Domain nucleation across the metal-insulator transition of self-strained V2O3 films

TL;DR

This study investigates how strain and film thickness influence the nucleation and growth of insulating domains during the metal-insulator transition in self-strained V2O3 films, revealing a critical thickness for complete transition.

Contribution

It provides detailed imaging and analysis of domain nucleation and growth in V2O3 thin films, highlighting the effects of substrate clamping and film thickness on the MIT behavior.

Findings

Insulating domains nucleate at the top of the film and grow downward.

A critical thickness of 50 nm prevents the MIT in the interior of the film.

A persistent bottom metallic layer remains below the critical thickness.

Abstract

Bulk V2O3 features concomitant metal-insulator (MIT) and structural (SPT) phase transitions at TC ~ 160 K. In thin films, where the substrate clamping can impose geometrical restrictions on the SPT, the epitaxial relation between the V2O3 film and substrate can have a profound effect on the MIT. Here we present a detailed characterization of domain nucleation and growth across the MIT in (001)-oriented V2O3 films grown on sapphire. By combining scanning electron transmission microscopy (STEM) and photoelectron emission microscopy (PEEM), we imaged the MIT with planar and vertical resolution. We observed that upon cooling, insulating domains nucleate at the top of the film, where strain is lowest, and expand downwards and laterally. This growth is arrested at a critical thickness of 50 nm from the substrate interface, leaving a persistent bottom metallic layer. As a result, the MIT cannot take place in the interior of films below this critical thickness. However, PEEM measurements revealed that insulating domains can still form on a very thin superficial layer at the top interface. Our results demonstrate the intricate spatial complexity of the MIT in clamped V2O3, especially the strain-induced large variations along the c-axis. Engineering the thickness-dependent MIT can provide an unconventional way to build out-of-plane geometry devices by using the persistent bottom metal layer as a native electrode.