Neutral-donor bound-exciton transitions were studied for isolated neutral donors that were made up of defect-pair complexes. The neutral-donor nature of these complexes was deduced from magnetic field measurements and from 2-electron transitions. Excited states of the neutral-donor bound excitons were observed in the form of rotator states that were analogous to rotational states of the H2 molecule. Annealing experiments showed that the higher-energy emission lines disappeared and that, at annealing temperatures near to 800C, all of the emission intensity appeared in the lowest-energy donor bound-exciton transition. Integrated intensity measurements revealed that the total emission intensity was approximately conserved. It was suggested that the higher-energy emission lines were due to neutral-donor bound-exciton transitions in which the pairs that made up the neutral donors were more distantly spaced. These were the ones that were first to break up during annealing. The conservation of emission intensity suggested that the pairs were not eliminated but took up a closer spacing. The measured activation energy of 3.6eV was consistent with this motion.

Neutral-Donor-Bound-Exciton Complexes in ZnO Crystals. Reynolds, D.C., Look, D.C., Jogai, B., Litton, C.W., Collins, T.C., Harsch, W., Cantwell, G.: Physical Review B, 1998, 57[19], 12151-5