Phase diagram and polarization of stable phases of (Ga$_{1-x}$In$_x$)$_2$O$_3$

TL;DR

This study uses density-functional calculations to revise the phase diagram of (Ga$_{1-x}$In$_x$)$_2$O$_3$, revealing phase competition, non-polarity of stable phases, and potential ferroelectric properties of the $ ext{Ga}_2 ext{O}_3$ $ ext{ε}$ phase for electronic applications.

Contribution

It provides a detailed phase diagram and polarization analysis of (Ga$_{1-x}$In$_x$)$_2$O$_3$, highlighting the non-polar nature of stable phases and identifying the ferroelectric potential of the $ ext{ε}$ phase.

Findings

Three competing phases in the phase diagram.

None of the stable phases are polar at any concentration.

The $ ext{ε}$ phase of Ga$_2$O$_3$ is pyroelectric with ferroelectric-grade polarization.

Abstract

Using density-functional ab initio calculations, we provide a revised phase diagram of (Ga$_{1-x}$In$_{x})_2$O$_3$. Three phases --monoclinic, hexagonal, cubic bixbyite-- compete for the ground state. In particular, in the $x$$\sim$0.5 region we expect coexistence of hexagonal, $\beta$, and bixbyite (the latter separating into binary components). Over the whole $x$ range, mixing occurs in three disconnected regions, and non-mixing in two additional distinct regions. We then explore the permanent polarization of the various phases, finding that none of them is polar at any concentration, despite the possible symmetry reductions induced by alloying. On the other hand, we find that the $\varepsilon$ phase of Ga$_2$O$_3$ stabilized in recent growth experiments is pyroelectric --i.e. locked in a non-switchable polarized structure-- with ferroelectric-grade polarization and respectable piezoelectric coupling. We suggest that this phase could be used profitably to produce high-density electron gases in transistor structures.