Improved quantum efficiency of 350 nm LEDs grown on low dislocation density AlGaN buffer layers

The effect of buffer layers with reduced defect density on the efficiency of AlGaN based UV LED structures has been investigated. We report on two approaches of defect reduction: Firstly, AlGaN was grown on thin GaN nucleation islands which exhibit a three-dimensional facetted structure (3D nucleation). Secondly, AlGaN buffer layers with in-situ SiNx nano-masking interlayers inserted were employed. Both approaches result in reduced widths of asymmetric high-resolution X-ray diffraction ω-scan peaks. Full LED structures were grown on these buffer layers and are compared to structures grown on a purely 2D grown low Al-content AlGaN nucleation layer. The influence of the different buffer layer technologies, and hence defect density, on the quantum efficiency has been investigated by excitation-density and temperature dependent photoluminescence. The data indicate an increase in the PL efficiency from below 5% for 2D nucleation to 15% for 3D nucleation and up to 50% for structures grown on SiNx interlayer buffers. These results are in line with electroluminescence measurements taken on-wafer. The output power at 40 mA emitted through the substrate for the 3D nucleated LED structure is 0.3 mW which corresponds to an increase by a factor of 9 compared to similar LED structures grown in 2D mode. The same LED structure grown on an AlGaN buffer containing SiNx interlayers showed a 30× enhancement in efficiency with an output power of 0.51 mW at 20 mA, rising linearly to 1.05 mW for a current of 40 mA (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)