Defect engineering in AlGaN-based UV optoelectronic heterostructures grown on c-Al 2 O 3 by plasma-assisted molecular be
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Defect engineering in AlGaN-based UV optoelectronic heterostructures grown on c-Al2O3 by plasma-assisted molecular beam epitaxy Sergei Rouvimov1,2, Valentin N. Jmerik1, Dmitrii V.Nechaev1, Valentin V. Ratnikov1, Alexey A. Toropov1, Eugenii A. Shevchenko1, Pavel N. Brunkov1, Mikolai V. Rzheutski3, Eugenii V. Lutsenko3, and Sergey V. Ivanov1 1 Ioffe Physical-Technical Institute, 194021, 26 Politekhnicheskaya str., St.-Petersburg, Russia, 2 University of Notre Dame, Notre Dame, Indiana 46556, U.S.A. 3 Stepanov Institute of Physics of NAS Belarus, Independence Ave. 68, Minsk 220072, Belarus ABSTRACT AlGaN-based SQW heterostructures grown by plasma-assisted molecular beam epitaxy on cAl2O3 substrates have been studied with high resolution transmission electron microscopy (HR TEM), photoluminescence spectroscopy and x-ray diffraction. The high-temperature (780°C) synthesis of the AlN buffer layer nucleated on c-Al2O3 by a migration enhanced epitaxy and including several ultra-thin GaN interlayers grown under moderate N-rich conditions was shown to be the optimum approach for lowering the threading dislocations density down to 108-109 cm-2. HR TEM study has confirmed the fine structure of single quantum wells (SQW) formed by a sub-monolayer digital alloying technique and revealed different kinds of compositional inhomogeneities in the AlxGa1-xN barrier layers of the heterostructures, including the formation of Al-rich barriers induced by the temperature-modulated epitaxy and the spontaneous compositional disordering along the growth axis for x=0.6-0.7. The influence of these phenomena on the parameters of the mid-UV stimulated emission observed in the SQW structures has been studied as well. INTRODUCTION Improving the structural quality of AlGaN-based heterostructures with high Al content (x>0.4) is a crucial issue for fabricating high efficiency UV optoelectronic devices working in the sub300 nm wavelength range. Several approaches to defect engineering have been employed in a plasma-assisted MBE (PA MBE) to reduce the threading dislocation (TD) density in the top (active) region of the heterostructures grown on c-Al2O3 substrates. It has resulted in lowering the optical threshold power density from several MW/cm2 down to 2 150 kW/cm for the optically pumped AlGaN mid-UV laser heterostructures grown on c-Al2O3 [1,2]. Recently, the threshold has been lowered below 100 kW/cm2 for analogous AlGaN/Al2O3 heterostructures grown by metalorganic chemical vapor deposition (MOCVD) [3]. The paper studied the generation and filtering of TDs during PA MBE of AlN/GaN buffer heterostructures on c-Al2O3 substrates using high-resolution transmission electron microscopy (HR TEM) and X-ray diffraction (XRD) analysis. Different types of compositional inhomogeneities in AlxGa1-xN waveguide layers of the AlGaN QW laser structures, in particular the ultra-thin Al-enriched barrier layers induced by substrate-temperature modulated epitaxy (TME) and the spontaneously formed aperiodic compositional modulations along the c-axis for x in the 0.6-0.7
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