Correlation between structural properties and optical amplification in InGaN/GaN heterostructures grown by molecular bea
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microscopy experiments is described in Ref. [8]. For the time integrated high-excitation investigations we used a dye laser pumped by an excimer laser, providing pulses with a duration of 15 ns at a rate of 30 Hz and a total energy of up to 20 µJ at 340 nm. The samples were mounted in a bath cryostat at 1.8 K. Gain measurements were performed using the variable-stripe-length method [9]. In this paper we focus on four typical InGaN/GaN heterostructures which are chosen from a variety of samples. All samples are grown on sapphire, followed by a 1.8 µm GaN (MOVPE) layer for samples A-C and capped by a 30 nm GaN layer. Samples A and B are double heterostructures including a 40 nm thick InGaN layer, with an Indium concentration of 10% and 13.7%, respectively, as determined by XRD [10]. Sample C is a 10x5 nm InGaN multiple-quantum well (MQW) with 4 nm GaN barriers. The growth conditions for the InGaN wells were the same as for sample B. Finally, sample D contains an InGaN layer of 120 nm thickness and 21% Indium not grown on a MOVPE-GaN template. Results
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Sample A
Energy (eV)
2.970
λ =325 nm
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Normalized PL Intensity (arb. u.)
Temperature (K) 10K 30K 50K 70K 80K 90K 100K 110K 120K 140K 160K 180K 200K 220K 240K 260K 290K 2.4
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Energy (eV)
Fig. 1: PL spectra of sample A at different temperatures. The inset shows the position of the high-energy peak vs. temperature.
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Fig. 1 shows the temperature dependence of the photoluminescence of sample A. One can distinguish between two different PL peaks, which change their relative intensities with temperature. At very low temperatures (10 K-70 K) and at room temperature the high-energy peak dominates the spectra. Furthermore this luminescence exhibits a “S-shaped” emission position shift with temperature as can be seen in the inset of Fig. 1. This behavior (redshift-blueshift-redshift) was first explained by Cho et al. [11] in terms of inhomogeneity and carrier localization in the InGaN. Recently, it has been shown that the “S-shape” behavior becomes less pronounced with increasing excitation power which can also be understood in terms of local potential fluctuations in the InGaN [12]. We think that thermalization and carrier freeze out in potential fluctuations leads to this unexpected temperature behavior. In this meaning the high-energy peaks is not only one emission line, but the spatially integrated luminescence of locally different recombination energies, which mirrors the distribution of the potential fluctuations. We assign the low-energy peak being dominant between 90 K and 180 K as recombination from electronic states deeper in the band gap.
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Intensity (arb. u.)
Intensity (arb. u.)
3X X4 2X 6 7 X5 X X 1X
T=4 K λ=290 nm Laser
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Wavelength (nm) 0
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Wavelength (nm) Norm . Intensity (log. u.)
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