Large Effect of Temperature Cycling on the Electrical and Optical Properties of TiO₂:H Thin Films

Abstract

In this study, hydrogenated titanium dioxide (TiO 2 :H) thin films of thicknesses between 33 and 300 nm were grown via the reactive radiofrequency magnetron sputter deposition technique. These thin films were characterized with respect to electrical resistivity, Seebeck coefficient, and optical absorption coefficient. However, heating to temperatures above 430 K results in irreversible property changes of the thin films. The characteristics of these changes depended on the atmosphere, in which the samples were thermally treated (10 −7 mbar vacuum or 1 bar of 99.99% N 2 ). In order to explain these findings, we investigated our samples not only in the asdeposited state but also after thermal cycling over different temperature ranges. Mott's three-dimensional (3D) variable range hopping model was identified as the most appropriate electrical conductivity model in the asdeposited state for the temperature range of 223−430 K, after which it changes irreversibly to the small polaron hopping model in the 223−615 K temperature range. This thermally induced change appears to be due to changes in the number of intrinsic (interstitial titanium Ti int , titanium vacancies V Ti , and oxygen vacancies V o ) and extrinsic (hydrogen dopants) defects in the material. Spectroscopic ellipsometry measurements support this assumption. For this, we developed a dielectric dispersion model for TiO 2 :H thin films, which combines the Cody−Lorentz model with an additional Lorentz oscillator. In the as-deposited state, an additional peak of the absorption coefficient appears at 1.3−1.4 eV, which disappears for samples thermally treated in N 2 atmosphere but is retained for samples thermally treated in vacuum.