Muonium as a shallow center in GaN

TL;DR

This study reports the first observation of a paramagnetic muonium state in GaN with a shallow energy level, suggesting hydrogen could contribute to n-type conductivity in the material.

Contribution

It provides new experimental evidence of muonium as a shallow center in GaN, indicating potential implications for hydrogen-related doping effects.

Findings

Muonium exhibits a highly anisotropic hyperfine structure.

The ionization energy of muonium is less than 14 meV.

Muonium may act as a shallow donor in GaN.

Abstract

A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.