We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons—chemically analogous to protons in this context—form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.

Muonium, and by analogy hydrogen, is shown to form a shallow-donor state in ${\text{In}}_2{\text{O}}_3$ and ${\text{SnO}}_2$. The paramagnetic charge state is stable below $\sim 50\text{K}$ in ${\text{In}}_2{\text{O}}_3$ and $\sim 30\text{K}$ in ${\text{SnO}}_2$ which, coupled with its extremely small effective hyperfine splitting in both cases, allows its identification as the shallow-donor state. This has important implications for the controversial issue of the origins of conductivity in transparent conducting oxides.