Exciton Lifetimes in GaN
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*Department of Physics and Measurement Technology, Linktiping University, S - 58183 Link6ping, Sweden
**Department of Electrical and Electronic Engineering, Meijo University, Nagoya 468, Japan
ABSTRACT. We have performed time resolved photoluminescence measurements of the exciton recombination in different GaN samples at low temperatures. In epitaxial layers the decay time of the free exciton is typically faster than 100 ps. This is due to a dominating non-radiative recombination process. In thick bulk samples we have resolved and measured the decay time of the free exciton with a value of about 200 ps. We believe that this value is close to the radiative lifetime for free excitons in GaN. We have also shown that excitation transfer occurs between free and bound exciton states. We have furthermore measured the decay of the donor and acceptor bound excitons, and obtained values of the decay time of 250 ps and 1200 ps, respectively. INTRODUCTION GaN and nitride based compounds have in recent years attracted a great interest. This is due to the potential for nitrides as excellent materials for devices, mainly in optoelectronic applications. The radiative lifetimes, and the influence of Coulomb correlation on such lifetimes as well as on gain spectra are very important parameters for future laser applications, in a conventional laser structure. A long radiative lifetime will make it more easy to obtain the carrier
concentration needed for a specific gain. On the other hand, a long radiative lifetime increases the possibility for a dominating non-radiative recombination, as compared to the radiative recombination. Optical time resolved spectroscopy of bulk material can provide important information on the radiative lifetimes. At low. temperatures the photoluminescence (PL) spectrum in bulk semiconductors is typically completely dominated by the recombination of bound excitons (BE). The BE's, both the donor BE (DBE) and the acceptor BE (ABE) are thermally quenched at about 50 K due to the relatively low exciton binding energy. At higher temperatures the emission is typically dominated by the recombination of free excitons (FE). So far only scattered studies of time resolved measurements have been published [1-3]. These results are also somewhat contradictory mainly dependent on the large difference in sample quality used for the studies. In this work we have mainly studied three different samples. Sample A is an epitaxial double heterostructure, with a GaN layer surrounded by AIN and AlGaN barriers. This sample shows a PL spectrum typical for epitaxial GaN layers, where the emission at low temperatures is dominated by the recombination of DBE's. The second sample, Sample B, is also an epitaxial layer but with lower substitutional defect concentration, so that the emission is dominated by the recombination of FE's. The third sample, sample C, is a thick (500 gim) bulk crystal grown by vapour phase epitaxy. This sample is of higher optical quality with narrow emission lines. 709 Mat. Res. Soc. Symp. Proc. Vol. 395 01996
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