Unusual Electronic Transport and Magnetism in Titanium Oxide based Semiconductors and Metals
Venkatesan, Thirumalai V
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The main objective of this thesis was to explore the structural, electrical, magnetic and optical properties of titanium based novel oxide thin films, such as transparent conducting oxides (TCOs) and diluted magnetic semiconductors (DMSs), so as to be able to realize optoelectronics and spintronics applications. I demonstrated that niobium doped titanium dioxide (TiO2) in its epitaxial anatase phase grown at certain condition is an intrinsic transparent conducting oxide, with both its conductivity and transparency comparable to that of the commercial transparent electrode In-Sn-O being widely used in current optoelectronic devices. I investigated the growth parameter dependence of structure and conductivity of this material. It was found that the growth temperature is a crucial parameter for the structural quality as well as the electron mobility, while the oxygen partial pressure is essential for the conduction electron concentration. The excellent conductivity of niobium doped TiO2 should be attributed to the extremely high solubility of niobium in the TiO2 matrix as well as a very shallow donor level created in the TiO2 band gap. I investigated several important oxide based DMS systems, such as niobium and cobalt dual doped TiO2, transition metal (TM) element doped SrTiO3 etc. I found that niobium dual doping is an effective way to introduce carriers into the classical Co: TiO2 system, which provides the feasibility of studying the RKKY interaction in this system by chemical doping. Our detailed characterization of TM doped SrTiO3 questioned the intrinsic nature of the ferromagnetism observed by other groups. By a systematic study of Hall effect on superparamagnetic Co-(La,Sr)TiO3 thin films, I was able to demonstrate that the magnitude of the anomalous Hall effect is a way to distinguish between intrinsic and extrinsic DMS. A Kondo effect was observed in niobium doped TiO2 grown at certain condition. The origin of magnetic moments in this system was suggested to be from the cation vacancy defects. This observation of defect magnetism in conventional non-magnetic TiO2 may shed light on the occurrence of ferromagnetism in oxide diluted magnetic semiconductors.