| Strongly correlated materials, in which electron-electron interactions play a central role, often have remarkable properties and transitions. As an extensive researched correlated transition-metal oxide, vanadium dioxide (VO2) exhibits dramatic metal-insulator transition (MIT) at 68℃, along with remarkable changes of lattice stucture, electronic, optical and magnetic properties. Besides the normal thermal induced phase transition, The MIT process of VO2 can also be triggered by electronic field, radiation field, strain, carrier injection and atomic doping. These attempts to modulate the MIT process are always associated with the status of the VO2 material including its morphology, crystal orientation and surface modification, which supply various experimental routes for phase transition modulation and demonstrate abundant physical contents for VO2 studies. Accordingly, it is meaningful to understand the inter-relations of lattice structure & electronic changes in the transition of VO2, and the nature of electron-electron interactions. Moreover, due to the significant transition properties and near room-temperature transition temperature, VO2 provides promising applications in smart windows, memory devices, FETs, gas sensors, etc.The main work of this paperis as follows:1) An improvement of the electron beam gun evaporator of the solid-state source molecular beam epitaxy is present. By simulating the distrubtion of the electric field and tracing the electrons motion between solid source and filament, the configuration of the e-beam gun is optimized to ensure that the electrons are foucusd on the top-tip of the solid source, thus stable and controllable beam flux is acquired. On this basis,2-inch VO2 thin film on sapphire are prepared by using this oxygen-assisted MBE. Strain controlled epitaxy VO2 thin films are prepared on TiO2, MgF2 substrates, which show excellent electrical and optical properties accorss the phase transition boundary.2) A new configuration of filed-effert transistor based on VO2 is designed by covering the suface of the single-crystall VO2 thin films with ionic liquild. By controlling the bias voltage and gating time, it is able to modulate the MIT transition of VO2. The investigation on the phase transition of electrolyte gated wafer-scale VO2 thin films suggests that the modulation is relied on the depth of the thin films. With the assistance of gating voltage, oxygen atoms in VO2 films are seized by Ionic liquild under high interface electric field. The formation of oxygen vancancies in lattice are equivalent to electron doping, thus triggling the MIT transition of VO2. By dynamic tracking the unsynchronized change of integral electric, optical and structure properties of VO2 druing the transition, it is revealed that the formation of oxygen vancancies asscitated with VO2 metalization is a slowly and continuous process, which starts from the ionic liquid/VO2 interface and gradually permeates to the VO2/substrate interface, with a maximal depth of tens of nanometers.(3) The effect of H atoms doping on the MIT process of VO2 is studied. By control the H concentration, it is able to reversibly modulate the MIT behavior of VO2. It is observed that light H doping will induce the insulator M-VO2 to metallic H-VO2, heavy H doping can induce the metallic H-VO2 to insulator H-VO2. Releasing the H atoms from VO2 crystal lattice by heating treatment will transfer the H-VO2 back to intrinsic M-VO2. It is suggested that the M-VO2. metallic H-VO2 and insulator H-VO2 are three different phases. While they are interconvertible and reversible by control the H concentration in VO2 host, along with noteable changes in electronic and optical properties. The simulations based on first principle calculation indicate that these structures and electronic changes are the results of continuous up-shifts of Fermi level by electron doping. |
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