%0 Journal Article
%T Room Temperature Optical Constants and Band Gap Evolution of Phase Pure M1-VO2 Thin Films Deposited at Different Oxygen Partial Pressures by Reactive Magnetron Sputtering
%A Meng Jiang
%A Yamei Li
%A Shaotang Li
%A Huaijuan Zhou
%A Xun Cao
%A Shanhu Bao
%A Yanfeng Gao
%A Hongjie Luo
%A Ping Jin
%J Journal of Nanomaterials
%D 2014
%I Hindawi Publishing Corporation
%R 10.1155/2014/183954
%X Spectroscopic ellipsometry study was employed for phase pure VO2(M1) thin films grown at different oxygen partial pressures by reactive magnetron sputtering. The optical constants of the VO2(M1) thin films have been determined in a photon energy range between 0.73 and 5.05£¿eV. The near-infrared extinction coefficient and optical conductivity of VO2(M1) thin films rapidly increase with decreasing O2-Ar ratios. Moreover, two electronic transitions can be uniquely assigned. The energy gaps correlated with absorption edge at varied O2-Ar ratios are almost the same (~2.0£¿eV); consequently, the absorption edge is not significantly changed. However, the optical band gap corresponding to semiconductor-to-metal phase transition decreases from 0.53 to 0.18£¿eV with decreasing O2-Ar ratios. 1. Introduction Vanadium dioxide (VO2), one of the most interesting transition metal oxides, exhibits a reversible first-order semiconductor-to-metal phase transition (SMT) at a critical temperature ¡ãC (for bulk single crystal VO2) [1]. VO2 has a tetragonal rutile structure with the P42/mnm space group (R phase) above the phase transition temperature, where the partially filled d// band localized at the Fermi level and the rutile phase is metallic [2]. Below the phase transition temperature, it transforms to a monoclinic structure with the P21/c space group (M1 phase), in which the partially filled d// band splits into an unoccupied part being pushed past the band and a filled part with the d// band dropping below the Fermi level, thus opening up a bandgap of ~0.6£¿eV between and the filled part of d// band [2]. Dramatic changes in the electrical and optical properties across the SMT make VO2 thin films suitable for many applications, such as switching devices, sensors, and smart windows [3¨C6]. It has been noted that oxygen partial pressure has effects on the structural and resistivity transition behaviors of VO2 [7]. Although the optimized oxygen partial pressure to fabricate VO2 films on glass and the optical properties of those samples were investigated [8], the optical constants, especially extinction coefficient , which is crucial in understanding band structures, are not involved. Moreover, two energy gaps and are not distinguished as well. Low visible transparency and unfavorable yellowish colour, which are correlated with absorption edges, limit the application of VO2 smart windows. For most practical applications the phase transition temperature needs to be in the vicinity of room temperature (~25¡ãC) and the may be assumed to be correlated with the optical band gap .
%U http://www.hindawi.com/journals/jnm/2014/183954/