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 T_c 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 T_c 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 T_c, the unit cell of the high-temperature rutile Ti_xV_1-xO₂ phase is aligned with that of the rutile TiO₂ substrate whereas, below T_c, 180°-domains of the M₁-phase of Ti_xV_1-xO₂ are observed. The structural relationship between TiO₂ substrate and the high respective low-temperature phase of the Ti_xV_1-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.