High rate deposition of photoactive TiO2 films by hot hollow cathode

Abstract

In this paper we present a plasma deposition technique that allows the reactive deposition of oxide layers with extremely high deposition rate. The new approach combines reactive sputtering by DC hollow cathode discharge with thermal evaporation from the hot surface of the hollow cathode. As an example of successful fast deposition, photoactive films of titanium dioxide (TiO2) with various thicknesses were deposited using this technique. The uncooled titanium nozzle served as a hot hollow cathode and simultaneously as an inert gas (Ar) inlet. The reactive gas (O2) was introduced into the vacuum chamber through a separate inlet. During deposition, the temperature of the titanium hollow cathode reached up to 1600 °C, depending on the discharge parameters. This made it possible to combine the ion sputtering of hot titanium cathode with its thermal surface evaporation, which significantly increased the TiO2 deposition rate. The highest achieved deposition rate was 567 nm/min (34 μm/h), which (with respect to the geometry of this process) corresponds to total volume of the deposited TiO2 material 1.2 mm3/min per 1 kW of absorbed power. Despite extremely high thermal flux to the substrate, TiO2 films were successfully deposited even on temperature-sensitive PET foil. The as-deposited and post-annealed TiO2 films prepared on fluorine doped tin oxide (FTO) substrates and glass were subject to further analyses including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and photoelectrochemical (PEC) measurements. Whereas the as-deposited TiO2 films had an amorphous (or nearly amorphous) structure, which exhibited only weak photoactivity, after annealing their PEC activity increased by an order of magnitude.