A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time-resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

The electronic structure and phase stability of MgO, ZnO, CdO, and related alloys in the rocksalt ͑B1͒, zincblende ͑B3͒, and wurtzite ͑B4͒ crystal structures were examined within first-principles band structure theory; the thermodynamically stable phases are reproduced for each material. The band alignment and bandgap deformation potentials were analyzed, showing an increase in the valence band maximum from Mg to Zn to Cd. Ternary alloy formation was explored through application of the special quasirandom structure method. The B1 structure is stable over all ͑Mg,Cd͒O compositions, as expected from the preferences of the binary oxides. The ͑Mg,Zn͒O alloy undergoes a tetrahedral to octahedral transition above 34% Mg content, in agreement with experiment. For ͑Zn,Cd͒O, a transition is predicted above 62% Cd content. These results imply that band-gap manipulation of ZnO from alloying with Mg ͑Cd͒ will be limited to 4.0 eV ͑1.6 eV͒, while preserving the tetrahedral coordination of the host.

Time-resolved photoluminescence has been employed to study the mechanisms of band-edge emissions in Mg-doped p-type GaN. Two emission lines at about 290 and 550 meV below the band gap (Eg) have been observed. Their recombination lifetimes, dependencies on excitation intensity, and decay kinetics have demonstrated that the line at 290 meV below Eg is due to the conduction band-to-impurity transition involving shallow Mg impurities, while the line at 550 meV below Eg is due to the conduction band-to-impurity transition involving doping related deep-level centers (or complexes).

Time-resolved photoluminescence (PL) has been employed to study the optical transitions and their dynamic processes and to evaluate materials quality of InGaN epilayers grown by metalorganic chemical vapor deposition. Our results suggest that the PL emissions in InGaN epilayers result primarily from localized exciton recombination. The localization energies of these localized excitons have been obtained. In relatively lower quality epilayers, the localized exciton recombination lifetime τ, decreases monotonically with an increase of temperature. In high quality epilayers, τ increases with temperature at low temperatures, which is a well-known indication of radiative exciton recombination. Our results demonstrate that time-resolved PL measurements uniquely provide opportunities for the understanding of basic optical processes as well as for identifying high quality materials.

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