Structural and Optical Properties of Group III-Nitride Quantum Wells Studied by (S)Tem and CL
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**4. Physikalisches Institut, Universitat Stuttgart, D-70550 Stuttgart, Germany
ABSTRACT Wurtzite InGaN/GaN and AlGaN/GaN heterostructures grown on sapphire by metal organic vapor phase epitaxy were studied using scanning transmission electron microscopy (STEM), cathodoluminescence (CL) combined with secondary electron (SE) imaging, high resolution x-ray diffractometry (HRXRD), and atomic force microscopy (AFM). SE imaging and AFM were used to study the surface morphology. The results indicate the presence of the following structural defects on the surface of InGaN/GaN heterostructures: hexagonal mesa-like structures, hexagonal pyramids and micropipes, while the surface of the AlGaN/GaN heterostructures are mirror-like smooth. The local optical properties of defects and defect free regions were studied using spatially resolved CL at low temperature. In addition, the dependence of the optical properties of both sorts of heterostructures on the quantum well width or chemical composition of ternary materials was investigated. The structural properties of the heterostructures were studied by STEM and HRXRD. Convergent beam electron diffraction (CBED) and corresponding simulations, convergent beam imaging (CBIM), and high resolution x-ray diffraction (HRXRD) were used to study the strained layers. Dislocations and interface properties were characterized using bright-field imaging, while the chemical compositions fluctuations were analyzed by Z-contrast imaging and energy dispersive x-ray microanalysis (EDX). INTRODUCTION Group III nitrides have attracted strong interest and have been recognized as very important wide bandgap semiconductors for fabricating optoelectronic devices operating in the blue/UV region and electronic devices capable of operating under high power and high temperature conditions [1, 2]. In the past years, much effort was spent to grow and characterize this material system. The realization of highly efficient blue and green light emitting LED's and first progress on blue and UV light emitting laser devices using MOVPE grown group III nitrides have been demonstrated [3]. Because of the absence of suitable substrates on which the group-III nitrides can be grown lattice matched, a high density of crystalline defects is expected in these materials. Despite the successful fabrication of AlxGal1 xN/GaN and InxGal1 xN/GaN heterostructures the basic properties of AlxGal-xN are to a large extent unknown. It is of current interest to understand the growth mechanisms, the defect structure and their role in electrical and electro-optical properties. Additionally the growth of InGaN is hindered by several difficulties. They include: (1) The miscibility gap in this ternary material or the difference in the atomic radii of In and N [4]. Regarding to that, quite different growth conditions (compared to those of GaN) such as low growth temperature and extremely high V/III ratios are required to achieve significant In concentrations in the layers [5]. (2) The weak In-N bond necessitating the use of a high equilibriu
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