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Alexei Nateprov
Investigation of growth, structural and electronic properties of V2O3 thin films on selected substrates
Supervisor: Prof. Dr. Siegfried Horn [Experimental physics II]
Date of oral examination: 12/01/2006
87 pages, english
Transition metal oxides are of continuous interest in modern solid state physics due to their exceptional electronic, magnetic and optical properties. Many transition metal oxides show phase transitions from metallic to insulating or semiconducting states. The electronic structure of these materials is strongly influenced by the partially filled d-orbitals and related strong Coulomb interaction. Vanadium oxides are one the most typical examples of transition metal oxides. In particular, vanadium sesquioxide (V2O3) which is often referred to as an prototype of a strongly correlated electron system. This compound shows a sharp metal to insulator transition (MIT) at TMIT~160-170 K, quantified by a six orders of magnitude increase of the resistance. During last several decades a lot of experimental and theoretical studies were done to understand the origin of this transition. However, a consistent model, accounting for the features of the MIT- as well as of the related magnetic and structural phase transitions, is absent till now, thus making these phenomena and the material itself still highly actual nowadays. The present work is devoted to the experimental study of the MI transition in V2O3 thin films, grown on different substrates. The main goal of the work was to develop a technology of growth of V2O3 thin films on substrates with different electrical and structural properties (diamond and LiNbO3), designed for specific applications. Because of the large lattice mismatch between V2O3 and diamond, the film growth presents a rather complicated problem, which was not solved before the present work started. Another important problem of growth technologies is the temperature of substrate during the deposition. Especially, because of high mobility of atoms of light elements (like Li) at higher temperatures, the diffusion of these elements into the growing film is enhanced, yielding a deterioration of the properties of V2O3. We present here a new growth strategy for V2O3 thin films, based on the UHV technique, which successfully solves the above mentioned difficulties. Using this technology, thin films of V2O3 with reproducible parameters were grown on diamond and LiNbO3 substrates for the first time. The structural and electrical properties of the obtained films were characterized in detail with a special focus on their potential applications. The MIT of V2O3 was investigated by SAW using first directly deposited V2O3 thin film onto a LiNbO3 substrate. A softening of the sound velocity shift, which occurs at about 60 K above the resistivity jump in the paramagnetic metallic phase, provides experimental evidence for a precursor of the MIT of V2O3.