Modulation-Doping a Correlated Electron Insulator

TL;DR

This study demonstrates modulation doping in VO2 thin films to control the metal-insulator transition electronically without structural changes, enabling new possibilities for device applications.

Contribution

We introduce modulation doping in VO2 heterostructures to achieve high carrier densities and tunable phase transitions without structural modifications.

Findings

Carrier densities >5x10^21 cm^(-3) achieved without structural change

MIT temperature decreases with increased doping

Insulating state remains robust at high doping levels

Abstract

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard x-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5x10^21 cm^(-3) without inducing any measurable structural changes. We find that the MIT temperature (T_MIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.