Reduction of the threading edge dislocation density in AlGaN epilayers by GaN nucleation for efficient 350 nm light emitting diodes

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We report on the reduction of the threading edge dislocation density of Al 0.15 Ga 0.85 N buffer layers for efficient 350nm light-emitting diodes (LEDs). Structures were grown by metalorganic vapor phase epitaxy (MOVPE) on (0001) sapphire substrates using three-dimensional (3D) facetted GaN nucleation islands. The flattening of the overgrowing AlGaN buffer layers could be controlled by choosing appropriate growth conditions resulting in smooth surfaces. High-resolution X-ray diffraction (HRXRD) ω-scans show that a prolonged 3D growth phase leads to a narrowing of the asymmetric diffraction peaks and hence to an effective reduction of the density of edge-type threading dislocations. Photoluminescence (PL) and electroluminescence (EL) measurements show directly the beneficial effect of the improved crystal quality on the optical emission properties. The output power of LED structures grown on an optimized buffer was increased by a factor of 6 compared to structures grown on a two-dimensional (2D) low Al content AlGaN nucleation layer.