Ai Yanagihara

Directional radiation beam from yellow-emitting InGaN-based nanocolumn LEDs with ordered bottom-up nanocolumn array

Yellow-emitting (572 nm) InGaN-based nanocolumn LEDs, consisting of orderly arranged bottom-up nanocolumns in a triangular lattice, were fabricated. We observed a spectral linewidth of 37.3 nm at 226 A/cm2, which was exceptionally narrow for yellow emission. The periodic arrangement of nanocolumns leads to the photonic crystal effect. We measured the angular dependence of the emission spectrum, using which the photonic band diagram was experimentally traced, evincing that the yellow light was diffracted at the photonic band edge. As a result, a highly directional radiation beam with a radiation angle of ±20° suitable for display applications was successfully demonstrated.

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.

GaN nanocolumn light-emitters, growth, and optical characterization

Selective area growth of GaN nanocolumn arrays on Si substrates was developed. Orange-emitting nanocolumns on GaN/Al2O3 templates were optically characterized, fabricating red-color InGaN-based nanocolumn LEDs. Successful monolithic integration of four emission-colors nanocolumn LEDs was demonstrated.

Effect of structural properties on optical characteristics of InGaN/GaN nanocolumns fabricated by selective-area growth

The effect of the structural properties on the optical characteristics was investigated for In0.3Ga0.7N nanocolumns (NCs) grown on GaN NCs as a function of GaN column diameter, D GaN. With increasing D GaN, the photoluminescence spectra changed from single-peak to double-peak emissions at the diameter D 0 where InGaN axial NCs change to InGaN–InGaN core–shell NCs. For the core–shell NCs, the volume recombination probabilities of the InGaN cores did not change with D GaN. Whereas the surface recombination probability of the InGaN cores exponentially decreased because of the spontaneous formation of InGaN shells for D GaN > D 0, it drastically increased for D GaN ≤ D 0.

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.

Emission Characteristics Based on Nanocolumn Photonic Crystal Effect of Orderly Arrayed InGaN/GaN Nanocolumns

In this paper, we discuss the emission characteristics based on the nanocolumn photonic crystal effect. The GaN nanocolumn arrays, at the top region of which InGaN/GaN MQWs were integrated, were optically pumped resulting in lasing oscillation, as shown in Fig. 2. Clear thresholds were observed in the light intensity vs. excitation density characteristics (see Fig.2 (c)). The lasing wavelength changed from 492.3 to 501.1 nm with increasing the column diameter (D) from 183 to 205 nm at the lattice constant (L) of 245 nm, as the result of the photonic band edge shift. Figure 3 shows the photonic band diagram of a nanocolumn system; note that the experimental lasing wavelength corresponded to the photonic band edge at Γ11. The normalized band edge wavelength L/λ is expressed as a function of the structural parameter of D/L 13) as shown in Fig. 4 (a), where the experimental lasing wavelength are plotted by circles.