Current-induced formation of stable M₂-phase vanadium dioxide

Studies are reported on the transformation of M₁-phase vanadium dioxide into the stable M₂ structure for a micro-fabricated device. The device is composed of a VO₂ channel of length 50 μm, width 40 μm, and thickness 120 nm, and electrical contacts made of titanium and gold. Transformation is achieved in the channel region between electrical contacts by heating the entire structure to 40 C and simultaneously driving current. The channel is observed to partially transform along its length. Micro-Raman measurements reveal the two regions to be the M₁ and M₂-phases of VO₂. X-ray photoemission measurements show the presence of gold in both regions, but with higher content in the M₂ material. The M₁ portion of the channel is found to undergo a phase transition to the conductive R phase at ∼ 63 C based on resistance and micro-Raman measurements as temperature is increased. The M₂-phase is found to remain stable to temperature cycling between 25 and 120 C. We attribute both the reduced M₁→R phase transition temperature and the stable formation of M₂-phase VO₂ to the presence ofgold.