Raman scattering of TixV1-xO2 thin films on (110) rutile TiO2 in the low and high temperature phase adjacent to the metal-insulator transition

Abstract

Vanadium dioxide (VO2) undergoes a reversible first-order metal-to-insulator transition (MIT) from a high-temperature metallic phase to a low-temperature insulating phase at a critical temperature Tc of 68°C. The MIT is accompanied by a structural phase transition. In addition to the metallic high-temperature rutile phase, several insulating phases may be involved depending on doping, interfacial stress, or external stimuli. Unambiguously identifying the crystal phases involved in the phase transition is of key interest from the point of view of application as well as fundamental science. We study the impact of Ti doping of VO2 thin films on (110) rutile TiO2 substrates. We conduct a careful analysis of structural properties by combining results of x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The transition temperature Tc of the deposited thin films decreases with increasing Ti-content. All our thin film samples undergo a structural phase transition from the monoclinic M1-phase to the rutile R-phase with increasing temperature without passing the intermediate monoclinic M2-phase. A careful analysis of polarization and angle-dependent Raman data reveals that, above Tc, the unit cell of the high-temperature rutile TixV1-xO2 phase is aligned with that of the rutile TiO2 substrate whereas, below Tc, 180°-domains of the M1-phase of TixV1-xO2 are observed. The structural relationship between TiO2 substrate and the high respective low-temperature phase of the TixV1-xO2 determined by Raman spectroscopy is in excellent agreement with TEM results on these samples. Raman spectroscopy is a powerful tool for studying structural changes of VO2-based samples in the vicinity of MIT.