Effects of Piezoelectric Fields in GaInN/GaN and GaN/AlGaN Heterostructures and Quantum Wells
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Figure 1: Time-integrated photoluminescence spectra of a series of GaN/AIGaN quantum wells. The dashed line indicates the position of the GaN bandgap.
and GaN/A1GaN double heterostructures (DH) as well as single quantum wells (SQW's) of different thicknesses were investigated. Time-resolved spectroscopy with resonant excitation of the quantum wells was performed using a setup already described elsewhere [7].
EXPERIMENTAL RESULTS Time-integrated low-temperature luminescence spectra of GaN/AlGaN quantum wells exhibited a fairly complex behavior. Besides the AIGaN emission at 3.75 eV, we observe two emission lines from thick quantum wells but only a single line from thin QW's. As shown in Fig. 1, thin quantum wells (1 nm and 2 nm) show a clear blue-shift of their emission due to size quantization. In thicker layers (5 nm and 10 nm) the higher energy emission line is about 70 meV above the GaN bandgap, whereas the lower energy line shifts to energies well below the GaN bandgap with increasing thickness. Measurements of the luminescence decay time (Fig. 2) reveal that both the single line in thin QW's as well as the higher energy line in thicker layers have decay times of about 300 ps. The lower energy line in the thicker layers becomes extremely slow for the 10 nm sample, with a decay time of 30 is. Time-resolved spectra of the lower energy line as depicted in Fig. 3 show that there is a red-shift of the emission peak of 27 meV for long delay times after pulsed excitation. A very similar behavior was observed for several sets of GaInN/GaN QW's, with the decay time of the lower energy line gradually increasing into the upper microsecond range with increasing well thickness. 514
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delay time (ns) Figure 2: Double-logarithmic plot of the luminescence decay for a 2 nm, a 5 nm, and a 10 nm GaN/AlGaN quantum well. The thickest layer shows a decay on a microsecond timescale. DISCUSSION AND MODEL Basically, there are three possible explanations for the origin of the low-energy emission line found both in GaN/AlGaN and in GaInN/GaN structures. 1. In GaInN/GaN MQW structures a red-shifting emission has already been attributed to localized excitons or quantum dots.
2. The extremely slow luminescence decay could be an indication for spatially indirect donor-acceptor-like transitions. 3. Piezoelectric fields due to mismatch-induced strain could lead to a Stark-shift of the emission and to a strong reduction of the oscillator strength. Donor-acceptor-like impurity-related transitions can be excluded since it is hard to see how such transitions would shift to lower energy with increasing well width. Localization or quantum dots can not be the origin of this behavior for two reasons: i) for the GaN/AlGaN samples such transitions could not be below the bulk GaN bandgap; ii) the e
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