Semiconductor-metal phase transition in doped ion beam synthesized VO 2 nanoclusters
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1174-V06-35
Semiconductor-metal phase transition in doped ion beam synthesized VO2 nanoclusters H. Karl, J. Dreher and B. Stritzker Institut für Physik, Universität Augsburg, D-86135 Augsburg, Germany. ABSTRACT We have synthesized W and Mo doped VO2 nanoclusters embedded in 200 nm thick thermally grown SiO2 on 4-inch silicon wafers by sequential ion implantation of the elements V, W, Mo and O. The implantation energies have been chosen to locate the mean projected range in the centre of the SiO2 thin film. A post implantation rapid thermal annealing step in flowing Ar at 1000°C for 10 min leads to the growth of doped VO2 nanoclusters. The optical properties of the nanoclusters were analyzed by temperature dependent spectral ellipsometry in the spectral range of 320 to 1700 nm. It will be shown, that the semiconductor-metal phase transition hysteresis width starting at 50K in the undoped case can be systematically closed by increasing dopand concentration.
INTRODUCTION Some of the vanadium oxides show a semiconductor-metal transition (i.e. VO2 at 68°C or V2O3 at -123°C) [1,2] for that reason they attract increasing interest both in scientific research and technological applications. The near room temperature transition temperature of VO2 is very attractive for many technological applications. The phase transition is marked by a strong decrease in electrical resistivity and an important change in optical transmittance and reflectance in the near infrared spectral region. Due to their functionality these materials might find applications in infrared optical modulators and switches [3,4], smart windows, waveguides and adaptable photonic crystals. With the semiconductor-metal transition the material undergoes also a structural phase transition (e.g. VO2 transforms from a monoclinic in the semiconducting to a tetragonal structure in the metallic state). In particular the semiconductor-metal phase transition temperature of VO2 can be altered and adjusted by doping making it possible to adapt that material to specific application requirements. Due to the structural phase transition single crystals and thin films experience damage when cycled through the phase transition. This disadvantageous effect is largely reduced for material with nano crystalline morphology, where the energetic conditions prevent the generation of crystal defects. Apart these practical considerations nano-crystallinity gives rise to new physical phenomena. So it has been found, that nanoscopic grain size causes hysteresis of the semiconductor-metal phase transition and changes the transition temperature [5]. Most of the experiments have been performed on nanocrystalline thin films where the vanadium oxide crystallites are attached to each other forming grain boundaries. There is only a limited number experiments on well separated nanocrystallites embedded in an electrically insulating and optically transparent matrix. Those nano-composite thin films allow it to study effects of the localization of the phase transformation to a nanoscopic volume. This
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