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 θ-2θ scan and pole figure plots confirmed the epitaxial relationship, which were rutile (1 0 1)∥sapphire (1 1 0), (0 1 0)_f∥(0 0 1)_s, and anatase (0 0 1)∥SrTiO₃(0 0 1), (1 0 0)_f∥(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 being expanded. According to the results of spectroscopic ellipsometry, the films show very high 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.