Optical EngineeringInfluence of deposition parameters on optical properties of TiO2 films
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Titanium dioxide is extensively used as high index material for multilayer optical thin film device applications operating in the visible and near infrared region. The performance (high reflectance/transmittance and laser-induced damage) of the device is decided by the absorption in the films. The refractive index and extinction coefficient of TiO2 films is strongly influenced by the deposition parameters and stoichiometry of the evaporation material. In the present investigation, TiO2 films have been deposited by reactive electron beam evaporation of TiO, Ti2O3, and TiO2 in a neutral and ionized oxygen atmosphere. A Heitmann-type discharge source has been fabricated in the laboratory and used to ionize oxygen. Deposition parameters such as oxygen partial pressure (5 x 10-5 to 5x 10-4 torr), rate of deposition (60 to 210 Åmin-1), and substrate temperature (25 to 250°C) were varied during the preparation of the films. The optical constants of TiO2 films were estimated from the spectrophotometer data. In-situ optical monitoring of TiO2 films with suboxides as the starting material showed the presence of considerable absorption in the films deposited in neutral oxygen, even under favorable deposition conditions. Postdeposition heating was necessary to reduce the absorption in the films. TiO2 films with minimum absorption have been made using TiO2 starting materials. Absorption-free films have also been obtained using ionized oxygen with the starting materials, even at higher substrate temperatures. The observed variation in optical properties has been explained on the basis of mismatches between the film growth and rate of oxidation.