Mater. Res. Soc. Symp. Proc. Vol. 1741 © 2015 Materials Research Society DOI: 10.1557/opl.2015. Defect engineering in AlGaN-based UV optoelectronic heterostructures grown on c-Al 2 O 3 by plasma-assisted molecular beam epitaxy Sergei Rouvimov 1,2 , Valentin N. Jmerik 1 , Dmitrii V.Nechaev 1 , Valentin V. Ratnikov 1 , Alexey A. Toropov 1 , Eugenii A. Shevchenko 1 , Pavel N. Brunkov 1 , Mikolai V. Rzheutski 3 , Eugenii V. Lutsenko 3 , and Sergey V. Ivanov 1 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 c- Al 2 O 3 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-Al 2 O 3 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 10 8 -10 9 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 Al x Ga 1-x N 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 sub- 300 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-Al 2 O 3 substrates. It has resulted in lowering the optical threshold power density from several MW/cm 2 down to 150 kW/cm 2 for the optically pumped AlGaN mid-UV laser heterostructures grown on c-Al 2 O 3 [1,2]. Recently, the threshold has been lowered below 100 kW/cm 2 for analogous AlGaN/Al 2 O 3 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-Al 2 O 3 substrates using high-resolution transmission electron microscopy (HR TEM) and X-ray diffraction (XRD) analysis. Different types of compositional inhomogeneities in Al x Ga 1-x N 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 range, as well as the fine structure of the AlGaN QWs grown by sub-monolayer digital alloying (SDA) technique [2] were detected by HR TEM and analyzed from the view point of PA MBE growth and their effect on the laser structure performance studied by photoluminescence (PL) spectroscopy. 247