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

In this study, hydrogenatedtitanium dioxide (TiO2:H)thin films of thicknesses between 33 and 300 nm were grown via thereactive radio-frequency magnetron sputter deposition technique. Thesethin films were characterized with respect to electrical resistivity,Seebeck coefficient, and optical absorption coefficient. However,heating to temperatures above 430 K results in irreversible propertychanges of the thin films. The characteristics of these changes dependedon the atmosphere, in which the samples were thermally treated (10(-7) mbar vacuum or 1 bar of 99.99% N-2). Inorder to explain these findings, we investigated our samples not onlyin the as-deposited state but also after thermal cycling over differenttemperature ranges. Mott's three-dimensional (3D) variablerange hopping model was identified as the most appropriate electricalconductivity model in the as-deposited state for the temperature rangeof 223-430 K, after which it changes irreversibly to the smallpolaron hopping model in the 223-615 K temperature range. Thisthermally induced change appears to be due to changes in the numberof intrinsic (interstitial titanium Ti-int, titanium vacanciesV(Ti), and oxygen vacancies V-o) and extrinsic(hydrogen dopants) defects in the material. Spectroscopic ellipsometrymeasurements support this assumption. For this, we developed a dielectricdispersion model for TiO2:H thin films, which combinesthe Cody-Lorentz model with an additional Lorentz oscillator.In the as-deposited state, an additional peak of the absorption coefficientappears at 1.3-1.4 eV, which disappears for samples thermallytreated in N-2 atmosphere but is retained for samples thermallytreated in vacuum.