Sign of the piezoelectric field in asymmetric GaInN/AlGaN/GaN single and double quantum wells on SiC
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BSTRACT We study both GaInN/GaN/AlGaN quantum wells with an asymmetric barrier structure grown on SiC substrate and GaN/AlGaN asymmetric double quantum well (ADQW) structures. In the first case, a time-resolved study reveals an enhanced oscillator strength when the AlGaN barrier is on top of the GaInN quantum well. In comparison to our previous study of the same structure grown on sapphire, we find that the sign of the field is the same in both cases: the field points towards the substrate. In the case of ADQW, we observed not only intrawell transitions of both a 4 nm and a 2 nm QW separated by a 2.5 nm AlGaN barrier but also an interwell transition between the two QWs in the photoluminescence. The lifetimes and emission energies of the transitions can be well explained by the existence of the piezoelectric field built in the QWs.
INTRODUCTION GaInN/AlGaN/GaN-based quantum wells (QWs) have played a key role in the rapid development of short-wavelength light emitters [1]. To explain the puzzling optical properties of the quantum wells, the piezoelectric field effect has been recently discussed actively [2,3,4,5]. In this work, we explore the effect in more detail studying two structures designed specially: first, asymmetric barrier structures, i.e., a GaInN quantum well sandwiched between an AlGaN and a GaN layer, and secondly, GaN/AlGaN asymmetric double quantum well structures. The asymmetry introduced in hese structures exhibits clearly the dominating influence of the piezoelectric field on the optical transitions in the quantum wells.
EXPERIMENTAL Our samples were grown on (0001)-oriented SiC substrates using low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The GaInN layers were grown below temperature of 800°C with N2 as a carrier gas. The growth temperature of GaN and AlGaN layers was 1000°C. Reciprocal space mapping of X-ray diffraction intensity shows that GaInN grown on GaN buffers is coherently strained up to thickness of some 100 nm [6]. A nominally undoped 7 nm GaInN QW is sandwiched between asymmetric
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barrier layers, which consist of a 300 nm GaN buffer layer, a 60 nm GaN cap layer, and a 20 nm AlGaN layer below or above the quantum well (see Fig. 1). The asymmetric double quantum well structure consists of a 2 nm and a 4 nm GaN QW, which are grown in succession on a 700 nm AlGaN buffer layer and separated by a 2.5 nm AlGaN layer. Time-resolved photoluminescence (TRPL) spectroscopy with resonant excitation of the quantum wells was performed at 5 K using a setup already described elsewhere [3].
Fig. 1. Schematic pictures of sample structure of GaInN/GaN quantum wells with an AlGaN barrier.
RESULTS Asymmetric barrier structure on SiC substrate In this section, we study GaInN/GaN QWs grown on SiC substrates with an additional AlGaN barrier above or below the quantum well. Fig. 2 shows lowtemperature photoluminescence spectra of two QWs with asymmetric structure and a simple QW without the AlGaN barrier. The simple QW has an emission maximum at 3.08 eV and a phonon replica is recogni
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