Optical, Structural, and Electrical Properties of Vanadium Dioxide Grown on Sapphire Substrates with Different Crystallo
- PDF / 372,942 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 65 Downloads / 215 Views
Optical, Structural, and Electrical Properties of Vanadium Dioxide Grown on Sapphire Substrates with Different Crystallographic Orientations M. Nazari,1,2 Y. Zhao,2,3 Y. Zhu,2,3 V. V. Kuryatkov,2,3 A. A. Bernussi,2,3 Z. Fan,2,3 and M. Holtz1,2 1 Department of Physics and Nano Tech Center, Texas Tech University, Lubbock, TX 79409 2 Nano Tech Center, Texas Tech University, Lubbock, TX 79409 3 Department of Electrical Engineering, Texas Tech University, Lubbock, TX 79409 ABSTRACT The phase transition of VO2 grown on sapphire having different crystallographic growth planes is examined experimentally. Measurements of electrical resistivity are compared with spectroscopic ellipsometry studies, to obtain complex index of refraction and plasma frequency, and transmission in the terahertz frequency range, each as a function of temperature. INTRODUCTION The metal-insulator transition (MIT) of vanadium dioxide (VO2) has drawn intense scientific interest for understanding fundamental mechanisms of these transformations.1 Single crystal VO2 exhibits an abrupt decrease in electrical resistivity, by up to five orders of magnitude, within 0.1 ºC upon heating through the MIT temperature of TMIT ~ 68 ºC. This phase transition is accompanied by concomitant changes in the optical properties in the terahertz (THz), far infrared (IR), and visible spectral ranges.2-5 The MIT is accompanied by a structural phase transition from monoclinic (M1) crystal structure at low-temperature to tetragonal rutile (R) in the high-temperature metallic phase.6,7 These properties render VO2 a promising candidate for a number of interesting applications in electronics, photonics, and sensors. It is anticipated that VO2 films grown on the different orientation of sapphire substrates will exhibit different crystal orientation and quality, including grain boundary, grain size, built-in strain, and V-V bond orientation.8 These differences may impact free carrier density, TMIT, hysteresis, and abruptness of the transition, thus affecting the device modulation performance. Recent investigations of the effect of stress on VO2 have resulted in new understanding of the material phase diagram.9-14 Stress along the M1 (011) crystal axis, corresponding to (110)R,15 has been shown to induce a phase transition from the M1 phase to M2, in which alternating V chains pair without twisting, while the others twist without pairing.16,17 The resulting diversity of phenomena has not yet been explored, particularly for thin films deposited on substrates which are generally more amenable to device applications.
Figure 1. SEM images of VO2 films grown (a) c-plane, (b) m-plane, and (c) r-plane sapphire substrates. The scale bars are 1 μm.
239
We report electrical and optical studies of VO2 deposited on sapphire substrates having different crystallographic orientations for the planar growth surfaces. The VO2 crystallographic orientation is found to depend on the substrate used even under identical deposition conditions. The structural variation has an impact on the electrical and
Data Loading...
