Annealing and Persistent Photoconductivity Effects in Amorphous and Crystalline Vanadium Oxide Films
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Annealing and Persistent Photoconductivity Effects in Amorphous and Crystalline Vanadium Oxide Films Jason Kekas, Anuj Dhawan, Praveen Gollakota, and John Muth Electrical & Computer Engineering, NC State University, Raleigh, NC, 27695 ABSTRACT Vanadium dioxide films have long attracted attention because of the phase transition between the semiconducting state and the metallic state at 68 degrees centigrade. Vanadium oxide films have been prepared by a variety of methods, including the annealing and oxidation of evaporated vanadium under controlled conditions. In this study we explore a different route by producing amorphous and crystalline VOx films by pulsed laser deposition and annealing under a variety of conditions. We found that that in amorphous films grown at lower temperatures and then annealed, persistent photoconductivity effects can be observed, while in epitaxial crystalline films grown at high temperature, persistent effects were not observed. Furthermore, it was found that for some crystalline films persistent optical effects were observed and that optical hysteresis was dependent on the amount of time after the film anneal.
INTRODUCTION Vanadium oxides are useful for a variety of application ranging from bolometers, optical switches, and catalysts for chemical reactions. Each of these classes of applications, especially if to be used in electronic applications, is important to produce stable, reproducible materials. For example, for bolometer applications where the absorbed infrared radiation results in a change in the resistance, one would like the resistance to return to its original value quickly after the illumination is removed. For optical switches, upon a change in state, the transmission or reflectance should be reproducible after multiple state changes. However the vanadium oxide material system is very complex, consisting of many different phases, and the vanadium atom itself can have a variety of valence states [1]. Of the many possible phases, VO2 is perhaps the most interesting and studied since the phase transition between semiconducting and metallic state occurs near 68oC [2]. Since VO2 can also be grown hydrothermally [3], producing high quality single crystals, when one studies thin films one can use the usually superior properties of the hydrothermally grown material as a point of comparison. In thin film form, one
usually finds that the phase transition is broadened, and can be shifted due to perturbations such as strain or doping. A wide variety of methods are employed for producing vanadium oxide films including hydrothermal process [3], sol gel process [4], oxidation of vanadium metal [5], and pulsed laser deposition [6]. In this study, we produce crystalline vanadium oxide films by pulsed laser deposition and investigate VO2 films that exhibit good crystalline structure by x-ray, and demonstrate thermooptic hysteresis. Two routes are investigated, growing films at high temperature in a reactive atmosphere, and growing films at low temperatures, and controlling t
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