Takao Oto

High-Optical-Power InGan Superluminescent Diodes with "j-shape" Waveguide

We demonstrate blue-violet InGaN superluminescent diodes (SLDs) with bent-waveguide (j-shape) geometry, emitting optical power exceeding 200 mW for SLD with 1-mm-long waveguide. The adopted j-shape geometry prevents devices from lasing, even at the highest applied currents (450 mA). However, high-resolution measurements reveal the appearance of spectral ripples at currents on the order of 250 mA. The maximum output power increases with the waveguide length.

Co-existence of a few and sub micron inhomogeneities in Al-rich AlGaN/AlN quantum wells

Inhomogeneity in Al-rich AlGaN/AlN quantum wells is directly observed using our custom-built confocal microscopy photoluminescence (μ-PL) apparatus with a reflective system. The μ-PL system can reach the AlN bandgap in the deep ultra-violet spectral range with a spatial resolution of 1.8 μm. In addition, cathodoluminescence (CL) measurements with a higher spatial resolution of about 100 nm are performed. A comparison of the μ-PL and CL measurements reveals that inhomogeneities, which have different spatial distributions of a few- and sub-micron scales that are superimposed, play key roles in determining the optical properties.

Carrier-density dependence of photoluminescence from localized states in InGaN/GaN quantum wells in nanocolumns and a thin film

We have measured and analyzed the carrier-density dependence of photoluminescence (PL) spectra and the PL efficiency of InGaN/GaN multiple quantum wells in nanocolumns and in a thin film over a wide excitation range. The localized states parameters, such as the tailing parameter, density and size of the localized states, and the mobility edge density are estimated. The spectral change and reduction of PL efficiency are explained by filling of the localized states and population into the extended states around the mobility edge density. We have also found that the nanocolumns have a narrower distribution of the localized states and a higher PL efficiency than those of the film sample although the In composition of the nanocolumns is higher than that of the film.

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.

Optical gain characteristics in Al-rich AlGaN/AlN quantum wells

The optical gain characteristics of Al-rich AlGaN/AlN quantum wells (QWs) were assessed by the variable stripe length method at room temperature. An Al0.79Ga0.21N/AlN QW with a well width of 5 nm had a large optical gain of 140 cm−1. Increasing the excitation length induced a redshift due to the gain consumption and the consequent saturation of the amplified spontaneous emission. Moreover, a change in the dominant gain polarization with Al composition, which was attributed to switching of the valence band ordering of strained AlGaN/AlN QWs at Al compositions of ∼0.8, was experimentally demonstrated.

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.

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.

Emission color control for densely packed InGaN-based nanocolumns and demonstration of independent drive of multicolor (RGBY) micro-LED array (Conference Presentation)

The monolithic integration of InGaN-based multi-color light-emitting diodes (LEDs) exerts a great impact on the full-color application field. The two dimensional arrangement of three primary colors micro-LED is expected to be used as a semiconductor video panel [1]. The emission color of InGaN/GaN triangular latticed nanocolumn arrays with the same lattice constant (L) is controlled by the nanocolumn diameter (D) [2]. For a blue light emission, the narrow nanocolumns should be utilized, resulting in a low filling factor of nanocolumns. However, the use of high filling factor of nanocolumn system is suitable for a stable device fabrication. In this study, therefore, we fabricated the densely-packed regularly arranged InGaN nanocolumns (D/L > 0.9). For the nanocolumn arrays with high filling factors, it was found that the emission color shifted from blue to red with increasing L from 80 to 350 nm. The emission color change is attributed to the different mechanism from Ref. [2], which is investigated to be clarified. Using the emission color change for the high filling factor nanocolumn system, four-color (red, green, blue, and yellow; RGBY) micro light emitting diodes (LEDs) were integrated in a 20×20 μm2 area (hereinafter called “unit”) and the units were two-dimensionally arrayed in a 16×16 square lattice in 400×400 mm2 area. These nanocolumn micro-LEDs were independently driven using matrix wiring electrodes, exhibiting RGBY light emissions. [1] K. Kishino et al., Appl. Phys. Express 6, 012101 (2013). [2] H. Sekiguchi et al., Appl. Phys. Lett 96, 231104 (2008).

Enhancement of light emission and internal quantum efficiency in orange and red regions for regularly arrayed InGaN/GaN nanocolumns due to surface plasmon coupling

We demonstrate enhanced light emission in the orange and red regions from regularly arrayed InGaN/GaN nanocolumns due to the surface plasmon (SP) coupling. A maximum photoluminescence (PL) enhancement ratio of 5.2 is observed by coating the nanocolumns with an Au thin film. In addition, a 2.1-fold increase in the internal quantum efficiency is obtained. Comparison of an electromagnetic field simulation and a theoretical calculation based on the SP dispersion indicates that the SP originates from a standing wave mode arising from the periodic Au/dielectric interface. The column-diameter dependence of the PL enhancement ratio can be reasonably explained by considering the simulated electric field intensity. The periodic plasmonic nanostructure is effective for improving the emission efficiencies of InGaN-based light emitters in the orange and red regions.