Group III-nitride semiconductors have been recognized as very important materials for opto-electronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) for applications in the visible and ultra-violet (UV) regions. To produce highly efficient light emission in LEDs, it is important to realize pure and almost perfect semiconductor materials in epitaxial growth. In most LED materials, it is desirable to have dislocation densities of 10 4 cm -2 or less for highly efficient light emission.In practice, GaN layers are grown on sapphire substrates (Al 2 O 3 ) by metalorganic chemical vapor deposition (MOCVD) at high temperature. Because of the difference in crystal structure and atomic spacing of sapphire substrate and GaN layer, so-called lattice mismatch, the GaN layers are formed and grown with a high density of threading dislocations with a typical density of 10 8 -10 9 cm -2 . This high density of threading dislocations associated with these GaN devices was a major obstacle in commercializing blue and green LEDs. Extended defects such as threading dislocations decrease the efficiency of LEDs because threading dislocations are associated with non-radiative recombination centers. Therefore, the efficiency of LEDs made in hetero-epitaxial growth decreases dramatically as the dislocation density increases.But it was discovered that adding a certain amount of In (indium) to the GaN layers increases the LED efficiency dramatically despite very high dislocation densities. This discovery has been the most important advance in allowing high efficiency blue and green LEDs. To explain the unexpected highly efficient light emission in InGaN based LEDs despite the existence of high dislocation density above 10 8 -10 9 cm -2 , a high density of In clustering or In rich regions was proposed to be essential in InGaN based active layers. Because the bandgap of the In rich regions is smaller than that of the surrounding InGaN materials, radiative carrier recombination is favored in these regions. The existence of In clustering has been often observed by photo-luminescence, cathodoluminescence, and high resolution transmission electron microscopy (HR-TEM) measurements. Especially, cross-sectional TEM and X-ray fluorescence measurements have been believed to be direct tools for detecting In clustering and used to optimize the density of In clusters or In rich quantum dots in the InGaN active layers to increase the internal quantum efficiency of InGaN based LEDs.The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). The average In mole fraction by three-dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis.The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be nonuniform in the InGaN active layer. In clustering or In ...

Abstract:In this paper, we have observed an atomic-scale structure and compositional variation at the interface of the InGaN/GaN multi-quantum wells~MQW! by both scanning transmission electron microscopy~STEM! using high-angle annular dark-field mode and atom probe tomography~APT!. The iso-concentration analysis of APT results revealed that the roughness of InGaN/GaN interface increased as the MQW layers were filled up, and that the upper interface of MQW~GaN/InGaN to the p-GaN side! was much rougher than that of the lower interface~InGaN/GaN tot he n-GaN side!. On the basis of experimental results, it is suggested that the formation of interface roughness can affect the quantum efficiency of InGaN-based light-emitting diodes.