GaN and related alloys are excitonic materials, i e excess carriers form excitons even at room temperature up to quite high carrier densities. A proper study of the recombination of excitons in GaN requires material of a suitable purity, in order to minimize the influence of defect-related processes. Thick (> 1 mm) freestanding GaN layers with a residual donor concentration < 1016 cm-3 have been used in this work, allowing a more detailed study of the transient photoluminescence (TRPL) properties of excitons in GaN than previously reported.
The TRPL decay of the free A exciton (FE) no-phonon (NP) line is very short at low temperature, of the order 100 ps. On the other hand the LO phonon replicas (1 - LO and 2 - LO) of this transition have a much longer decay time, about 1300 ps at 2 K. The situation with the donor bound excitons (DBEs) is similar, the no-phonon lines have an initial short decay of about 300 ps at 2 K, while the corresponding LO replicas have a decay time about 1400 ps. The so called two-electron transitions (TETs) related to the DBEs behave in a similar way, these lines exhibit a long decay time, above 1 ns. The decay times of the DBEs are found to be somewhat dependent on the residual doping level, they are found to shorten when the donor concentration is in the 1016 cm-3 range or higher due to excitation transfer processes.
The interpretation of the transient PL data obtained from 2 K to 300 K will be discussed, assuming temperature dependent transfer between the various levels, and with reference to the real sample structure. The oxidized GaN surface is known to promote nonradiative recombination, and a built-in electric field (depletion field) is present close to the surface. The effects of the near surface phenomena contribute to the different decays found for NP lines and LO and TET replicas.
An interesting property is the polarization selection rules for various optical phonon replicas for free as well as for bound excitons. Experimental high resolution PL spectra describing these properties will be presented and discussed. The relative strength of the LO phonon replicas for bound excitons, in particular the drastic enhancement (about a factor 100) of the 2-LO replica of the DBEs, will also be discussed.
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