Characteristics of VO2 films depending on substrate materials and VO2 / GaN hetero-junctions, 기판 물질에 따른 VO2 박막의 특성 및 VO2 / GaN 헤테로 접합 특성 연구

by Seung Hwi Ahn (MS Candidate)

Wednesday Nov 15, 2017, 3:30 PM → 4:00 PM Asia/Seoul

119호 (제1과학관)

Vanadium dioxide VO2 is an interesting transition metal oxide which shows a metal-insulator transition behav- ior at Tc ~ 340 K. Below Tc, VO2 is a monoclinic and insulating phase, while it changes to a tetragonal and metallic phase above Tc. Because vanadium can exist in various oxide states such as V2O3, VO2 and V2O5, it is important to control the oxygen content in VO2 films. Therefore, we first tried to find an optimal growth con- dition of the VO2 film using a pulsed laser deposition (PLD) method. We carried out the measurements of surface morphology, X-ray diffraction, resistance versus temperature for VO2 films grown at various conditions. As a result, we found that the VO2 (020) film could be successfully grown on Al2O3 (0001) substrate. The VO2 film exhibited the surface roughness of ~2 nm and a large resistance change of ~103 at Tc. In addition, we deposited VO2 films on MgF2 (001), (111), (110), and (210) substrates. VO2 films displayed different transition temperatures with the crystal orientation of the MgF2 substrate. We think that such behavior is probably related to lattice mismatch between VO2 and MgF2 with the crystal orientation and the resultant lattice strain. Finally, we prepared VO2 / GaN hetero-junctions with ohmic or Schottky electrode contacts and measured their current- voltage (I-V) characteristics with the temperature. The rectification behavior of VO2 / GaN hetero-junction was largely changed with temperature. At room temperature, VO2 / GaN didn’t exhibit a rectification behavior, but it showed a clear rectification behavior above Tc. On the other hand, the VO2 / GaN junction contacted with Au electrode displayed a rectification behavior even at room temperature because GaN / Au is a Schottky contact. As the temperature increased above Tc, the rectification behavior became stronger. This is presumably due to a change of VO2 / GaN junction from an Ohmic contact to a Schottky contact at Tc. Our results will be exploited in optoelectronic device and/or Schottky diode device applications.

Thesis Advisor: Prof. Joonghoe Dho