the use of TDD (titanium dichloride diethoxide) dissolved in toluene. Amorphous films were selectively deposited on silanol regions. Line width variation of the pattern was improved to be well below the current electronics design rule, 5%. Dielectric constant of an as-deposited TiO2 thin film, dielectric properties of TiO2/SiO2/Si interface and leakage current density were evaluated by measuring I-V and C-V characteristics of the MOS (metal-oxide-semiconductor) device. Rather low leakage current was observed under high electric fields and the TiO2 thin film with the thickness -18 nm showed dielectric constant of -22 at 100 kHz, which is more than 5 times as large as that of a usual SiO2 dielectric layer. However, the resistivity of the TiO2 film was estimated to be not high enough and the dielectric constant depended on frequency, both of which are disadvantages for a gate dielectric.

Electrostatic sol-spray deposition (ESSD) and characterisation of nanostructured TiO2 thin films

A novel film fabrication technique, i.e. electrostatic sol-spray deposition (ESSD), has been used in this study to prepare TiO2 thin films of various surface morphologies and crystalline structures. Scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and UV/Vis absorption spectroscopy were employed to characterize the as-deposited and annealed TiO2 thin films. It has been found that silicon dopant can increase the anatase-to-rutile phase transformation temperature up to about 1050°C. Two possible mechanisms are proposed to explain the phase stabilisation effect, i.e. reduction of the oxygen vacancy concentration by dissolution of SiO2 in the TiO2 lattice and segregation of a SiO2 second phase. The energy bandgap of amorphous SiO2 doped TiO2 films decreases with increasing annealing temperature.

Effect of rapid thermal annealing on the structural and electrical properties of TiO2 thin films prepared by plasma enhanced CVD

Titanium oxide thin films were deposited on p-type Si(100), SiO2/Si, and Pt/Si substrates by plasma enhanced chemical vapor deposition using high purity Ti(O-i-C3H7)4 and oxygen. As-deposited amorphous TiO2 thin films were treated by rapid thermal annealing (RTA) in oxygen ambient, and the effects of RTA conditions on the structural and electrical properties of TiO2 films were studied in terms of crystallinity, microstructure, current leakage, and dielectric constant. The dominant crystalline structures after 600 and 800 ‡C annealing were an anatase phase for the TiO2 film on SiO2/Si and a rutile phase for the film on a Pt/Si substrate. The dielectric constant of the as-grown and annealed TiO2 thin films increased depending on the substrate in the order of Si, SiO2/Si, and Pt/ Si. The SiO2 thin layer was effective in preventing the formation of titanium silicide at the interface and current leakage of the film. TEM photographs showed an additional growth of SiOx from oxygen supplied from both SiO2 and TiO2 films when the films were annealed at 1000 ‡C in an oxygen ambient. Intensity analysis of Raman peaks also indicated that optimizing the oxygen concentration and the annealing time is critical for growing a TiO2 film having high dielectric and low current leakage characteristics.