Effects of He-irradiation on the metal-to-insulator transition of vanadium dioxide nanoclusters
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Effects of He-irradiation on the metal-to-insulator transition of vanadium dioxide nanoclusters H. Karl, J. Peng and B. Stritzker Institut für Physik, Universität Augsburg, D-86135 Augsburg, Germany. ABSTRACT In this work nanoclusters of vanadium dioxide (VO2) buried in 200 nm thick SiO2 on silicon have been irradiated with increasing fluences of He ions. The projected range of He was chosen to be 650 nm in order to avoid residual He in the VO2 nanoclusters and the surrounding SiO2. The VO2 nanoclusters have been synthesized by sequential ion implantation of the elements vanadium and oxygen followed by a rapid thermal annealing step. Irradiation with He ions leads to generation of reversible lattice point defects in the nanocrystallineVO2 precipitates. Simultaneously there is no electronic doping by He incorporation. The effect of the local- and long-range structural disorder on the metal-to-insulator phase transition has been investigated as a function of He fluence by µ-Raman spectroscopy and temperature dependent spectral ellipsometry. The disappearance of a low-frequency Raman mode indicates increasing disorder in the long-range crystal structure due to He irradiation. At the same time the thermal hysteresis of the metal-to-insulator transition narrows.
INTRODUCTION Vanadium dioxide (VO2 ) shows a metal-to-insulator phase transition (MIT) near room temperature at 68°C which is characterized by a tremendous decrease in electrical resistivity and an altered optical transmittance and reflectance mainly in the near infrared spectral region [1, 2]. The physical properties of VO2 bulk material have been studied since a few decades. Especially the question whether the origin of the MIT is better explained by a Mott- or Peierls transition or a mixture of both is still under debate. Since it has been recently shown that this MIT is very fast and can be controlled not only by temperature, but also by external parameters like electric field [3] or mechanical strain [4, 5], VO2 attracts increasing interest for technological applications. In case of application in micro- and nano-scale optoelectronic devices the restricted dimensions of integrated VO2 material can alter severely the properties observed for bulk material. It has been found, that decreasing size of the nanocrystallites leads to an increase of the temperature hysteresis of the MIT and changes the transition temperature [3, 6]. Moreover the structural phase transition going along with the MIT can mechanically damage single crystals and thin films of VO2 making nano-scale structures of this material mandatory to avoid fatigue damage during repeated cycles through the MIT. Synthesis by high fluence ion implantation allows embedding nanocrystalline (nc) VO2 precipitates in electrically insulating and optically transparent matrix materials like SiO2 [7]. These nc-VO2/SiO2 nano-composite thin films enable on the one hand the study of localization effects when restricting the MIT on nanoscopic volumes and on the other hand the investigation of novel opto
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