| Novel functionalities of Vanadium dioxide (VO2), such as, several orders of magnitude transition in resistivity and IR transmittance, provide an exciting opportunity for the development of next generation memory, sensor, and field-effect based devices. A critical issue in the development of practical devices based on metal oxides is the integration of high quality epitaxial oxide thin films with the existing silicon technology which is based on silicon (100) substrates. However, silicon is not suitable for epitaxial growth of oxides owing to its tendency to readily form an amorphous oxide layer or silicide at the film-substrate interface. The oxide films deposited directly on silicon exhibit poor crystallinity and are not suitable for device applications. To overcome this challenge, appropriate substrate templates must be developed for the growth of oxide thin films on silicon substrates.;The primary objective of this dissertation was to develop an integration methodology of VO2 with Si (100) substrates so they could be used in "smart" sensor type of devices along with other multifunctional devices on the same silicon chip. This was achieved by using yttria-stabilized zirconia (YSZ) template layer deposited in situ. It will be shown that if the deposition conditions are controlled properly, YSZ can be grown epitaxially on silicon substrates even if the native oxide is not etched completely prior to deposition. I have used this approach to integrate VO2 thin films with Si (100) substrates using pulsed laser deposition (PLD) technique. The deposition methodology of integrating VO2 thin films on;silicon using various other template layers will also be discussed. The detailed x-ray diffraction, transmission electron microscopy (TEM), electrical characterization results for the deposited films will be presented. In the framework on domain matching epitaxy, epitaxial growth of VO2 (tetragonal crystal structure at growth temperature) on both tetragonal and cubic YSZ has been explained. Our detailed phi-scan X-ray diffraction measurements corroborate our understanding of the epitaxial growth and in-plane atomic arrangements at the interface. It was observed that the transition characteristics (sharpness, over which electrical and optical property changes are completed, amplitude, transition temperature, and hysteresis) are a strong function of microstructure, strain, and stoichiometry. We have shown that by choosing the right template layer, strain in the VO2 thin films can be fully relaxed and near-bulk VO2 transition temperatures can be achieved. We have demonstrated this by using NiO as a buffer layer on Al2O3 (0001) substrate.;We have also used swift heavy ion irradiation to induce controlled modifications in the semiconductor-to-metal transition characteristics of VO2 single-crystal thin films with varying ion fluences. At very high energies of ions (200 MeV), the electronic stopping (∼2009 eV/A) dominates over nuclear stopping (∼16 eV/A). Under these extreme electronic excitation conditions caused by electronic stopping and the passage of swift heavy ions through the entire thickness of the film, we expect creation of certain unique type of defects and disordered regions. Detailed characterization using X-ray diffraction, Raman spectroscopy, infra-red transmission spectroscopy, x-ray photoelectron spectroscopy (XPS), and electrical measurements were performed to investigate the characteristics and role of these defects on structural, optical, and electrical properties of VO2 thin films. XPS andelectrical resistivity measurements suggest that the ion-irradiation induces localized defect states which appear to correlate well with the creation of disordered regions in the VO2 thin films. The high energy heavy ion-irradiation changes the transition characteristics drastically from a first-order to a second-order transition (electronic -- Mott type). The low temperature conductance data for these ion-irradiated films fits well with the quasi-amorphous model for resistivity of VO2 where ion-irradiation is believed to create mid band-gap defect states. |
