Two-step current-temperature-induced electrical and optical modifications in VO_2 films around the metal-insulator transition

摘要

We report on two-step current-induced effects on the electrical, optical, and structural properties of VO_2 films around the Metal-Insulator Transition (MIT) in synergy with ambient temperature (T). Simultaneous electrical resistance and transmittance measurements of VO_2 semitransparent thin films as a function of T show that the electric current modifies the MIT that takes place in two steps: an abrupt change that increases upon increasing current, implying the formation of larger metallic domains within the current path, accompanied by a smoother change that follows the temperature change. Resistance measurements of thicker bulk-like VO_2 films have been also investigated exhibiting similar two-step behavior. By monitoring the specimen temperature (T_0) during resistance measurements, we show that the abrupt resistance step, accompanied by instantaneous heating/cooling events, occurs at temperatures lower than T_(MIT) and is attributed to current-induced Joule heating effects. Moreover, by monitoring T_0 during current-voltage measurements, the role of T in the formation of two-step current modified MIT is highlighted. X-ray diffraction with in situ resistance measurements performed for various currents at room temperature as a function of T_0 has shown that the current can cause partially MIT and structural phase transition, leading to an abrupt step of MIT. The formation of a rutile metallic phase of VO_2 under high applied currents is clearly demonstrated by micro-Raman measurements. By controlling current in synergy with T below T_(MIT), the VO_2 film can be driven to a two-step current-induced MIT as gradually a larger part of the film is transformed into a rutile metallic phase.