The selective growth of rutile thin films using radio-frequency magnetron sputtering

Using unbalanced radio-frequency (RF) magnetron sputtering crystalline rutile films were synthesised on glass substrates at (combined Ar and O2) pressures of 0.4 Pa or less, at RF powers of 500 and 600 W with substrate to magnetron distances of 40 mm or longer. Anatase films were deposited at the greater pressure of 1.2 Pa (substrate to magnetron distance of 40 mm) or shorter substrate to magnetron distance of 20 mm (at 0.4 Pa). A mixture of anatase and rutile was formed at 0.5 Pa with all other conditions being as for those required for rutile or the power was reduced along with the substrate to magnetron distance (500 W and 20 mm). The crystallite sizes of rutile obtained were 1 – 3 nm. It is proposed that the greater the energy imparted to the substrate surface by the impinging positive species the greater the activation energy to crystalline phase formation that can be overcome. Hence the formation of rutile over anatase is favoured at greater power, longer magnetron to substrate distances and decreased pressure. Moreover, not only is it possible to control the phase of TiO2 formed it appears to be possible to control the degree of oxygen non-stoichiometry in the rutile films formed. Smaller O2 partial pressures, shorter substrate to magnetron distances and greater RF power are believed to produce an environment of reduced reaction of sputtered Ti species with O2 and to result in the formation of non-stoichiometric rutile structures resulting in increased band gap energies and decreased refractive indices.

► Crystalline rutile films synthesized using unbalanced radio-frequency magnetron sputtering.
► Low pressure, high RF power and long distances between substrate and magnetron are used.
► UV-Vis optical simulation predicts film thickness.
► Band-gap energy is affected by the partial oxygen pressure.
► Rutile has finer crystallite size than anatase.