Tackling Disorder in $\gamma$-Ga$_2$O$_3$

TL;DR

This paper combines density functional theory and machine learning to model the complex disordered structure of $eta$-Ga$_2$O$_3$, validated by various spectroscopic experiments, advancing understanding of its electronic properties for electronic applications.

Contribution

It introduces a novel atomistic model of $eta$-Ga$_2$O$_3$ using machine learning and DFT, validated by experimental spectroscopy, to better understand its disordered structure and electronic behavior.

Findings

Strong absorption onset at 5.1 eV at room temperature

Good agreement between theoretical and experimental excitation energies

Advances in modeling complex disordered oxides

Abstract

Ga$_2$O$_3$ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga$_2$O$_3$ offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. $\gamma$-Ga$_2$O$_3$ presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure -- electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the $\gamma$-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of $\gamma$-Ga$_2$O$_3$. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by PLE spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure.