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 (D⁰,X) emission were identified. An extensive temperature-varied study of the donor-bound exciton emission shows that at low temperatures (T<~25K) the main channel of thermal dissociation of the (D⁰,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 free 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 (A⁰,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 [(EFE2S)A-(EFE1S)A=18.3±0.4meV]. 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 (Ebind1S=24.81±0.52meV) and first excited (Ebind2S=6.51±0.12meV) state. By matching the calculated values with experimentally determined energy separation between these states, we obtain the A-band hole nonpolaron mass mₕ*=0.52±0.04.