Stabilizing polar phases in binary metal oxides by hole doping

TL;DR

This paper demonstrates that hole doping can stabilize polar ferroelectric phases in binary metal oxides like HfO2 by localizing holes on oxygen sublattices, lowering electrostatic energy, and maintaining large polarization.

Contribution

It introduces a new electrostatic mechanism for stabilizing ferroelectric phases via hole doping, supported by first-principles calculations.

Findings

Hole localization on oxygen sublattices stabilizes polar phases.

Hole doping lowers electrostatic energy in ferroelectric oxides.

Large spontaneous polarization persists under high hole concentrations.

Abstract

The recent observation of ferroelectricity in the metastable phases of binary metal oxides, such as HfO2, ZrO2, Hf0.5Zr0.5O2, and Ga2O3, has garnered a lot of attention. These metastable ferroelectric phases are typically stabilized through epitaxial growth, alloying, or defect engineering. Here, we propose hole doping plays a key role in stabilizing the polar phases in binary metal oxides. Using first-principles density-functional-theory calculations, we show that holes in these oxides mainly occupy one of the two oxygen sublattices. This hole localization, which is more pronounced in the polar phase than in the nonpolar phase, lowers the electrostatic energy of the system, and makes the polar phase more stable at sufficiently large concentrations. We demonstrate that this electrostatic mechanism is responsible for stabilization of the ferroelectric phase of HfO2 aliovalently doped with elements that introduce holes to the system, such as La and N. Finally, we show that the spontaneous polarization in HfO2 is robust to hole doping, and a large polarization persists even under a high concentration of holes.