Microstructural and electrical investigation of polymorph stabilization and multistate transition in interface engineered epitaxial VO2 films

Modulation of metal to insulator transition (MIT) in vanadium dioxide (VO2) remains one of the most important cornerstones for its wide scale applications in electrical devices. Incorporating VO2 in and beyond room temperature ranges requires energy efficient systems with high device performance and low transition losses. To achieve this performance, we studied pulsed laser deposition (PLD) grown VO2 films on sapphire substrate, with and without the TiO2 buffer layer. An insight into microstructural modification and MIT was provided using in-situ characterization techniques. We observed that misfit strain and external stimuli induces polymorphs and phases like VO2 (M2), VO2 (T) and V2O3 in the epitaxial films along with higher stabilization of intermediate M2 in the TiO2 system. To understand the effect of interface engineering on transition temperature and voltage, in-plane and out-of-plane device architectures were fabricated and characterized using electrical measurements. We discovered that TiO2 interface enables realization of two step electrical transition, Tc close to room temperature and lowering of transition voltage and work function. Also using DFT calculation, we studied the effect of substrate misfit strain on structural and electronic properties of polymorphs. Since M1 and R have different structure, thus the strain experienced before and after transition on the same substrate will be different. Using the orientation relation of PLD grown films, we predict the MIT temperature, validating the Tc observed through experimental electrical measurements.