Abstract
Control over the concurrent occurrence of structural (monoclinic to tetragonal) and electrical (insulator to the conductor) transitions presents a formidable challenge for VO2-based thin film devices. Speed, lifetime, and reliability of these devices can be significantly improved by utilizing solely electrical transition while eliminating structural transition. We design a novel strain-stabilized isostructural VO2 epitaxial thin-film system where the electrical transition occurs without any observable structural transition. The thin-film heterostructures with a completely relaxed NiO buffer layer have been synthesized allowing complete control over strains in VO2 films. The strain trapping in VO2 thin films occurs below a critical thickness by arresting the formation of misfit dislocations. We discover the structural pinning of the monoclinic phase in (10 ± 1 nm) epitaxial VO2 films due to bandgap changes throughout the whole temperature regime as the insulator-to-metal transition occurs. Using density functional theory, we calculate that the strain in monoclinic structure reduces the difference between long and short V-V bond-lengths (ΔV−V) in monoclinic structures which leads to a systematic decrease in the electronic bandgap of VO2. This decrease in bandgap is additionally attributed to ferromagnetic ordering in the monoclinic phase to facilitate a Mott insulator without going through the structural transition.
Highlights
The metal-insulator transition in strongly correlated materials such as vanadium dioxide (VO2) is usually coupled with the symmetry-lowering structural transition, which is tetragonal rutile P 42/mnm to monoclinic P 21/c
To trap the strain uniformly in the VO2 thin films, we synthesized epitaxial films below the critical thickness where strain energy is insufficient to trigger the nucleation of misfit dislocations
The NiO is used as the buffer layer because the VO2 film on top of it can be almost fully relaxed above the critical thickness through domain matching epitaxy (DME) paradigm with near bulk behavior[26,27,28]
Summary
The metal-insulator transition in strongly correlated materials such as vanadium dioxide (VO2) is usually coupled with the symmetry-lowering structural transition, which is tetragonal rutile P 42/mnm to monoclinic P 21/c. The coexistence of electrical and structural transitions presents practical challenges in fabricating electronically-correlated VO2 based solid-state devices[3] In this respect, the development of materials displaying an isolated electrical transition without an accompanying structural transition provides an ideal solution. We have designed a unique non-equilibrium isostructural monoclinic (no temperature dependency) VO2 phase on a practical substrate, which demonstrates an uninterrupted insulator-metal transition without undergoing a structural change. This pseudomorphic structure is compared with a fully relaxed VO2 thin film grown on the same heterostructure above the critical thickness. The demonstration of structurally-stabilized VO2 thin films in this study presents a promising route to enhance the lifetime, endurance, and reliability of VO2-based smart thin-film heterostructure devices
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