Jun Yoshida

Influence of GaN column diameter on structural properties for InGaN nanocolumns grown on top of GaN nanocolumns

The influence of GaN column diameter DGaN on structural properties was systematically investigated for InGaN nanocolumns (NCs) grown on top of GaN NCs. We demonstrated a large critical layer thickness of above 400 nm for In0.3Ga0.7N/GaN NCs. The structural properties were changed at the boundary of DGaN=D0 (∼120 nm). Homogeneous InGaN NCs grew axially on the GaN NCs with DGaN≤D0, while InGaN-InGaN core-shell structures were spontaneously formed on the GaN NCs with DGaN>D0. These results can be explained by a growth system that minimizes the total strain energy of the NCs.

Spatial emission distribution and carrier recombination dynamics in regularly arrayed InGaN/GaN quantum structure nanocolumns

Emission mechanisms in regularly arrayed InGaN/GaN quantum structures on GaN nanocolumns were investigated, focusing on the spatial emission distribution at the nanocolumn tops and the carrier recombination dynamics. The double-peak emission originated from the dot- and well-like InGaN areas with different In compositions was observed. From the results regarding the spatial emission distribution, we proposed a simple analytical approach to evaluating the carrier recombination dynamics using the rate equations based on the two energy states. The considerable six lifetimes can be uniquely determined from the experimental results. Carrier transfer from the high- to the low-energy state is dominant at high temperatures, producing the increased total emission efficiency of the inner low-energy area. In addition, the internal quantum efficiency should not be simply discussed using only the integrated intensity ratio between low and room temperatures because of the carrier transfer from high- to low-energy states.

Triangle-lattice InGaN/GaN nanocolumn arrays exhibiting photonic crystal effect (Conference Presentation)

GaN nanocolumns are extensively studied as promising nano-materials for high-performance visible emitters because of their dislocation filtering and strain relaxation effects. The size and position of nanocolumns were precisely controlled using Ti-mask selective-area growth (SAG) by RF-MBE, fabricating uniform arrays of pn-junction InGaN/GaN nanocolumns. The periodic arrangement in the nanocolumn arrays led to nanocolumn photonic crystal (PhC) effect. It is however, necessary to integrate a wave-guiding scheme in the nanocolumn system to activate efficiently the PhCs. In the experiment, triangle-lattice GaN nanocolumn arrays with the lattice constant from 280 to 350 nm were grown, followed by the growth of InGaN/GaN superlattice buffer, MQW, and p-type GaN cladding layers. In the upper region of pn-junction nanocolumns from SL to p-GaN, the nanocolumn diameter increased and introduced the increase in the equivalent refractive index, which acts to confine the optical field there. Thus, the optical mode propagated laterally, interacting with the nanocolumn PhC. The diffraction at the photonic band edge resulted in high-directional beam radiations from the nanocolumn system. The photonic band edge was systematically investigated for various nanocolumn arrays with L=280–250 nm. The experimental photonic band diagram for the triangular-lattice pn-junction InGaN/GaN nanocolumn array exhibited a clear photonic band edge.