Shallow donors hydrogen model

An exciton bound to a shallow neutral donor of interstitial zinc (Fig 1 a) and of interstitial lithium (Fig. lb) is presented, for example, in our spectra. In some instances the radiative recombination of an exciton bound to a neutral defect may not lead to the ground state of the respective defect but to an excited state of the carrier at this occupied center (2 - electron transition). In a hydrogenic model we can calculate an ionization energy of the neutral donor state of interstitial zinc to 0.05 eV and of interstitial lithium to 0.033 eV. 

If this level lies in the conduction band (as in ZnO and InN) hydrogen forms a shallow donor level. If the level lies in the band gap (as for the other three materials) then hydrogen forms a deep centre. The predictive nature of this model is currently being tested for other semiconductors, in particular oxides other than ZnO. 

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