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In this study, the growth mechanisms of n-type gallium nitride (GaN) cap layers for embedding nanowire-based multi-quantum-shell (NW-MQS) with a tunnel junction were investigated using the metal–organic vapor-phase epitaxy method. Herein, instead of a p-type GaN layer, n-type GaN cap layers were applied as a contact layer to the anode electrode via a tunnel junction. NW-MQSs were prepared in a rectangular grid arrangement aligning with the m- and a-axis of GaN. The growth mode of the cap layers was controlled in three stages using different growth pressures and temperatures. The first cap layer was a faceted growth mode primarily comprising the r-plane, the second was a lateral growth mode that expanded the area of the c-plane, and the third was an acceleration of lateral growth. Finally, cap layers with a flat surface were realized. Void formation in the cap layers occurred only between the NW-MQSs along the m-axis of GaN. This observation can be attributed to the difference in the coalescence of the cap layers in the a- and m-axis directions. We believe that the NW-MQS structure with the cap layer that was optimized in this study can be used as a highly efficient optical device.

In this study, we discuss the influence of SiH4 flow rates on the structural and optical properties of GaN nanowires (NWs) with multiple-quantum-shells (MQSs). To this end, we prepared two n-GaN core NW samples with different SiH4 flow rates. Subsequently, MQS active layers of the same structure were grown on each n-GaN core NW under identical growth conditions. The samples were characterized by scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray, and cathodoluminescence (CL) mapping. From the experimental results, we ascertained that a Si-rich layer was created between the sidewall of the NWs and MQSs, in which the number of Si atoms was mainly determined by the SiH4 flow rate. These Si atoms diffused into the MQSs during the growth, and significantly impacted the structural and optical properties, such as the shape and crystalline quality of the MQSs and NWs, and the CL intensity of the MQSs. On the basis of experimental results, we conclude that the SiH4 flow rate during the NW growth plays a critical role in the performance of an MQS-based optoelectronic semiconductor device.

Embedded growth of an n-GaN cap layer on multi-quantum shells (MQSs) and nanowires through a tunnel junction (TJ) was investigated for the improvement of current injection to the m-plane of the MQSs. Different growth conditions for an n-GaN cap layer were systematically studied to suppress Mg diffusion and void formation, which are serious problems in this structure. At a high temperature of 900 °C and above, the growth rate on a semi-polar r-plane decreased, and large voids were formed at the bottom part of the nanowires owing to the diffusion of Ga atoms from the r-plane to the m-plane. When the growth temperature decreased to 800 °C, the growth rate on both the m-plane and the r-plane increased, and the size of voids decreased. Simultaneously, Mg diffusion also disappeared because of the low growth temperature of 800 °C. It was found that the growth with an extremely low V/III ratio of 20 at low temperatures increases the lateral growth rate on the m-plane, and void formation is fully suppressed when the MQS height is 700 nm or less.