Electrical Transition in Isostructural VO2 Thin-Film Heterostructures (original) (raw)

Control over the concurrent occurrence of structural (monoclinic to tetragonal) and electrical (insulator to the conductor) transitions presents a formidable challenge for Vo 2-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 VO 2 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 VO 2 films. The strain trapping in VO 2 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 VO 2 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 (Δ V−V) in monoclinic structures which leads to a systematic decrease in the electronic bandgap of Vo 2. 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. The metal-insulator transition in strongly correlated materials such as vanadium dioxide (VO 2) is usually coupled with the symmetry-lowering structural transition, which is tetragonal rutile P 4 2 /mnm to monoclinic P 2 1 /c. The fundamental understanding and control over electrical and structural transitions in VO 2 , which occur often simultaneously, are of immense scientific importance with profound impact on technological applications ranging from smart switching to infrared sensing devices. Over the years, numerous efforts have been made in this direction, primarily focusing on the manipulation of these transitions via defect and interface engineering 1-5. However, the switching speed and endurance of such devices are often limited by the complexities that emerge from the kinetically slower occurrence of the structural transition (10 picoseconds) as compared to the electrical transition (0.1 picoseconds) 6-8. This leads to the decoupling between these coexisting transitions in the presence of strain, dopants, and defects in the thermal spectrum and deleteriously affects the performance of such systems 2,4,8,9. The coexistence of electrical and structural transitions presents practical challenges in fabricating electronically-correlated VO 2 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. This can be achieved by strain management in VO 2 thin films 10-13. It has been shown that the primary mechanisms of metal-insulator transitions are based on electron-electron interactions (Mott transition) and electron-lattice interactions (Peierls transition). The ratio of these can be effectively steered through strain-induced tuning of c/a lattice ratio in VO 2 thin films 1,14-18. This is a result of an interplay between these competing mechanisms of electron-electron interaction and electron-phonon interaction, leading to a tunable electrical transition 1,12,15,19. Previously, several researchers including our group have shown that it is possible to separate structural and electrical transitions 9,10,20-23. However, this raises the question of whether it is possible to totally prevent the occurrence of the structural transition, which had been predicted previously by density functional theory (DFT) calculation suggesting a thermally stable monoclinic metallic phase of VO 2 24. The insulating state in monoclinic VO 2 results from electron-electron correlations and electron-phonon interactions. These correlations can be manipulated by charge, spin, orbital, and lattice degrees of freedom. This means that the ratio of the Mott (electron-electron correlations) and Peierls (electron-phonon interactions) transitions can change depending on these factors. Thus, if