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0892-FF20-01.1

Time-resolved Spectroscopy of Excitons Bound at Shallow Neutral Donors in HVPE GaN B. Monemar1, P. P. Paskov1, J. P. Bergman1, T. Malinauskas2, K. Jarasiunas2, A. A. Toropov3, T. V. Shubina3, A. Usui4 1

Department of Physics and Measurement Technology, Linköping University, S-581 83 Linköping, Sweden 2 Institute of Materials Science and Applied Research, Vilnius University, LT-2040 Vilnius, Lithuania 3 A. F. Ioffe Physico-Technical Institute, St. Petersburg, 194021, Russia 4 R&D Division, Furukawa Co., Ltd. Tsukuba, Ibaraki 305-0856, Japan ABSTRACT Time-resolved photoluminescence (TRPL) spectroscopy has been performed in the temperature range 2 K to 300 K on thick (300 µm – 1 mm) nominally undoped bulk HVPE grown GaN layers. The PL spectra are dominated by the free excitons (FEs) and the two neutral donor bound excitons (DBEs) at about 3.4712 eV and 3.4721 eV at 2 K, assigned to the residual O and Si donors, respectively. The zero-phonon decay curves for both FEs and DBEs are nonexponential, with a fast initial decay and a slower tail at longer times. The initial decay time of the FE is about 130 ps at 2 K in such samples, the corresponding initial decay time for the DBEs for O and Si is close to 300 ps. The different lines corresponding to the so called twoelectron transitions for the DBE show a different behaviour, with a longer decay time than the corresponding principal DBE states. The LO phonon replicas of the DBEs also have a decay time in excess of 1 ns in lightly doped samples. INTRODUCTION Excitons bound at shallow neutral donors (DBEs) are the main optical signatures of the shallow donors in semiconductors [1]. The important characteristics of the narrow DBE lines are the spectral position (i. e. the photon energy) and the radiative decay time, which is inversely proportional to the oscillator strength of the transition [2]. Previous works on high quality samples have identified the spectral positions of the DBEs related to the two main shallow donors in strain free GaN, i. e. the Si donor at 3.4723 eV and the O donor at 3.4713 eV [3], although some uncertainty still exists in the literature [4, 5]. The so-called two-electron satellite (TES) spectra at lower energies have also been studied, providing accurate values for the neutral donor excited states as well as the respective donor binding energy [3, 4]. The decay characteristics of these DBEs have also been reported in a large number of papers, with very different results [6-9]. The short decay times (1 ns, although some influence of nonradiative recombination can be seen above 100 K in sample 1. The decay data for sample 2 are similar to those for sample 1 for the DBEs and FEs at 2 K. A slightly longer decay time (10 – 15 % longer) is observed for the O donor BE at 2 K, as

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compared to the Si donor. At high temperatures the FE decay times for sample 2 become much longer, however, and show a value longer than 3 ns at 300 K. Interestingly the 1LO and 2LO replicas of the DBEs have a transient behaviour remarkably differen