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An unusual photoluminescence line X has been observed in ZnO at an energy between that of the common donor-bound excitons (DBEs) and the free excitons (FEs). In the presence of a high carrier concentration, induced by a second below-band gap laser, the DBEs decrease in intensity, due to screening, and both the FEs and X increase. Thus, X has free-exciton, rather than bound-exciton, character. However, its electric-field vector lies in the plane perpendicular to the c axis, as is also found for the DBEs. The appearance of X is discussed in terms of the polariton picture.


Copyright © 2001, American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 79.23, and may be found at



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