Neutral-donor-bound-exciton transitions have been observed in ZnO. The isolated neutral donors are made up of defect pair complexes. The neutral-donor nature of these pair complexes was determined from magneticfield measurements and from two-electron transitions. Excited states of the neutral-donor bound excitons were observed in the form of rotator states analogous to rotational states of the H 2 molecule.
512Т. L. LOUCKS 143 changes in the exchange potential should also affect the results for Mo and we have seen (Ref. 8) that this is not necessary to get agreement with experiment. This, coupled with the fact that Mo and W differ in several experiments, leads us to attribute these differ ences mainly to relativistic effects.The absorption and reflection spectra of ZnO crystals have been studied in the region of intrinsic absorp tion, and the data have been interpreted in terms of the wurtzite model given by Birman. Three intrinsic exciton transitions have been observed, one associated with each of the three valence bands. Several boundexciton transitions have also been observed. The data have been analyzed, and a number of exciton param eters in ZnO are reported which are somewhat modified from those previously reported.
The crystal structure of ZnO is wurtzite and the stacking sequence of atomic layers along the ''c'' axis is not symmetric. As a result, a ZnO crystal surface that is normal to the c axis exposes one of two distinct polar faces, with ͑0001͒ being considered the O face and ͑0001͒ the Zn face. Photoluminescence ͑PL͒ measurements on the two faces reveal a striking difference. Two transitions are observed in PL that are dominant from the O face and barely observed in PL from the Zn face. These lines are identified as phonon replicas of a particular D 0 ,X transition using energy separations, excitation dependence, and time-resolved PL measurements. In addition, PL emission from free excitons is found to be more intense from the O face than from the Zn face.
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