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Oxide muonics I: Modelling the electrical activity of hydrogen in semiconducting oxides
Authors: S.F.J Cox, J.S. Lord, S.P. Cottrell, J.M. Gil, H.V. Alberto, A. Keren, D. Prabhakaran, R. Scheuermann, and A. Stoykov
Ref.: J. Phys.: Condens. Matter 18, 1061-1078 (2006)
Abstract: 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, Ag(2)O and Cu(2)O, paramagnetic muonium centres show varying degrees of delocalization of the Singly Occupied orbital, their hyperfine constants spanning 4 orders Of Magnitude. PbO and RuO(2), 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 Ag(2)O, 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 Cu(2)O, 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.
