Semiconductor-metal transition in epitaxial VO_2 thin films on TiO_2 (100) considering a phase equilibrium

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Vanadium dioxide (VO_2), standing out from the thermochromic vanadium oxide family (VO,V_2 O_3,V_2 O_5 ,VO_2), is intensively studied for its abrupt and reversible semiconductormetal transition (SMT) that happens slightly above room temperature (T_c 340 K). Interestingly, the transition can be triggered not only by temperature change, but also by many other forms of excitations, e.g. infrared irradiation, external strain, and applied voltage. The underlying mechanism of the transition is still in debate, and a model to describe the transport properties near T_c is needed for utilizing this material. Recent studies have revealed that near the transition, there is a phase coexistence between the semiconducting monoclinic phase and the metallic rutile phase in this material. In this presentation, the growth of epitaxial VO_2 thin films on TiO_2 substrates by chemical vapor deposition and the characterizations of their structural and electrical properties will be discussed. Detailed studies on DC resistivity, current-voltage characteristics and impedance spectroscopy of deposited films at various ambient temperatures have been performed. By considering the ensemble average of the transport properties of VO_2 films near T_c as a superposition of the transport properties of the two phases, and assuming a distribution of transition temperatures across the whole film, we have derived parameterized models to fit the temperature dependence of both resistivity and impedance of VO_2 thin films in the vicinity of SMT. Voltage triggered breakdown in micron-scale two-terminal planar VO_2 devices has been tested and modeled. An avalanching-like breakdown mechanism is elicited by considering a competition between Joule heating and heat dissipation, that could explain the faster-than-expected transition observed in this kind of devices. To further study the physics of SMT, the design and implementation of a cross-spectrum analyzer for measuring VO_2 transport noise near T_c will be demonstrated. Our preliminary results reveal a significant increase in the noise power spectral density near the transition, possibly induced by the metastability of current percolation.

Electronic Thesis or Dissertation
Materials science