Muonium as a shallow center in GaN
  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. 
K. Shimomura, R. Kadono, K. Ohishi, M. Mizuta, M. Saito, K. H. Chow,   B. Hitti, and R. L. Lichti