Oxide muonics: I. Modelling the electrical activity of hydrogen in semiconductingoxides

Abstract

A shallow-to-deep instability of hydrogen defect centres in narrow-gap oxidesemiconductors is revealed by a study of the electronic structure and electrical activity oftheir muonium counterparts, a methodology that we term ‘muonics’. In CdO,Ag2O and Cu2O, paramagnetic muonium centres show varying degrees of delocalization of the singlyoccupied orbital, their hyperfine constants spanning 4 orders of magnitude. PbO andRuO2, on the other hand, show only electronically diamagnetic muon states, mimickingthose 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 atsome variance with the effective-mass model but illustrates the possibility ofhydrogen doping, inducing n-type conductivity as in the wider-gap oxide, ZnO. ForAg2O, the principal donor level is deeper (0.25 eV) but ionization is nonetheless complete byroom temperature. Striking examples of level-crossing and RF resonance spectroscopyreveal a more complex interplay of several metastable states in this case. InCu2O, muonium has quasi-atomic character and is stable to 600 K, although the electron orbitalis substantially more delocalized than in the trapped-atom states known in certainwide-gap dielectric oxides. Its eventual disappearance towards 900 K, with an effectiveionization energy of 1 eV, defines an electrically active level near mid-gap in thismaterial.