TiO₂ Coating for SnO₂:F Films Produced by Filtered Cathodic Arc Evaporation for Improved Resistance to H⁺ Radical Exposure

Abstract

Titanium dioxide thin films were deposited by filtered cathodic arc evaporation (FCAE) from a Ti target in an oxygen atmosphere onto (a) fluorine-doped tin oxide substrates SnO₂:F (FTO) and (b) glass microscope slides. The growth rate calculated from film thickness profilometry measurements was found to be approximately 0.8 nm/s. The films were highly transparent to visible light. x-Ray photoemission spectroscopy analysis of the Ti 2p electron binding- energy shift confirmed the presence of a TiO₂ stoichiometric compound. The results for the root-mean-square (RMS) surface roughness of the films deposited onto FTO substrates evaluated by atomic force microscopy suggested nanostructured film surfaces. When exposed to hydrogen plasma, TiO₂ films revealed insignificant changes in the optical spectra. The initial sheet resistance of the SnO₂:F layer was 14 Ω/sq. The deposition of the top TiO₂ layer (45 nm thick) over the FTO electrode resulted in an increase of the sheet resistance of 2 Ω/sq. In addition, the sheet resistance of the double-layer FTO/TiO₂ transparent conductive oxide (TCO) electrode increased by 1 Ω/sq as a result of H⁺ plasma exposure. Regardless of the TiO₂ film’s low conductivity, a thin protective layer could be coated onto FTO films (presumably 15 nm thick) due to their high transparency, offering high resistance to aggressive H⁺ plasma conditions. In this paper we show that ∼50-nm-thick TiO₂ coating on FTO films provides sufficient protection against deterioration of transparency and conductivity due to hydrogen radical exposure.