Graphene buffer layer impacts enhanced metal-insulator-transition (MIT) in remote epitaxial VO₂ thin film

VO₂ is an archetypal correlated material, which exhibits a dramatic MIT near room temperature. The MIT characteristics and reconfigurability of VO₂-based heterostructures and associated devices via electric field control have attracted significant attention and effort from both fundamental understanding and technologically important applications. However, the precise synthesis of VO₂ epitaxial thin film is a daunting challenge, as it is difficult to obtain films with high structural quality and good stoichiometry. Oxygen vacancies in the as-grown VO_2-δ thin film without post-treatment often deteriorates the electrical features of the MIT process, such as a broader switching temperature window and lower on/off ratios. In this talk, we demonstrate that much enhanced MIT properties can be attained in an as-grown VO₂ thin film if few monolayers of graphene are used as a buffer layer placed on top of a Al₂O₃ substrate before VO₂ growth. Electrical transport measurements show clearly that the on/off ratio is enhanced 10 fold and the switching temperature window is significantly narrowed for VO₂ thin films with a graphene buffer layer as compared to VO₂ grown directly on Al₂O₃. We utilize a suite of structural, chemical and spectroscopic characterization methods to reveal the mechanisms behind the improvement in MIT behavior due to the presence of graphene during the growth.