Near-Bandgap Photoluminescence Decay Time in GaN Epitaxial Layers Grown on Sapphire
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EXPERIMENTAL DETAILS Samples Our GaN samples were grown on 0001-oriented sapphire substrates using low pressure metalorganic vapor phase epitaxy (LP-MOVPE) and employing an AIN buffer layer. The layer thickness was in the range 1 to 3 pm. The samples exhibited X-ray diffraction linewidths of 50 to 80 arcsecs and photoluminescence linewidths of around 2 to 4 meV. The net donor concentration of nominally undoped GaN was less than 1 • 1017 cm- 3 . Measurement Setup The carrier dynamics were investigated using a picosecond time-resolved photoluminescence setup, where the samples were excited with 5 ps pulses from a cavity-dumped frequency-doubled synchronously mode-locked dye laser. The luminescence was detected with a Hamamatsu R3809U microchannel-plate photomultiplier and processed using time-correlated single-photon-counting electronics. By employing suitable deconvolution techniques, an overall time resolution of less than 20 ps was reached. The samples were mounted in a variable-temperature cryostat, allowing for a temperature range from 2 K up to 400 K. 559 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
EXPERIMENTAL RESULTS Low temperature: Bound excitons At low temperature (5 K) our samples exhibit a rather sharp emission with a maximum at 3.488 eV close to the GaN band edge. The intensity of donor-acceptor pair transitions etc. is lower by more than a factor of 10. A close look at highly resolved spectra (Fig. 1) reveals that, depending on temperature, there are at least 3 lines, which are about 6 and 8 meV apart. From the dynamic behavior (see below) and in accordance with the recent literature [2, 3] we interpret these as being due to a shallow-donor-bound exciton (D°X) and two symmetry-split states of the free exciton (A, B). 3.0
DX
I
2.5 ,n
GaN / A12 0 3
2.0
0
10K1.5
C
1.020K
0.0• 3.46
1
30K 401K
0.5
3.47
3.48
B
3.49
energy (eV)
3.5
3.51
3.52
Figure 1: Temperature dependent PL spectra in the excitonic region close to the band gap at low temperature. The decay times of the emission lines labeled D°X and A at low temperature are depicted in Fig. 2. Since the decay time of the lower energy line D°X decreases rapidly between 15 and 30 K our previous interpretation as shallow-donor-bound excitons is confirmed. It is interesting to note that obviously there is no equilibrium between bound and free excitons at these low temperatures, since otherwise their decay times would be identical. Therefore we have to solve the full rate equations including the capture and thermal emission times in order to fit the measured data as shown in Fig. 2. For the fit we have used the spectroscopic binding energy of 6.2 meV as the activation energy. There is some deviation between theory and experiment for the lower branch of the decay times. This is probably due to the fact that experimentally one expects to observe a biexponential decay, which may not have been resolved in our experiments. As a subtle detail, we observe a slight increase of the bound exciton decay time between 5 and 15
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