Mechanism for Radiative Recombination in In 0.15 Ga 0.85 N/GaN Multiple Quantum Well Structures
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G 2.5 Mat Res. Soc. Symp. Proc. Vol. 537 © 1999 Materials Research Society
resolution TEM pictures, while the upper InGaN/ GaN interface had a short range fluctuation of about 1-2 monolayers. The spectral PL experiments discussed here involve continuous wave (cw) laser excitation, as well as more advanced transient studies. For the transient PL measurements we had several UV laser systems available, tunable both above and below the GaN bandgap. For detection we employ photon counting techniques for the time domain 100 ps to 4 .is, and a streak camera for the faster time domain 15 ps to 2 ns. PLE spectra were obtained with a Xenon lamp and a monochromator as light source. Transient photoluminescence spectra We have investigated the transient behavior of the PL over a large time scale and a large dynamic range. The results for the shorter time scale will be reported separately [6]. The PL decay over a longer time scale (ns - ýts) has been measured with photon counting technique over the entire spectral range covered by the PL emission. Typical results are shown in Figs 1(a) and 1(b) for selected samples at low temperature. In Fig I we compare the time resolved spectra of an undoped (a) and a doped sample (b) ( n = 2 x 1018 cm-3), respectively. It is noted that the spectra for the doped samples are considerably narrower, with the impression that the lower energy part of the spectrum in Fig I (a) is simply missing in the spectrum l(b). This will be discussed further below. There is also a rather strong shift downwards for the peak energy with delay time, which is expected due to screening by the excited e-h pairs, and also due to spectral diffusion from carrier hopping before recombination.
a)Undoped
b)Doped 4GaInN/GaN
4GalnN/GaN
10
I7 At .
MQW
10
At --0.8 n T2K
Tý2
2
2
"• 102
102
S101
10
100 __100
_
2.5 3.0 3.5 2.5 3.0 3.5 Photon Energy [eV] Photon Energy [eV] Fig. 1. Timeresolved PL spectra, with 0.8 ns between each spectrum, for two MQW samples obtained at 2 K with excitation at 3.6 eV. In a) are shown the spectra for a undoped sample while b) shows the corresponding data for a doped sample. Note the strong spectral shift between the two samples.
4
10o
105 104
•2 10
102 102 a
101
100
10 10
2.6 2.8 .II......0 3.0 Photon Energy [eV]
1
- 2.6 2.8 3.0 Photon Energy [eV]
Fig. 2. Timeresolved PL spectra for an MQW sample with the InGaN layer grown at 750 C, obtained at 2 K (a) and at 300 K (b), respectively. The spectra at 300 K are broadened towards higher energy, and also have shorter decay time. In Fig 2 we show a comparison of the timeresolved spectra at 2 K and 300 K for an MQW sample with the InGaN layers grown at 750 *C. The spectra are quite similar concerning downward shift with delay time, but the 300 K spectrum is broadened considerably and selectively on the high energy side, even at long delay times. This may be understood by the presence of strong localization potentials in the system. Also, the decay is faster at 300 K, presumably due to the presence of a nonradiative recombi
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