Rutile Thin Films Research Articles

Sol–Gel Preparation and Third-Order Nonlinear Optical Properties of TiO2 Thin Films

Abstract Rutile and anatase thin films have been prepared by the sol–gel method using Ti(OC3H7i)4. Third-order nonlinear optical properties of both TiO2 thin films have been investigated by the third-harmonic generation (THG) method and the effect of the polymorph of TiO2 on the third-order nonlinear optical susceptibility, χ(3), has been examined. The measured χ(3) values of rutile and anatase thin films were 1.4 × 10−12 and 9.7 × 10−13 esu, respectively. The χ(3) values corrected for the porosity of the film were 4.0 × 10−12 (rutile) and 2.4 × 10−12 esu (anatase), which are about 100 times as high as that of SiO2 glass used as standard sample (2.8 × 10−14 esu). The measured and corrected χ(3) values were discussed in comparision with those calculated on the basis of several models.

Electrical and optical properties of TiO2 anatase thin films

Electrical and optical spectroscopic studies of TiO2 anatase thin films deposited by sputtering show that the metastable phase anatase differs in electronic properties from the well-known, stable phase rutile. Resistivity and Hall-effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self-trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.

Electromagnetic modes and prism-film coupling in anisotropic planar waveguides of epitaxial (101) rutile thin films

We report optical waveguiding in single-crystal, epitaxial (101) oriented rutile (TiO2) thin films grown on (112̄0) sapphire (α-Al2O3) substrates using the metal-organic chemical vapor deposition technique. The electromagnetic field distributions and propagation constants for asymmetric planar waveguides composed of an anisotropic dielectric media applicable to these films are derived. Modifications to the prism-film coupling theory for this anisotropic case are also discussed. By application of this model to (101) oriented rutile thin films, we directly obtain values of the ordinary and extraordinary refractive indexes, n0 and ne, of the rutile thin films as well as film thicknesses. We obtain typical values of the refractive indexes (n0=2.5701±0.0005; ne=2.934±0.001) near to those for bulk rutile single crystals indicating the exceptional quality of these films.

Prism-Film Coupling in Anisotropic Planar Waveguides of Epitaxial (101) Rutile Thin Films

AbstractWe report optical waveguiding in single-crystal, epitaxial (101) oriented rutile (TiO2) thin films grown on (1120) sapphire (α-Al2O3) substrates using the MOCVD technique. The propagation constants for asymmetric planar waveguides composed of an anisotropic dielectric media applicable to these films are derived. Modifications to the prism-film coupling theory for this anisotropic case are also discussed. By application of this model to (101) oriented rutile thin films, we directly obtain values of the ordinary and extraordinary refractive indexes, no and ne, of the rutile thin films as well as film thicknesses. We obtain typical values of the refractive indexes (no=2.5701±0.0005; ne=2.934±0.001) near to those for bulk rutile single crystals indicating the exceptional quality of these films.

Study of defects and interfaces on the atomic scale in epitaxial tio2thin films on sapphire

Abstract TiO2 thin films grown on (11 0) sapphire (α-Al2O3) at 800°C by the metallo-organic chemical vapour deposition technique have been characterized by transmission electron microscopy. The TiO2 thin films are single-crystal rutile. The epitaxial orientation relationship between the rutile thin films (R) and the sapphire substrates (S) is (101)[010]R‖(110)[0001]s. Growth twins are commonly observed in the films with (101) twin planes and twinning directions. The atomic structure of twin boundaries and TiO2-α-Al2O3 interfaces have been investigated by high-resolution electron microscopy. When the interfaces are viewed in the direction of [010]R-[0001]s, the interfaces appear structurally coherent along the direction of [10]R-[100]s. The small misfit (0.5%) is accommodated at interface steps. In contrast, in the direction of [10]R-[100]s, the interfaces are semicoherent. Growth mechanisms are discussed, based on information about the atomic structure of the interfaces.

Microstructure of TiO2 rutile thin films deposited on (110) α−Al2O3

TiO2 rutile thin films grown on (110) sapphire (α−Al2O3) by the MOCVD technique have been characterized by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The TiO2 rutile thin films grew on the sapphire with two epitaxial orientations. The epitaxial orientation relationships between the rutile films (R) and the sapphire substrate (S) were found to be (1) (101)[010]R ‖ (110)[0001]s. Detailed atomic structures of near-interface regions have been investigated by HREM, providing a clear picture of the initial stage of film growth. HREM images show that about 70% of the nuclei at the interface are the (101) rutile, but most of them are very small, about 5 nm (or 2% of the film thickness) in the growth direction. The film growth was dominated by the (200) orientation. Nucleation and growth of the films will be discussed in terms of the lattice mismatch at the interface and growth rates along the two orientations. Planar defects such as twin boundaries and special grain boundaries are commonly observed in the films, especially in regions close to the substrate. The twin plane and twinning direction are {101} and 〈101〉, respectively. Special grain boundaries are found to be correlated with nucleation and twinning.

Fabrication of Oriented Polycrystalline Titania Films by Sol-Gel Process

Various hydrosols of rutile and anatase particles were prepared by aging or autoclaving of amorphous titanium hydroxide precipitated from solutions. Crystalline gel films were formed by drying the sols on a glass substrate and separated from the substrate for sintering. The particles in the films, both rutile and anatase, have a tendency to orientate with the tetragonal caxis parallel to the film plane even before sintering. The degree of orientation increased with grain growth during firing. Rutile polycrystalline thin films, about 10μm in thickness, were obtained by sintering at 1300°C.

Electrical Properties of Rutile (TiO2) Thin Film

Rutile thin films of Au(Ti, Al)–TiO2–Pt sandwich system were prepared on a substrate glass by evaporation of Ti metal and oxidation of it by heat treatment. Through AC and DC measurements, the capacitance obtained was about 2000 pF/mm2, the dielectric constant was estimated as 60∼110, tan δ was 1∼10%, the conductivity was about 10-13Ω-1cm-1 and the electrical strength was 5∼10 V (0.3∼0.5 MV/cm), respectively. In the micro-sec- and micro-sec-pulse conduction measurement, the relation of current and time became I∝t-n, in which the value of n was 0.7∼0.8. The polarity effect on the current was considered to be due to the work functions of electrodes. The current carrier was estimated to be electronic from the measurement of photoconduction and the polarity effect in the pulse conduction.

Mechanism of Heterogeneous Deposition of Thin Film Rutile

The kinetics of rutile thin film deposition on silicon from the reaction has been studied over a range of temperature (673°–1320°K), titanium tetrachloride partial pressure , and oxygen partial présure in a horizontal flow system with a rf‐heated susceptor. Between 673° and 1120°K the reaction has been found to proceed by a Rideal‐Eley mechanism with the surface reaction rate constant following the relationship exp , over a range of partial pressures and flow rates. Homogeneous reaction becomes appreciable above 1120°K.