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Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

Coaxial GaInN/GaN multiple-quantum-shells (MQSs) nanowires (NWs) were grown on an n-type GaN/sapphire template employing selective growth by metal-organic chemical vapour deposition (MOCVD). To improve the cathodoluminescence (CL) emission intensity, an AlGaN shell was grown underneath the MQS active structures. By controlling the growth temperature and duration, an impressive and up to 11-fold enhancement of CL intensity is achieved at the top area of the GaInN/GaN MQS NWs. The spatial distribution of Al composition in the AlGaN undershell was assessed as a function of position along the NW and analysed by energy-dispersive X-ray measurement and CL characterisation. By introducing an AlGaN shell underneath GaInN/GaN MQS, the diffusion of point defects from the n-core to MQS is effectively suppressed because of the lower formation energy of vacancies-complexes in AlGaN in comparison to GaN. Moreover, the spatial distribution of Al and In was attributed to the insufficient delivery of gas precursors to the bottom of the NWs and the anisotropy diffusion on the nonpolar m-planes. This investigation can shed light on the effect of the AlGaN undershell on improving the emission efficiency of NW-based white and micro-light-emitting diodes (LEDs).

GaInN/GaN multiple quantum shells (MQS) nanowires and p-GaN shells were embedded with n-GaN layers through tunnel junction (TJ) shells using metalorganic chemical vapor deposition (MOCVD) method. The MQS nanowires were selectively grown on n-GaN/sapphire or GaN substrates. The fabrication process of laser structures with different resonators of 600500, 750, 1000 μm, and cavity widths of 7, 12, and 17 μm were investigated with insulating layer on the sidewalls of the ridge. The structures of the fabricated devices were characterized by scanning electron microscope (SEM) and current-voltage-light output characteristics were evaluated. Two different methods for mirror formation, etching and cleavage, were developed for the laser devices. During the investigation, a superior mirror formation suffered from the difference in etching rate between GaInN and GaN, generating concaves in the MQS region. Bluegreen light emission was observed from the entire ridge surface of the MQS index-guided laser structures. A maximum current density of emission at 17.9 kA/cm2 has been confirmed in the devices. The electroluminescence and cathodoluminescence measurements demonstrated that the r-plane and c-plane at the top of the MQS are dominant at low current densities, and the m-plane emission becomes stronger as the current density increases.

A tunnel junction and a n-GaN cap layer grown on the multi-quantum shells (MQS) /nanowires are introduced to decrease the resistivity and optical loss. The selective-area growth of the MQS/nanowire core-shell structures on the template was performed by metalorganic vapour phase epitaxy (MOVPE). Further, the MQS structure was covered with the tunnel junction and the n-GaN cap layer. Here, the growth conditions of the n-GaN cap layer were systemically investigated. The effect of p-GaN shape on the morphology of grown n-GaN cap layer was also assessed.

The selective-monolithic growth of coaxial GaInN/GaN NWs was investigated by changing the TEG flow rate, barrier and well growth temperature during MQS growth. In incorporation increased with a higher TEG flow rate. However, In-rich flakes were formed the NWs resulting in the deterioration of crystal quality. Using a higher growth temperature of quantum barriers, abnormal growth at the top of NWs was eliminated. As a result, the CL emission intensity was enhanced. Furthermore, the occurrence of In desorption was suppressed by decreasing the growth temperature of quantum wells. Therefore, these results are promising for NW-based white LEDs.

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.

Tunable emission wavelengths from 415 nm to 600 nm were demonstrated in coaxial GaInN/GaN multiple quantum shells (MQS) grown on three-dimensional nanostructures. Uniform aperture arrays with different diameters and pitches were patterned on GaN/sapphire substrates by nanoimprint and e-beam lithography. Thereafter, the coaxial GaInN/GaN MQS structures were selectively grown on the nanostructures using metalorganic chemical vapor deposition (MOCVD). To achieve long wavelength emissions, a detailed investigation in terms of height, filling factor, diameter of apertures, and pitch was performed for clarifying the In incorporation in nanostructures. Scanning transmission electron microscopy (STEM) features demonstrate that the diffusion of precursors on SiO2 mask area plays an important role in the coaxially growth of MQS structures on naonopyramids. A growth model based on the diffusion length and pitch was proposed to explain the experimental results. It was clarified that the probability of In incorporation was promoted in nanopyramids with a small pitch (less than 3 times of the diffusion length), wherein direct impinged adatoms on nanopyramids also diffused and participated in the growth on the neighbor. With a decrease in the intervals of aperture patterns, emission longer than 600 nm can be achieved for white and micro light-emitting diodes.

Broadened emission was demonstrated in coaxial GaInN/GaN multiple quantum shell (MQS) nanowires that were monolithically grown by metalorganic chemical vapor deposition. The non-polar GaInN/GaN structures were coaxially grown on n-core nanowires with combinations of three different diameters and pitches. To broaden the emission band in these three nanowire patterns, we varied the triethylgallium (TEG) flow rate and the growth temperature of the quantum barriers and wells, and investigated their effects on the In incorporation rate during MQS growth. At higher TEG flow rates, the growth rate of MQS and the In incorporation rate were promoted, resulting in slightly higher cathodoluminescence (CL) intensity. An enhancement up to 2&ndash, 3 times of CL intensity was observed by escalating the growth temperature of the quantum barriers to 800 &deg, C. Furthermore, decreasing the growth temperature of the quantum wells redshifted the peak wavelength without reducing the MQS quality. Under the modified growth sequence, monolithically grown nanowires with a broaden emission was achieved. Moreover, it verified that reducing the filling factor (pitch) can further promote the In incorporation probability on the nanowires. Compared with the conventional film-based quantum well LEDs, the demonstrated monolithic coaxial GaInN/GaN nanowires are promising candidates for phosphor-free white and micro light-emitting diodes (LEDs).

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.

Nonpolar GaInN/GaN multiple-quantum shells (MQSs) on nanowires (NWs) were investigated for high-efficiency light-emitting diodes (LEDs). The growth conditions of NWs were systematically optimized via a continuous growth mode in metal-organic chemical vapour deposition (MOCVD). The In incorporation rate on the m-planes decreased as the growth temperature elaborated, whereas the crystalline quality is improved. The cathodoluminescence (CL) results revealed that longer growth time of the GaInN well can induce additional In-rich droplets and degrade the emission properties of MQSs. The CL emission intensity and the peak wavelength increased as the number of MQS pairs increased from one to three pairs, which was attributed to the increased In incorporation as the diameter enhanced. The linearly enhanced CL emission intensity with barrier thickness was ascribed to the increase of the electron-hole states from the GaN barrier to the wells, resulting in a larger recombination probability. The scanning transmission electron microscopy (STEM) results demonstrated that a thicker barrier shell can suppress the formation of In-rich droplets. Overall, the feasibility of obtaining high-quality m-plane coaxial GaInN/GaN MQSs structures are promising for NW-based white and micro LEDs.