Observation of shallow-donor muonium in Ga2O3: Evidence for hydrogen-induced conductivity

The electrical nature of muonium in the transparent conducting oxide material Ga2O3 is investigated via muon-spin rotation and relaxation spectroscopy. It is found to be a shallow donor, with an effective donor depth of 15 < E-D < 30 meV and a hyperfine splitting of 0.13 +/- 0.01 MHz. This is in contrast to the deep level observed in the majority of semiconductors but supports the recent suggestion that muonium should be a shallow donor across the class of transparent conducting oxides. These observations suggest that hydrogen will also be a shallow donor in Ga2O3, with important implications both for unintentional conductivity and deliberate n-type doping of this material.