Shinya Nunoue

Room Temperature Pulsed Operation of Nitride Based Multi-Quantum-Well Laser Diodes with Cleaved Facets on Conventional C-Face Sapphire Substrates

We demonstrate room temperature pulsed operation of nitride based multi-quantum-well (MQW) laser diodes with cleaved mirror facets grown on a conventional C-face sapphire substrate. Cleavage was performed along the direction of the sapphire substrate, and the resultant facet was analyzed using an atomic force microscope (AFM) and theoretical calculation. A single peak emisson, at a wavelength of 417.5 nm, with a full width at half-maximum of 0.15 nm, was obtained. The threshold current density of the laser was 50 kA/cm2 and a voltage for the threshold current was 20 V.

Development of InGaN-based red LED grown on (0001) polar surface

We report on the optical properties of the InGaN-based red LED grown on a c-plane sapphire substrate. Blue emission due to phase separation was successfully reduced in the red LED with an active layer consisting of 4-period InGaN multiple quantum wells embedding an AlGaN interlayer with the Al content of 90% on each quantum well. The light output power and external quantum efficiency at a dc current of 20 mA were 1.1 mW and 2.9% with the wavelength of 629 nm, respectively. This is the first demonstration of a nitride-based red LED with the light output power exceeding 1 mW at 20 mA.

Luminescence Properties of Eu2+-Doped Green-Emitting Sr-Sialon Phosphor and Its Application to White Light-Emitting Diodes

We developed a green-emitting phosphor Sr3Si13Al3O2N21:Eu2+ that is highly luminescent under excitation by blue light. It shows a highly efficient green luminescence whose external quantum efficiency reaches 67% for 460 nm excitation, and has small thermal quenching. Using this phosphor, we obtained white light-emitting diodes (LEDs) whose luminous efficacy and color rendering index Ra at 5330 K are 62 lm/W and 87, respectively. These features show that this green-emitting phosphor has high potential for application to white LEDs.

Spontaneous Emission Enhancement in Pillar-Type Microcavities

A reduction in photoluminescence decay time of pillar-type microcavities was observed with decreasing the detuning between the resonance wavelength of the cavities and the luminescence peak wavelength of the active layers. The radiative recombination time was extracted from the measured photoluminescence decay time by a diffusion equation analysis. The obtained spontaneous emission enhancement factor was between 1.6 and 1.8. This is the first observation of a spontaneous emission enhancement in three-dimensionally confined semiconductor microcavities.

THERMAL ANALYSIS FOR GAN LASER DIODES

Improvement in temperature characteristics of GaN laser diodes is important for realizing reliable devices operating at high temperatures. Thermal characteristics of GaN lasers have been analyzed by using a three-dimensional thermal conduction model. Maximum operation temperature has also been calculated. It was shown that the reduction in the stripe width as well as the reduction in threshold current density and operation voltage is very important for improving the temperature characteristics of GaN laser diodes.

A revised Kubelka–Munk theory for spectral simulation of phosphor-based white light-emitting diodes

We developed a simulation method for designing the luminescence profiles of white light-emitting diodes (LEDs) with multicolor phosphor blend. By taking into account the effect of the light scattering by phosphor particles as well as the reabsorption effect by phosphor species, we found a model of white LEDs to calculate the light output as a function of various parameters such as the thickness of the resins, the concentration of phosphors, and the spatial distribution of phosphor particles. This method is widely applicable to phosphor-based white LEDs with either blue or UV excitation.

Effect of metal type on the contacts to n-type and p-type GaN

Abstract The electrical properties of various metal contacts to both n-type and p-type GaN were investigated to determine the underlying trend between the metallic contact workfunction and the resultant Schottky barrier height between the said contact and the GaN material. It is concluded that, contrary to the expected trend, Fermi level pinning is not only present but is quite strong, with the effect being greater in the p-type material. The S factor (the index of interface behaviour) was seen to be reduced from 1.00 to 0.21 and 0.01 for n and p-type GaN, respectively.

Device Simulator for Designing High-Efficiency Light-Emitting Diodes

A general device simulator has been constructed for designing high-efficiency light-emitting diodes (LEDs). The optical characteristics, including the spatial profiles of the light extracted from LEDs, are important for practical LED applications such as display and illumination. The radiation patterns are affected by device structures such as electrodes, which determine the current distribution, and by the light-absorbing layers, which intercept the internal light emission. The simulator analyzes light extraction efficiency and the radiation pattern, both of which are very important in designing an optical system using LEDs. The simulator is useful for designing high-efficiency LEDs and package configurations for various applications.

Dry etching of InGaAsP/InP structures by reactive ion beam etching using chlorine and argon

Very smooth InGaAsP/InP surfaces and sidewalls were obtained by using the enhanced sputtering effect of Ar addition to Cl2 etching gas in RIBE. The substrate temperature was raised to 180°C in order to remove involatile chlorides and impurities. Moreover, a vertical sidewall with no lateral etching under the mask was realized at the same time. This enables submicron dry etching for heterostructure devices.

Numerical Simulation on Light Output of UV-based White Light-Emitting Diodes with Multicolor Phosphor Blends

We developed a new simulation method for designing the luminescence profiles of phophor-based white light-emitting diodes (LEDs). By combining the rate equations for absorption/emission processes by phosphors with a differential equation for spatial distribution of light intensity, we take into account the cascade process of phosphor emission due to the reabsorption of photons. We found that our model is suitable for a systematic design method of white light sources with multicolor phosphor blends.