An impressive enhancement of cathodoluminescence was achieved in coaxial GaInN/GaN multiple-quantum-shells nanowires by employing an AlGaN undershell for trapping point defects.

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The superior crystalline quality of coaxial GaInN/GaN multiple-quantum shell (MQS) nanowires (NWs) was demonstrated by employing an AlGaN undershell during metal-organic chemical vapor deposition. Scanning transmission electron microscopy (STEM) results reveal that the NW structure consists of distinct GaInN/GaN regions on different positions of the NWs and the cores were dislocation-free. High-resolution atomic contrast STEM images verified the importance of AlGaN undershells in trapping the point defects diffused from n-core to MQSs (m-planes), as well as the improvement of the grown crystal quality on the apex region (c-planes). Time-integrated and time-resolved photoluminescence (PL) measurements were performed to clarify the mechanism of the emission within the coaxial GaInN/GaN MQS NWs. The improved internal quantum efficiency in the NW sample was attributed to the unique AlGaN undershell, which was able to suppress the point defects diffusion and reduce the dislocation densities on c-planes. Carrier lifetimes of 2.19 ns and 8.44 ns were derived from time-resolved PL decay curves for NW samples without and with the AlGaN undershell, respectively. Hence, the use of an AlGaN undershell exhibits promising improvement of optical properties for NW-based white and micro light-emitting diodes.

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A multi-quantum-shell (MQS) grown on a GaN nanowire is a promising three-dimensional active region and it is expected to show excellent performance, compared with conventional nitride-based LEDs. However, there are no suitable simulators for calculating optical properties of MQS-LEDs, because of their complex structure. In this study, a hybrid simulation, which is composed of the finite-difference time-domain method, the rigorous coupled wave analysis method, and the ray tracing method, is developed. Applying this useful tool to the calculation of the light extraction efficiency (LEE) of MQS-LEDs, we have found considerable light absorption loss by the large refractive index steps between the active layer, ITO layer and air in the commonly used MQS-LED structure with the ITO electrode. Thus, to eliminate the large refractive index steps, the MQS-LED buried with the n-GaN current diffusion layer, which has a high LEE, was proposed.