Photoactivity of Dopped TiO2 Thin Films by Ion Sputter Deposition

Titanium dioxide is well known as a photoactive material to be activated under ultraviolet irradiation and is either employed as a photocatalyst or exhibits superhydrophilic behavior after reducing the surface energy under illumination for self-cleaning or anti-fogging surfaces. For increasing the reactivity of the thin films under solar illumination, a reduced band gap is desired. Doping with transition metals or with nitrogen has been reported in the literature. However, the incorporation of nitrogen into the growing film is a much more complex process, which is presently not completely understood. TiO2 thin layers were produced by ion beam sputter deposition, which should allow the usage of temperature sensitive substrates. The deposition has been performed in the existing experimental setup, previously used for investigations on Ag and Ge thin film physical vapor deposition processes, where the influence of the sputtered and reflected particles and their accompanying energy flux on the grain size, surface morphology, and conductivity have been measured. By adjusting the geometry between the incident ion beam, the sputter target, and the substrate, independently from the primary ion energy and species, a controlled deposition of samples was possible. Conventional ion implantation was employed to implant either carbon or nitrogen ions below the surface for bandgap engineering. The resulting thin films have subsequently been investigated for optical properties, stoichiometry, structural properties, surface topography, and photoactivity. The aim was to find an optimized geometry and to determine the exact particle and energy flux necessary for these conditions. Thus, the process opens up for transfer or scaling when assuming future industrial applications.