Asuka Terai

Preparation and characterization of polycrystalline anatase and rutile TiO2 thin films by rf magnetron sputtering

Titanium dioxide films with the anatase and rutile single phase were formed on Si substrates by rf sputtering through a precise control of critical parameters. The structure of the films was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the optical properties were evaluated with spectroscopic ellipsometry (SE). Lattice distortion was found in both anatase and rutile films from TEM observation. The obtained refractive indices n exhibit higher values than those reported for thin films due presumably to the density structure of the sputtered films. Optical band gaps were calculated by Tauc plot using the obtained extinction coefficient separately for anatase and rutile, with values larger than those reported for bulk materials. The reasons for the larger band gap might be due to the strain from lattice distortion.

AB-INITIO CALCULATIONS OF THE OPTICAL BAND GAP OF TiO2 THIN FILMS

Titanium dioxide has been extensively studied in recent decades for its important photocatalytic application in environmental purification. The search for a method to narrow the optical band gap of TiO2 plays a key role for enhancing its photocatalytic application. The optical band gap of epitaxial rutile and anatase TiO2 thin films deposited by helicon magnetron sputtering on sapphire and on SrTiO3 substrates was correlated to the lattice constants. The optical band gap of 3.03 eV for bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of 3.20 eV for bulk-anatase increases to 3.51 on SrTiO3. In order to interpret this expansion, ab-initio calculations were performed using the Vienna ab-initio software. The calculations for rutile as well anatase show an almost linear increase of the band gap width with decreasing volume or increasing lattice constant a. The calculated band gap fits well with the experimental values. The conclusion from these calculations is, in order to achieve a smaller band gap for both, rutile or anatase, the lattice constant c has to be compressed, and a has to be expanded.

Simultaneous epitaxial growth of anatase and rutile TiO2 thin films by RF helicon magnetron sputtering

Epitaxial films of TiO2 with rutile structure on sapphire and anatase structure on SrTiO3 were simultaneously grown by RF helicon magnetron sputtering of a TiO2 target in Ar atmosphere. X-ray diffraction using y � 2y scan and pole figure plots confirmed the epitaxial relationship, which were rutile (1 0 1)Jsapphire (1 1 0), (0 1 0)fJ(0 0 1)s, and anatase (0 0 1)JSrTiO3(0 0 1), (1 0 0)fJ(1 0 0)s, where suffix f and s stand for the film and substrate, respectively. Moreover, observation by transmission electron microscopy identified the epitaxial film growth of single crystalline anatase and rutile structure with slight lattice distortion compared with bulk. The lattice constants of a and b; which were calculated from electron diffraction spots and lattice image in TEM of the films were contracted while that of c beingexpanded. Accordingto the results of spectroscopic ellipsometry, the films show very hig h refractive indices ( n) at the designated wavelength range in comparison with the past reports on TiO2 thin films. Although there are no bulk references in the anatase case, the values n of the rutile film are comparable to the bulk in the data-book. Such high refractive indices of the films indicate the compact texture of the epitaxial films fabricated by helicon sputtering. r 2003 Elsevier Science B.V. All rights reserved.

Spectroscopic Ellipsometry of Yttrium–Iron Garnet Thin Films Containing Gold Nanoparticles

We report complex dielectric function (e) for yttrium–iron garnet thin films containing Au nanoparticles. Films are prepared using a co-sputtering method with post-annealing. Real and imaginary parts of e of the films are obtained using spectroscopic ellipsometry in a photon energy region from near-infrared to ultra-violet. An anomalous dispersion of Re[e] in the visible region originating from localized surface plasmon of Au nanoparticles is clearly observed. The influence of e on the magneto-optical Kerr properties of the nanocomposite films is discussed.