Shallow versus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgO are compared. Whereas in ZnO a theoretically predicted shallow donor state is confirmed, in HgO we find a considerably deeper state. The respective ionization temperatures are around 40 K and 150 K and the donor ionization energies are 19 +/-1 and 136 +/-3 meV, deduced from the temperature dependence of the mu SR (muon spin-rotation) signal amplitudes. The mu SR spectra provide a comprehensive characterization of the undissociated paramagnetic states: the hyperfine parameters, which measure the electron spin density on and near the muon, differ by a factor of similar to 30. These define a hydrogenic radius of 1.1 mn in ZnO but indicate a much more compact electronic wavefunction in HgO, more akin to those of Mu* and the AA9 centre in Si. These data should largely carry over to hydrogen as a guide to its electrical activity in these materials.