Collapse of the low temperature insulating state in Cr-doped V$_2$O$_3$ thin films

TL;DR

This study investigates how Cr doping affects the electronic phases of V$_2$O$_3$ thin films, revealing a collapse of the insulating state at low doping levels and its recovery through annealing, with implications for device applications.

Contribution

It demonstrates the doping-dependent collapse and recovery of the insulating state in Cr-doped V$_2$O$_3$ thin films and links oxygen excess to these effects.

Findings

Collapse of insulating state at 1-3% Cr doping

Recovery of insulating state after annealing

Oxygen excess influences electronic phase stability

Abstract

We have grown epitaxial Cr-doped V$_2$O$_3$ thin films with Cr concentrations between $0$ and $20\%$ on $(0001)$-Al$_2$O$_3$ by oxygen-assisted molecular beam epitaxy. For the highly doped samples (> $3\%$), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between $1\%$ and $3\%$), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below $3\%$ and increases the room temperature resistivity towards the values of Cr-doped V$_2$O$_3$ single crystals. It is well-know that oxygen excess stabilizes a metallic state in V$_2$O$_3$ single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V$_2$O$_3$ films can be interesting candidates for field effect devices.