Piezoelectric Field Effect on Optical Properties of GaN/GaInN/AlGaN Quantum Wells
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X-ray diffraction intensity shows that GaInN grown on GaN buffers is coherently strained up to thicknesses of some 100 nm [10]. In this study, two groups of samples were designed and fabricated to introduce an asymmetry into quantum wells. First, a nominally undoped 7 nm GaInN QW was sandwiched between asymmetric barrier layers, which consist of a 300 nm GaN buffer, a 60 nm GaN cap layer, and an additional 20 nm AlGaN layer below or above the quantum well. The AlN and InN mole fraction of AlGaN and GaInN layers are estimated as 15 % and 6 %, respectively. The second sample group consists of 6 nm GaInN QW’s sandwiched between doped or undoped GaN barrier layers: in one sample both the GaN buffer and the GaN cap layer are doped with Si, and in another one only the GaN cap but not the buffer layer is doped. The Si-doping level is estimated as (1 -2) × 1018 cm-3. As reference samples, 6 nm and 3 nm GaInN QW’s with nominally undoped GaN barrier layers were grown. Time-resolved photoluminescence (TRPL) spectroscopy with resonant excitation of the quantum wells was performed at 5 K using a setup already described elsewhere [7]. RESULTS AND DISCUSSION
Asymmetric barrier structure This section focuses on GaInN/GaN QW’s with an additional AlGaN barrier above or below the quantum well. Low-temperature photoluminescence spectra of these two quantum wells with asymmetric barrier structure are summarized in Fig. 1. To start with time-integrated spectra (doted curves), the sample with an AlGaN barrier below the quantum well has an emission maximum at 3.060 eV, and the other one, in contrast, exhibits a broad emission band with two maxima at 3.146 eV and 3.236 eV. A more detailed picture is given by time-resolved spectra (solid curves). At short delay times, both samples show an emission line near 3.245 eV, but their temporal behaviors are clearly different afterwards: in the sample with the AlGaN barrier on top of the quantum well, an additional lower-energy emission line emerges with increasing time and finally reaches 3.146 eV, i.e. 120 meV below the emission line at early times, but the other sample with the AlGaN-barrier below the quantum well shows a dramatic red-shift by about
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AlGaN (20 nm) sapphire
6 4 2 0 2.9
(a)
GaN GaN
0 - 430 ps .43 - 1.3 ns 1.3 - 3.8 ns 3.8 - 7.7 ns 7.7 - 13 ns 13 - 27 ns 27 - 90 ns 90 - 200 ns 200 - 400 ns time-integrated
3.0
3.1 3.2 3.3 3.4 photon energy (eV)
3.5
(b)
AlGaN (20 nm) GaN
intensity (arb. units)
intensity (arb. units)
GaInN (7 nm)
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GaInN (7 nm) sapphire
0 - 70 ps 70 - 180 ps 180 - 450 ps 450 - 580 ps .58 - 1.8 ns 1.8 - 4.6 ns 4.6 - 13 ns 13 - 22 ns 22 - 33 ns
6 4 2 0 2.9
GaN
time-integrated 3.0
3.1 3.2 3.3 3.4 photon energy (eV)
3.5
Figure 1: Time-integrated (doted curves) and time-resolved (solid curves) low-temperature spect
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