Optimization of GaN/AlGaN Quantum Wells for Ultraviolet Emitters
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Optimization of GaN/AlGaN Quantum Wells for Ultraviolet Emitters A. Hangleiter, D. Fuhrmann, M. Greve, and U. Rossow Institut f¨ur Technische Physik, Technische Universit¨at Braunschweig, Mendelssohnstr. 2, D-38106 Braunschweig, Germany E-mail: [email protected] ABSTRACT We have studied GaN/AlGaN and GaInN/GaN quantum well structures grown on sapphire and SiC substrates using low-pressure MOVPE. The emission wavelength of the GaN/AlGaN QW’s was varied in the 320-400 nm range and that of the GaInN/GaN QW’s in the 390-480 nm range, by adjusting the QW width. Using temperature dependent resonant-excitation photoluminescence measurements we have determined the internal quantum efficiency (IQE) and analyzed its temperature dependence. For the GaN/AlGaN QW’s we achieve a reasonable IQE of in excess of 5 % only at fairly high excitation power, while GaInN/GaN QW’s provide IQE’s of up to 46 % at rather low power density, indicating a much stronger role of defects for the former. While thermal activation energies are dominated by intrinsic mechanisms for GaInN/GaN QW’s, lower activation barriers evident from the GaN/AlGaN data also hint at a more import role of defects. INTRODUCTION Light emission by GaInN-based heterostructures operating in the green, blue, and violet spectral region is known to be surprisingly efficient, while so far shorter wavelength GaN- or AlGaNbased structures suffer from low quantum efficiency due to nonradiative recombination at defects. For visible or near-UV LED’s based on GaInN quantum wells, external quantum efficiencies up to 35 % have been reported [1]. In contrast, GaN-, AlGaN-, and AlGaInN-based structures at emission wavelengths shorter than 360 nm exhibit external efficiencies below 1 %, even at high injection current densities [2]. The basic optical properties of nitride quantum wells are governed by the huge piezoelectric and spontaneous polarization fields inherent to such structures [3, 4, 5]. The emission energy [3, 5], the oscillator strength [5], a large Stokes-like shift between emission and absorption [6], the large emission linewidth [6], and many other properties are direct consequences of the large internal polarization fields [7]. It has become common in the literature to associate the high efficiency of GaInN-based quantum wells to carrier localization due to compositional fluctuations or even phase separation [8]. On the other hand, the low quantum efficiency of In-free nitride LED’s is being attributed to the lack of localization in binary QW’s. This interpretation is based on optical [8] and TEM data [9]. Nevertheless, a clear thermodynamic argument supporting the effect of localization is still missing. In this paper, we compare UV-emitting GaN/AlGaN quantum wells with blue and violet GaInN/GaN QW’s. From the temperature and injection dependence of the emission intensity and wavelength, we determine internal quantum efficiencies and localization energies. A detailed analysis allows us to identify the mechanisms dominating the room temperature efficiency. EXPER
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