Temperature-varied photoluminescence and magnetospectroscopy study of near-band-edge emissions in GaN

Near-band-edge emissions from wurtzite GaN films grown on sapphire by metalorganic chemical vapor deposition have been studied by temperature-varied photoluminescence and magnetospectroscopy. Free-exciton emissions of the A band (FEA) and B band (FEB) as well as the neutral donor-bound exciton (D-0,X) emission were identified. An extensive temperature-varied study of the donor-bound exciton emission shows that at low temperatures (T less than or equal to 25 K) the main channel of thermal dissociation of the (D-0, X) complex is through the release of a free exciton with thermal activation energy very close to the exciton binding energy. A temperature-varied study of the strong bound-exciton emission at 11.7 meV below the FEA line indicates that this complex dissociates through the release of a Gee exciton. We present arguments that due to the character of its temperature decay, this emission cannot originate from an ionized donor-bound exciton (D+,X). An exciton bound to a shallow acceptor (AO,X) is a likely candidate for this emission. A magnetospectroscopy study with magnetic field varied up to 9 T allows us to identify the first excited state (2S state) of the A-band free exciton at 3.5035 eV [(E-FE(2S))(A) - (E-FE(1S))(A) = 18.3 +/- 0.4 meV]. Using the Aldrich-Bajaj potential to account for electron-phonon interaction in GaN, we performed a Variational calculation of the binding energies for an A-band free-exciton ground (E-bind(1S) = 24.81 +/-0.52 meV) and first excited (E-bind(2S) = 6.51 +/-0.12 meV) state. By matching the calculated values with experimentally determined energy separation between these states, we obtain the A-band hole nonpolaron mass m(k)* = 0.52 +/-0.04.