Strain Effects in Epitaxial VO₂ Thin Films on Columnar Buffer-Layer TiO₂/Al₂O₃ Virtual Substrates

Epitaxial VO₂/TiO₂ thin film heterostructures were grown on (100) (m-cut) Al₂O₃ substrates via pulsed laser deposition. We have demonstrated the ability to reduce the semiconductor-metal transition (SMT) temperature of VO₂ to ∼44 °C while retaining a 4 order of magnitude SMT using the TiO₂ buffer layer. A combination of electrical transport and X-ray diffraction reciprocal space mapping studies help examine the specific strain states of VO₂/TiO₂/Al₂O₃ heterostructures as a function of TiO₂ film growth temperatures. Atomic force microscopy and transmission electron microscopy analyses show that the columnar microstructure present in TiO₂ buffer films is responsible for the partially strained VO₂ film behavior and subsequently favorable transport characteristics with a lower SMT temperature. Such findings are of crucial importance for both the technological implementation of the VO₂ system, where reduction of its SMT temperature is widely sought, as well as the broader complex oxide community, where greater understanding of the evolution of microstructure, strain, and functional properties is a high priority.