Growth Study and Characterization of TiO2 Based Thin Films for Low Temperature Synthesis of Transparent Conductive Films

Transparent conductive oxides (TCOs) are of increasing importance for displays, photovoltaics, architectural and window glass technologies. The most utilized TCOs are tin-doped indium oxide (ITO), which is suffering from scarcity, lower transmittance in the near infrared region, poor chemical stability in hydrogen atmosphere and toxicity. Due to these limitations, there is a huge interest to develop alternative TCOs which possess higher conductivity than indium or that use less scarce elements than indium. Currently, many potential candidates are under the extensive study to replace ITO including doped ZnO, SnO2 and TiO2. The core objectives of the work presented in this thesis were: (i) to develop a method for the low temperature (≤350 °C) synthesis of TiO2-based transparent conductive films, which is chemically stable, inexpensive and non-toxic material; and (ii) to investigate the growth-properties relationship to understand the factors dictating the doping process and properties of Nb-doped TiO2 films (TNO). The approach was to explore three doping methods of the films: intrinsic doping, extrinsic doping and, with the aim to combine the benefits of both, intrinsic-extrinsic co-doping. TNO films were deposited by RF sputtering on various substrates including glass, silicon and Kapton polyimide under different plasma power conditions in Ar and Ar-O2 discharge. Two types of niobium containing targets, Nb metal and Nb2O5 oxide were employed simultaneously with ceramic TiO2 target for the films growth. Thermal and laser annealing processes were carried out for the films crystallization and doping. The structural analysis by x-ray diffraction (XRD) revealed a poor control over the anatase phase (mixed anatase and rutile phase formation) when the substrate was heated during the films growth, which degraded the films transport properties. The adjustment of the process pressure and substrate temperature enhanced the conductivity of the annealed TNO films and allowed to tune the resistivity over a wide range (10-1-10-4 Ω.cm). The resistivity measured for the intrinsically and extrinsically doped films were of the order of 10-2 Ω.cm, which is higher than that of ITO (< 10-3 Ω.cm). The lowest resistivity was obtained for intrinsically-extrinsically co-doped TiO2 (7.45×10-4 Ω.cm) with optical transparency of 65-90 % in the wavelength range 400-750 nm. The findings of this work show that the conductivity of the films is strongly correlated with the crystalline structure particularly on the amount of anatase phase with respect to rutile and with the oxygen vacancies in the films. The approach adopted in this work for the generation of oxygen vacancies could be useful for other oxide-based films, where the oxygen vacancies-dependent properties are crucial, for room temperature ferromagnetism and photocatalytic applications. The obtained results can significantly contribute to the development of transparent electrodes by RF sputtering, a suitable technique for coating on large area substrates.