Local electronic structure of dilute hydrogen in $\beta$-Ga$_2$O$_3$ probed by muons

TL;DR

This study uses muon spin rotation to investigate the local electronic structure of dilute hydrogen in $eta$-Ga$_2$O$_3$, revealing two distinct muon states linked to donor and acceptor levels and their roles in electronic behavior.

Contribution

It provides the first detailed experimental identification of muon states in $eta$-Ga$_2$O$_3$ and correlates them with theoretical models of hydrogen behavior in the material.

Findings

Identified two muon states: Mu$_1$ (quasi-static) and Mu$_2$ (fast-moving).

Mu$_1$ forms an OMu-bonded state acting as an electron donor.

Mu$_2$ is in a hydride state, facilitating fast diffusion.

Abstract

The local electronic structure of muons (Mu) as dilute pseudo-hydrogen in single-crystalline $\beta$-Ga$_2$O$_3$ has been studied by the muon spin rotation/relaxation ($\mu$SR). High-precision measurements over a long time range of $\sim$25 $\mu$s have clearly identified two distinct Mu states: a quasi-static Mu (Mu$_1$) and fast-moving Mu (Mu$_2$). By comparing this result with predictions from the recently established ambipolarity model, these two states are respectively attributed to the relaxed-excited states associated with the donor ($E^{+/0}$) and acceptor ($E^{-/0}$) levels predicted by density functional theory (DFT) calculations for the interstitial H. Furthermore, the local electronic structure of Mu$_1$ is found to be an OMu-bonded state with three-coordinated oxygen. The structure is almost identical with the thermal equilibrium state of H, and it is found to function as an electron donor. The other Mu$_2$ is considered to be in the hydride state (Mu$^-$) from the ambipolarity model, suggesting that it is in fast diffusion motion through the short-lived neutral state due to the charge exchange reaction with conduction electrons (Mu$^0+e^-\rightleftarrows$Mu$^-$).