Electrical and optical properties of metal-insulator-transition VO2 thin films
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Vanadium dioxide thin films were deposited using in situ pulsed laser deposition technique on a-, c-, and r-plane Al2O3, and MgO(100) substrates. Microstructure of the films was varied between epitaxial VO2, polycrystalline VO2, mixtures of VO2 and V6O13 with VO2 as the main phase, and mixtures of VO2 and V6O13 with V6O13 as the main phase by controlling the deposition oxygen partial pressure. Detailed XRD, SPM, and FESEM measurements were performed in order to analyze the structure of the films. Resistivity as a function of temperature, current–voltage characteristics in electric current induced Joule heating transition process, and the optical transmittance both in insulator and metal states were measured. MIT effect led to 103–105 change in resistivity with varying transition temperature and hysteresis loop widths. The largest and the steepest transitions were found in the films with polycrystalline microstructure deposited at higher oxygen pressures. Epitaxial films had 2.5 times higher insulator state conductivity than polycrystalline films, which lead to a clearly smaller switching powers in MIT effect generated by Joule heating. However, the optical properties in both states were not considerable affected by conductivity or microstructure properties. The relationships between the microstructure, electrical and optical properties, as well as MIT switching effect together its dynamics in the films are discussed.
KeywordsVO2 Metal-insulator-transition Conductivity mechanisms Optical properties
This research work was a part of the NAPERO (Development of novel fabrication methods for nanoscale photonics and microelectronics components) project, funded by TEKES—National Technology Agency of Finland. Part of this work was carried out in the Micro- and Nanotechnology Center (MNT) of University of Oulu.