A shallow-to-deep instability of hydrogen defect centres in narrow-gap oxide
semiconductors is revealed by a study of the electronic structure and electrical activity of
their muonium counterparts, a methodology that we term 'muonics'. In CdO,
Ag2O
and Cu2O, paramagnetic muonium centres show varying degrees of delocalization of the singly
occupied orbital, their hyperfine constants spanning 4 orders of magnitude. PbO and
RuO2, on the other hand, show only electronically diamagnetic muon states, mimicking
those of interstitial protons. Muonium in CdO shows shallow-donor behaviour,
dissociating between 50 and 150 K; the effective ionization energy of 0.1 eV is at
some variance with the effective-mass model but illustrates the possibility of
hydrogen doping, inducing n-type conductivity as in the wider-gap oxide, ZnO. For
Ag2O, the principal donor level is deeper (0.25 eV) but ionization is nonetheless complete by
room temperature. Striking examples of level-crossing and RF resonance spectroscopy
reveal a more complex interplay of several metastable states in this case. In
Cu2O, muonium has quasi-atomic character and is stable to 600 K, although the electron orbital
is substantially more delocalized than in the trapped-atom states known in certain
wide-gap dielectric oxides. Its eventual disappearance towards 900 K, with an effective
ionization energy of 1 eV, defines an electrically active level near mid-gap in this
material.