Optical Switching of Coherent VO 2 Precipitates Embedded in Sapphire
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PRECIPITATES EMBEDDED IN
L. A. GEA, L. A. BOATNER, J. D. BUDAI, R. A. ZUHR Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA 37831
ABSTRACT In this work, we report the formation of a new type of active or "smart" surface that is produced by ion implantation and thermal processing. By co-implanting vanadium and oxygen into a singlecrystal sapphire substrate and annealing the system under appropriate conditions, it was possible to form buried precipitates of vanadium dioxide that were crystallographically oriented with respect to the host A12 0 3 lattice. The implanted V0 2 precipitate system undergoes a structural phase transition that is accompanied by large variations in the optical transmission which are comparable to those observed for thin films of V0 2 deposited on sapphire. Co-implantation with oxygen was found to be necessary to ensure good optical switching behavior.
1. INTRODUCTION Vanadium dioxide has been studied for a number of years in single-crystal form [1], and more recently, thin films of V0 2 have been investigated due to the large variations in their electronic and optical properties that occur as the oxide undergoes a thermally induced crystallographic transformation from a monoclinic to a tetragonal structure. This structural phase transition is also accompanied by a semiconductor-to-metal transition that is characterized by a change in the V0 2 electrical conductivity of over two orders of magnitude. Changes in the properties of vanadium dioxide that occur at the phase transition are not only of interest from the fundamental point of view. The transition temperature of -68°C is close to ambient temperature and can be easily altered by appropriate doping or by growing V0 2 in a thin-film geometry. This makes this material suitable for a variety of practical applications - particularly in the area of forming "s4mart" or interactive surfaces, i.e., surfaces that are capable of performing both sensing and actuating functions. In particular, the properties of V0 2 thin films have been investigated for technological applications that include: thermal sensors, optical switches, and optical storage media [2,3]- as well as mirrors for infrared lasers [41 and laser-pulse protection devices. In the present work, a new method for the formation of V0 2 precipitates that are "embedded" in the near-surface region of a transparent host matrix has been developed by employing ion implantation coupled with subsequent thermal processing [5]. The "surface-composite" V0 2/A12 0 3 system formed using this approach exhibits characteristics that are believed to be advantageous for both technological applications and for carrying out fundamental investigations. In the present case, the near-surface layer containing the active elements in the form of V0 2 precipitates is protected from environmental interactions and degradation effects. Furthermore, it is anticipated that new materials properties and effects are likely to arise at the phase transition as a result of interactions betwee
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