Optimisation of surface treatments of TiO2:Nb transparent conductive coatings by a post-hot-wire annealing in a reducing H2 atmosphere

Abstract

Transparent and electrically conductive niobium-doped TiO2 thin films have been deposited on glass surfaces by d.c.-pulsed reactive magnetron sputtering from a composite Ti:Nb target, using oxygen as reactive gas. A rapid 1 min annealing at 500 °C in an atomic hydrogen rich atmosphere, obtained by flowing H2 on a Ta filament resistively heated to 1750 °C in vacuum (hot-wire), proved to be very efficient in enhancing the electrical properties of these ~100 nmthick TiO2:Nb thin films. Dark conductivity (σd) and its activation energyweremeasured as a function of (inverse) temperature and the value of σd at room temperature was used to assess the effect of the H2 annealing on the transport properties. A 5-order ofmagnitude increase in electrical conductivitywas observed for optimised treatment conditions at a hydrogen pressure of 10 Pa. A maximum value of σd in the range of ~1.4 × 103 S/cm was attained for optimised conditions, where a level of ~6 at.% of H doping was measured close to the film surface. X-ray photoelectron spectroscopy, elastic recoil detection analysis, Rutherford backscattering and Raman spectroscopies were used to access information of composition and film structure for the explanation of the strong enhancement of the film's electrical conductivity and band-gap widening to 3.45 eV following hot-wire treatments. These thin films can be used as transparent conductive oxide contact layers for photovoltaic applications.