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
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 °C (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 [3–6]. 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°C) and the may be assumed to be correlated with the optical band gap .
References
[1]
F. J. Morin, “Oxides which show a metal-to-insulator transition at the neel temperature,” Physical Review Letters, vol. 3, no. 1, pp. 34–36, 1959.
[2]
T. C. Koethe, Z. Hu, M. W. Haverkort et al., “Transfer of spectral weight and symmetry across the metal-insulator transition in VO2,” Physical Review Letters, vol. 97, no. 11, Article ID 116402, 2006.
[3]
Z. Zhang, Y. Gao, H. Luo et al., “Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications,” Energy and Environmental Science, vol. 4, no. 10, pp. 4290–4297, 2011.
[4]
Y. F. Gao, S. B. Wang, L. T. Kang et al., “VO2-Sb:SnO2 composite thermochromic smart glass foil,” Energy and Environmental Science, vol. 5, pp. 8234–8237, 2012.
[5]
Y. Gao, H. Luo, Z. Zhang et al., “Nanoceramic VO2 thermochromic smart glass: a review on progress in solution processing,” Nano Energy, vol. 1, no. 2, pp. 221–246, 2012.
[6]
P. P. Boriskov, A. A. Velichko, A. L. Pergament, G. B. Stefanovich, and D. G. Stefanovich, “The effect of electric field on metal-insulator phase transition in vanadium dioxide,” Technical Physics Letters, vol. 28, no. 5, pp. 406–408, 2002.
[7]
C. H. Griffiths and H. K. Eastwood, “Influence of stoichiometry on the metal-semiconductor transition in vanadium dioxide,” Journal of Applied Physics, vol. 45, no. 5, pp. 2201–2206, 1974.
[8]
J. Li and J. Dho, “Anomalous optical switching and thermal hysteresis behaviors of VO2 films on glass substrate,” Applied Physics Letters, vol. 99, no. 23, Article ID 231909, 2011.
[9]
A. C. Jones, S. Berweger, J. Wei, D. Cobden, and M. B. Raschke, “Nano-optical investigations of the metal-insulator phase behavior of individual VO2 microcrystals,” Nano Letters, vol. 10, no. 5, pp. 1574–1581, 2010.
[10]
P. Schilbe, “Raman scattering in VO2,” Physica B, vol. 316-317, pp. 600–602, 2002.
[11]
J. Y. Chou, J. L. Lensch-Falk, E. R. Hemesath, and L. J. Lauhon, “Vanadium oxide nanowire phase and orientation analyzed by Raman spectroscopy,” Journal of Applied Physics, vol. 105, no. 3, Article ID 034310, 2009.
[12]
C. Marini, E. Arcangeletti, D. di Castro et al., “Optical properties of V1-xCrxO2 compounds under high pressure,” Physical Review B, vol. 77, no. 23, Article ID 235111, 2008.
[13]
J. M. Atkin, S. Berweger, E. K. Chavez et al., “Strain and temperature dependence of the insulating phases of VO2 near the metal-insulator transition,” Physical Review B, vol. 85, no. 2, Article ID 020101, 2012.
[14]
Y. D. Ji, T. S. Pan, Z. Bi et al., “Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases,” Applied Physics Letters, vol. 101, Article ID 071902, 2012.
[15]
P. Jin and S. Tanemura, “Formation and thermochromism of VO2 films deposited by RF magnetron sputtering at low substrate temperature,” Japanese Journal of Applied Physics, vol. 33, no. 3 A, pp. 1478–1483, 1994.
[16]
S. Zhang, I. S. Kim, and L. J. Lauhon, “Stoichiometry engineering of monoclinic to rutile phase transition in suspended single crystalline vanadium dioxide nanobeams,” Nano Letters, vol. 11, no. 4, pp. 1443–1447, 2011.
[17]
W. W. Li, Q. Yu, J. R. Liang et al., “Intrinsic evolutions of optical functions, band gap, and higher-energy electronic transitions in VO2 film near the metal-insulator transition region,” Applied Physics Letters, vol. 99, no. 24, Article ID 241903, 2011.
[18]
T. Yao, X. Zhang, Z. Sun et al., “Understanding the nature of the kinetic process in a VO2 metal-insulator transition,” Physical Review Letters, vol. 105, no. 22, Article ID 226405, 2010.
