Metal-Insulator Phase Transition in VO 2 : A Look from the Far Infrared Side
- PDF / 601,933 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 27 Downloads / 160 Views
0935-K01-09
Metal-Insulator Phase Transition in VO2: A Look from the Far Infrared Side Peter Uhd Jepsen1, Bernd M. Fischer2, Andreas Thoman3, Hanspeter Helm3, J. Y. Suh4, Rene Lopez4, and Richard F. Hanglund, Jr. 4 1 COM-DTU - Department of Communications, Optics & Materials, Technical University of Denmark, Ørsteds Plads, Building 345V, Kongens Lyngby, DK-2800, Denmark 2 Electrical Engineering, University of Adelaide, Adelaide, Australia 3 Department of Molecular and Optical Physics, University of Freiburg, Freiburg, D-79104, Germany 4 Department of Physics, Vanderbilt University, Nashville, TN 37235
ABSTRACT Vanadium dioxide (VO2) displays a well-known metal-insulator (MI) transition at a temperature of 68°C. In this study we use terahertz time-domain spectroscopy (THz-TDS) to investigate the optical properties of VO2 thin films in the vicinity of the MI transition temperature in the frequency range 0.1 – 1.5 THz. We observe the interesting effect that the phase of the transmitted THz field through the conducting VO2 film is delayed in comparison to the phase of the same THz signal transmitted through the insulating VO2 film. This is in contrast to the expected behavior of a homogeneous, conducting film. This observation shows that even at temperatures significantly above the transition temperature, the formation of a homogeneous, conducting film is incomplete. We demonstrate that effective-medium theory (EMT) in combination with a Drude model accounting for the conductivity of metallic domains formed in the VO2 film accounts for all our observations. We show that the Maxwell-Garnett EMT is consistent with our observations, whereas the Bruggeman EMT fails to account for our observations. INTRODUCTION Bulk vanadium dioxide (VO2) undergoes a phase transition from semiconductor to metal at a temperature near 68°C1. Associated with the phase transition is a change of conductivity by several orders of magnitude, strong changes of the optical properties at all wavelengths, as well as a structural transition from the low-temperature monoclinic phase to a tetragonal metallic phase. There is a marked hysteresis associated with both the electrical and the optical properties of VO2, indicating that the phase transition is of first order. The dynamics of the phase transition depends strongly on the morphology of the material. The transition temperature as well as the width of the hysteresis can be strongly modified by nanostructuring of the VO2 material2 and by interface effects in epitaxially grown thin films3. Most optical and electric characterization of VO2 has been carried out on thin films of either bulk or nanostructured VO2. In this work we discuss the properties of thin films of polycrystalline VO2. The phase transition in a thin insulating film of VO2 is initiated at seed points in the film, and the metallic domains then grow as the temperature is increased towards and above the transition
temperature. Hence in the vicinity of the transition temperature the insulating and metallic domains coexist. This spatial i
Data Loading...
