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.
Reynolds, D. C.,
Look, D. C.,
& Collins, T. C.
(2001). Polariton and Free-Exciton-Like Photoluminescence in ZnO. Applied Physics Letters, 79 (23), 3794-3796.