Vanadium Dioxide Thin Films for Thermo-Optical Switching Applications
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Vanadium Dioxide Thin Films for Thermo-Optical Switching Applications Lijun Jiang and William N. Carr New Jersey Institute of Technology Newark, NJ 07102, USA ABSTRACT Vanadium dioxide (VO2) thin films were fabricated by e-beam evaporation of vanadium thin films followed by thermal oxidation in oxygen ambient. The properties of the VO2 films were investigated for thermo-optical switching applications. Synthesized VO2 film displays a phase transition at 65 - 68 oC. It exhibits an abrupt change in optical reflectivity over the phase transition temperature range. Results for VO2 on a highly reflective metal layer are strongly dependent on the VO2 thickness. The optical switching has a major hysteresis of about 15 oC between the heating and cooling branches. The evolution of the surface morphology with the oxidation time was studied with a SEM. The VO2 film was patterned on microplatforms by metal lift-off technique. We conclude that the evaporation followed by oxidation is an effective method to produce active VO2 film for thermo-optical switching devices. INTRODUCTION Vanadium dioxide thin film undergoes a first order semiconductor-to-metal phase transition at about 68 oC [1]. At low temperature, the VO2 is a semiconductor with a monoclinic crystalline structure. It transforms into metallic state at high temperature with a tetragonal symmetry. The phase transition is accompanied with drastic changes in its optical and electrical properties. As large as four- to five-order of variation of the electrical resistivity has been observed for VO2 single crystal or polycrystalline films [2]. Optically, VO2 film is transparent at semiconductor state and highly reflective at metallic state. The phase transition temperature (Tc ≈ 68 oC) can be further reduced by doping [3]. All above properties make VO2 a good candidate material for a variety of applications such as infrared sensing, optical switching, and data storage devices [4]. Wide ranges of deposition methods have been investigated to fabricate VO2 thin films, including the reactive sputtering [5], reactive evaporation [6], chemical vapor deposition [7], pulsed laser ablation [8], as well as sol-gel methods [9]. However, most previous work has been focused on the investigation of basic properties of VO2 thin film as well as deposition parameters. There has not been enough information about the etching and patterning of VO2 film. In this paper, we study the integration of VO2 film with microfabrication process for micromachined thermo-optical switching devices. VO2 film was deposited by evaporation of vanadium thin film followed by thermal oxidation. The thermo-optical switching properties of our VO2 film were studied and compared with the films fabricated by others. In addition, the microstructure evolution during the oxidation was studied by scanning electron microscope.
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EXPERIMENTAL DETAILS Vanadium thin film was e-beam evaporated from pure vanadium target (99.9%) in vacuum on quartz substrate. After evaporation, the vanadium film was oxidized in room
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