Twin-domain Epitaxial Growth and Metal-insulator Transition of VO 2 Thin Film on C-Plane Sapphire
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Twin-domain Epitaxial Growth and Metal-insulator Transition of VO2 Thin Film on C-Plane Sapphire Changhong Chen, 1, 2 Yong Zhao,1 Xuan Pan,1 Mark Holtz,3 and Zhaoyang Fan1 1 Nano Tech Center and Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, TX 79409-3102, U.S.A. 2 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P.R. China 3 Nano Tech Center and Department of Physics, Texas Tech University, Lubbock, TX 794091051, U.S.A. ABSTRACT We report heteroepitaxial growth of VO2 thin film on c-plane sapphire by pulsed DC magnetron sputtering. X-ray diffraction experiment indicates that the 150 nm thick film is in triple-domain (020)-epitaxial structure with six-fold rotational symmetry in the basal plane; in particular, off-axis ) scans from (011) and (220) show twin and triple peaks in each group of the diffraction profiles due to angle E mismatch and V4+-V4+ dimerization, respectively. The epitaxial relationship between VO2 and c-plane sapphire can be concluded as be VO2 [010] ║Al2O3 [0001] and VO2 ( ) ║Al2O3 { }, with the in-plane lattice mismatch of 2.66% (tensile) along [ and the out-of-plane lattice mismatch of -2.19% (compressive). Temperature dependence of resistivity in van der Pauw method shows that the resistivity changes by ~5 orders of magnitude through the metal-insulator transition, and a narrow hysteresis window of ~3 K is obtained between cooling and heating cycles with respect to phase-transition temperatures at 347.1 and 350.1 K. INTRODUCTION Vanadium dioxide (VO2) undergoes a first-order metal-insulator transition (MIT) from a high-temperature tetragonal rutile-type (R; space group P42/mnm) metal phase to a lowtemperature monoclinic (M; space group P21/c) insulator phase around 340 K [1]. The electrical resistivity (U) jumps by 3-5 orders of magnitude through the phase transition. In addition, an applied electric field can also induce the phase transition to significantly change the electrical and optical properties [2, 3]. These characteristics make VO2 a promising material for a wide variety of applications including uncooled microbolometer [4], electrical switching [5], and optical switching and modulator in the infrared, terahertz wave, and microwave ranges [6-8]. Growth of the M-phase VO2 has been carried out by the methods of pulsed laser deposition, RF or DC magnetron sputtering, and ion-beam sputtering, and buffered silicon, quartz, or single crystal of TiO2 and sapphire is selected as the substrate to obtain a polycrystal or epitaxial VO2 thin film. Through the phase transition, the epitaxial VO2 shows larger change in the resistivity and narrower hysteresis than the polycrystal VO2. There are a few reports of the growth for the M-phase VO2 on the sapphire; however, neither detailed investigation nor convincing structure analysis can be referred. Here, we report the heteroepitaxial growth and multidomain structure analysis of VO2 thin film on c-plane sapphire by pulsed dc magnetron sputtering.
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