Symmetry breaking structural relaxation and optical transitions of native defects and carbon impurities in LiGa$_5$O$_8$

TL;DR

This study investigates defect structures, optical transitions, and symmetry-breaking relaxations in LiGa$_5$O$_8$, revealing revised defect levels and the role of carbon impurities in its electronic properties.

Contribution

It introduces a detailed analysis of symmetry-breaking effects on native defects and carbon impurities, revising defect levels and optical transition insights in LiGa$_5$O$_8$.

Findings

Revised transition levels for Li vacancies based on symmetry-breaking relaxations.

Comparison of polaronic and non-polaronic states in defect structures.

Identification of carbon impurities' potential impact on optical properties.

Abstract

LiGa$_5$O$_8$ in a spinel type structure has recently been claimed to be an unintentional p-type ultra-wide-band-gap oxide semiconductor. While previous computational work did not yet identify the origin of p-type doping and in fact predicted insulating behavior by compensation of deep acceptors by shallow donors, defect characterization in terms of its optical signatures remains important. Rather than focusing on thermodynamics transition levels, as in earlier work, this present paper focuses on the vertical transitions in a defect configuration diagram of defects in different charge states, representing absorption and emission processes involving carrier capture/emission from/to band edges. In addition, the structural relaxation of several native defects is revisited by allowing for more complex symmetry-breaking distortions in an effort to reconcile conflicting results in the previous literature. Special attention is given to the Li vacancy because it is the shallowest native acceptor. For this defect, the previously reported transition levels are revised on the basis of symmetry-breaking relaxations. The structural relaxations, band structures, and densities of states are compared between the symmetry-broken polaronic and symmetry-conserving non-polaronic states. Finally, we also study carbon impurities, which are likely to originate from growth methods involving organic precursors.