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

Abstract

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 $({\mathrm{FE}}_{A})$ and B band $({\mathrm{FE}}_{B})$ 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 $(Tl~25\mathrm{K})$ 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 ${\mathrm{FE}}_{A}$ 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 ${(A}^{0},X)$ 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 $[{(E}_{\mathrm{FE}}^{2S}{)}_{A}\ensuremath{-}{(E}_{\mathrm{FE}}^{1S}{)}_{A}=18.3\ifmmode\pm\else\textpm\fi{}0.4\mathrm{meV}].$ 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 ${(E}_{\mathrm{bind}}^{1S}=24.81\ifmmode\pm\else\textpm\fi{}0.52\mathrm{meV})$ and first excited ${(E}_{\mathrm{bind}}^{2S}=6.51\ifmmode\pm\else\textpm\fi{}0.12\mathrm{meV})$ state. By matching the calculated values with experimentally determined energy separation between these states, we obtain the A-band hole nonpolaron mass ${m}_{h}^{*}=0.52\ifmmode\pm\else\textpm\fi{}0.04.$