Using time-resolved photoluminescence spectroscopy, we determine the temperature dependence of the radiative lifetime for GaN layers with doping levels from 10 16 to 10 18 cm Ϫ3 . The experimental results are analyzed by a coupled rate-equation model taking into account band-to-band, free and bound excitons, as well as donor-to-band recombination. An analytic expression for the radiative lifetime of this coupled system is derived and fit to the data. Over the entire temperature range, radiative recombination is strongly affected by the decay of either bound or free excitons. The temperature dependence and the absolute values for the radiative lifetime are governed by the coupling between the individual populations. This coupling in turn is determined by the binding energies of the respective species. Using a binding energy of 26.4 meV for the free exciton, we obtain best-fit values of 30Ϯ6 and 36.3Ϯ2 meV for the donor and the donor-bound exciton, respectively.

Abstract.Photo-ionisation and -fragmentation of C6o by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection of C,,, n > 2, as opposed to successive Cz evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy of C~-o prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.

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