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 $\left({\mathrm{FE}}_A\right)$ and B band $\left({\mathrm{FE}}_B\right)$ 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 $(T<~25\mathrm{K})$ 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 ${(D}^{+},X).\mathrm{}$ 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 $[{(E}_{\mathrm{FE}}^{2S}{)}_A-{(E}_{\mathrm{FE}}^{1S}{)}_A=18.3\mathrm{}\pm 0.4\mathrm{meV}].$ 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 ${(E}_{\mathrm{bind}}^{1S}=24.81\mathrm{}\pm 0.52\mathrm{meV})$ and first excited ${(E}_{\mathrm{bind}}^{2S}=6.51\mathrm{}\pm 0.12\mathrm{meV})$ state. By matching the calculated values with experimentally determined energy separation between these states, we obtain the A-band hole nonpolaron mass $m_h^{*}=0.52\mathrm{}\pm \mathrm{0.04.}$