Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film

Vanadium dioxide (VO₂) is a strongly correlated material, and it has become known due to its sharp metal-insulator transition (MIT) near room temperature. Understanding the thermal properties and their change across MIT of VO₂ thin film is important for the applications of this material in various devices. Here, the changes in thermal conductivity of epitaxial and polycrystalline VO₂ thin film across MIT are probed by the time-domain thermoreflectance (TDTR) method. The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO₂ film to attenuate the impact from extra thermal interfaces. It is demonstrated that the method is feasible for the VO₂ films with thickness values larger than 100 nm and beyond the phase transition region. The observed reasonable thermal conductivity change rates across MIT of VO₂ thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate, which is different from the reported behavior of single crystal VO₂ nanowires. The recovery of the relationship between thermal conductivity and electrical conductivity in VO₂ film may be attributed to the increasing elastic electron scattering weight, caused by the defects in the film. This work demonstrates the possibility and limitation of investigating the thermal properties of VO₂ thin films by the TDTR method without depositing any metal thermoreflectance layer.