A study on the insulator-to-metal transition in undoped and tungsten-doped vanadium dioxide devices for electronics applications

Vanadium dioxide (VO2) is a strongly corelated material that undergoes reversible insulator-to-metal phase transition (IMT). Electrically driven IMT in VO2 devices is accompanied by an abrupt change in conductivity, negative differential resistance (NDR) and hysteresis. Such attributes are essential for designing tunable devices with reconfigurable characteristics for emerging electronics. In this work we studied the electrically driven IMT process in undoped and tungsten (W) doped vanadium dioxide (WxV1-xO2) devices and we explored its prospective applications in emerging electronics. The first chapter studies the phase transition phenomena in undoped and W-doped vanadium dioxide thin-films. A description of the VO2 electronic properties, phase transition mechanism, prospective applications in electronics along with thin-film characterization is also provided here. The second chapter depicts the fabrication processes of micro/nano-scale devices using standard techniques such as photo-lithography, plasma etching, electron-beam metal deposition. A discussion on the electrical and optical characterization methods are presented here. The static current-voltage (I-V) characteristics of undoped VO2 devices are studied in the third chapter. We explored the threshold-switching characteristics, NDR and charge transport properties in two-terminal devices in this chapter. The implementation of undoped VO2 devices as current controlled oscillators is described in the fourth chapter. We developed the equivalent circuit model for current controlled oscillators and the simulation showed good agreement with the experimentally obtained electrical oscillation waveforms. The optical tuning of oscillation waveforms was achieved through an optical fiber coupled laser. The application of VO2 devices as voltage controlled oscillators is presented in the fifth chapter. We developed the equivalent circuit model for the voltage controlled oscillator. The results of the simulation showed excellent agreement with experimentally generated oscillation waveforms. The graphical phase portrait analysis revealed the underlying mechanism of voltage controlled relaxation oscillations in VO2. The static I-V characteristics of W-doped VO2 devices is presented in the sixth chapter. A large decrease of the threshold voltage by more than 35 V was achieved with W-doping when compared to undoped VO2 devices. Furthermore, such change in VO2 thin-film stoichiometry reduced the hysteresis effect and tuned the NDR characteristics dramatically. The study on the effect of W-doping on the I-V characteristics revealed the underlying electro-thermal mechanism of electrically driven IMT. The finite-element model developed using this electro-thermal framework of phase change showed good agreement with the experiment. Very low voltage Schmitt-trigger and memory storage applications are shown here using the W-doped VO2 devices. In the seventh chapter the dynamic characteristics of W-doped VO2 devices are depicted. Here, room-temperature oscillations in current controlled and in voltage controlled modes, generated in W-doped VO2 devices are shown. This was achieved due to the low threshold voltages attributed to W-doping. Furthermore, electro-chemical impedance spectroscopy (EIS) results showed large tuning in W-doped devices compared to undoped devices. A conclusion is provided in the eight chapter.

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