Enhanced radiative recombination in AlGaN quantum wells grown by molecular-beam epitaxy
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Enhanced Radiative Recombination in AlGaN Quantum Wells Grown by Molecular-Beam Epitaxy B. A. Borisova^, S. N. Nikishina, V. V. Kuryatkova, V. I. Kuchinskiœb, M. Holtza, and H. Temkina aDepartment
of Electrical Engineering, Texas Tech University, Lubbock, TX 79409, USA ^e-mail: [email protected] bIoffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia Submitted September 6, 2005; accepted for publication September 21, 2005
Abstract—Dependences of the intensity of cathodoluminescence in multiple Al0.55Ga0.45N/Al0.45Ga0.55N quantum wells grown by molecular-beam epitaxy on the growth conditions are studied. An increase (by almost two orders of magnitude) in the intensity of the cathodoluminescence peak with an energy of 4.45 eV is observed as the quantum-well layer grows in the conditions of deep depletion with respect to ammonia. In this case, a tendency towards the mode of three-dimensional growth can be inferred from the pattern of diffraction of high-energy electrons; this effect is interpreted using a model of formation of AlGaN quantum dots. PACS numbers: 78.67.De, 78.67.Hc, 81.15.Hi DOI: 10.1134/S1063782606040154
1. INTRODUCTION The recent considerable progress in the epitaxial growth of high-quality layers and heterostructures based on AlGaN with the AlN content higher than 40% has made it possible to fabricate light-emitting diodes (LEDs) that emit in the far-ultraviolet region of the spectrum [1–6]. However, notwithstanding the attained milliwatt power of emission and good spectral characteristics [5, 6], the external quantum yield remains low (~1%). This effect is caused by a low internal quantum efficiency, which is governed by the low efficiency of radiative recombination of electron–hole pairs in the AlGaN wide-gap quantum wells (QWs) and the high density of threading dislocations [5, 7], which represent an efficient channel of nonradiative recombination. One of the methods for increasing the radiative– recombination efficiency consists in the formation of quantum dots (QDs). For example, the high quantum yield of LEDs based on InGaN is caused by QDs formed by the regions with a high content of InN [8]. These regions are formed owing to the low solubility of InN in GaN, which gives rise to phase separation. QDs can be obtained in the case of transition from the mode of two-dimensional (2D) growth to that of three-dimensional (3D) growth either using a surfactant [9] or as a result of high internal stresses caused by the mismatch between the crystal lattices of GaN and AlN [10–12]. The issue concerning the AlGaN formation has been touched on to a much lesser extent in the available publications. Hirayama et al. [13] reported on AlxGa1 – xN QDs grown on Al0.38Ga0.62N and formed by the low-
pressure MOCVD using Si as a surfactant. However, the formation of QDs led only to a decrease in the luminescence efficiency. In this study, we obtained an increase in the intensity of cathodoluminescence of the Al0.55Ga0.45N/Al0.45Ga0.55N multiple quantum w
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