Temperature-dependent photoluminescence excitation spectroscopic studies were made of the yellow band. The 5K photoluminescence excitation spectra demonstrated that the exciting light had to have a photon energy which was high enough to generate free carriers, or carriers localized on shallow impurities, in order to excite the yellow band effectively. With increasing temperature, progressively deeper energy levels could be thermally ionized, thus permitting extrinsic absorption by these deeper levels to generate the free holes which were required in order to excite the yellow-band emission. The broad below-bandgap photoluminescence excitation response then exhibited thermally activated onsets which were attributed to these free-to-bound transitions. One such onset corresponded to a well-known 0.205eV acceptor, and a second onset provided conclusive evidence for the existence of a previously unconfirmed 0.120eV impurity or defect level.

Photoluminescence Excitation Spectroscopy of Free-to-Bound Transitions in Undoped GaN. S.J.Rhee, S.Kim, E.E.Reuter, S.G.Bishop, R.J.Molnar: Applied Physics Letters, 1998, 73[18], 2636-8