Polariton and free-exciton-like photoluminescence in ZnO

An unusual photoluminescence line X has been observed in ZnO at an energy b etween that of the common donor-bound excitons (DBEs) and the free excitons (FEs). In the presence of a high carrier concentration, induced by a secon d below-band gap laser, the DBEs decrease in intensity, due to screening, a nd 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 p erpendicular to the c axis, as is also found for the DBEs. The appearance o f X is discussed in terms of the polariton picture. (C) 2001 American Insti tute of Physics.