Photoluminescence excitation spectroscopy of free-to-bound transitions in undoped GaN grown by hydride vapor phase epitaxy

Temperature-dependent photoluminescence excitation (PLE) spectroscopy has been carried out on the yellow band (YB) in GaN. The 5 K PLE spectra demonstrate that the exciting light must have photon energy large enough to generate free carriers or carriers localized on shallow impurities in order to excite the YB effectively. With increasing temperatures, progressively deeper energy levels can be thermally ionized, enabling extrinsic absorption by these deeper levels to generate the free holes required to excite the YB emission. The broad below-band gap PLE response then exhibits thermally activated onsets attributed to these free-to-bound transitions. One such onset corresponds to the well-known 205 meV acceptor, and a second onset provides conclusive evidence for the existence of a previously unconfirmed similar to 120 meV impurity or defect level in GaN. (C) 1998 American Institute of Physics. [S0003-6951(98)02544-3]