Katsushi Akita

531 nm Green Lasing of InGaN Based Laser Diodes on Semi-Polar {2021} Free-Standing GaN Substrates

Lasing in pure green region around 520 nm of InGaN based laser diodes (LDs) on semi-polar {2021} free-standing GaN substrates was demonstrated under pulsed operation at room temperature. The longest lasing wavelength reached to 531 nm and typical threshold current density was 8.2 kA/cm2 for 520 nm LDs. Utilization of a novel {2021} plane enabled a fabrication of homogeneous InGaN quantum wells (QWs) even at high In composition, which is exhibited with narrower spectral widths of spontaneous emission from LDs than those on other planes. The high quality InGaN QWs on the {2021} plane advanced the realization of the green LDs.

Continuous-Wave Operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {2021} GaN Substrates

Room-temperature continuous-wave operation of 520 nm InGaN-based green laser diodes on semi-polar {2021} GaN substrates was demonstrated. A threshold current of 95 mA corresponding to a threshold current density of 7.9 kA/cm2 and a threshold voltage of 9.4 V were achieved by improving the quality of epitaxial layers on {2021} GaN substrates using lattice-matched quaternary InAlGaN cladding layers and also by adopting a ridge-waveguide laser structure.

Improvements of external quantum efficiency of InGaN-based blue light-emitting diodes at high current density using GaN substrates

InGaN-based blue light-emitting diodes (LEDs) with different quantum well (QW) thicknesses were grown on freestanding GaN substrates with low threading dislocation densities (TDDs) and on c-plane sapphire substrates. In the case of thin QWs of 3nm in thickness, the external quantum efficiencies (EQEs) of LEDs on GaN substrates, as well as those on sapphire substrates, decreased with increasing forward current, indicating that carrier localization is in play on both types of substrates. For thicker 5-nm-thick QWs, the EQEs of LEDs grown on GaN substrates improved at high current densities, while those on sapphire substrates decreased even at low current densities. The LED with 5-nm-thick QWs on the GaN substrate mounted p-side down and molded with epoxy showed EQE as high as 26% at 125A∕cm2. Cathodoluminescence observations of the active layers on GaN substrates revealed that the expansion of nonradiative areas related to TDDs, which are responsible for the deterioration of the EQE of the LED on the sapphi...

High Al Composition AlGaN-Channel High-Electron-Mobility Transistor on AlN Substrate

AlGaN-channel high-electron-mobility transistor (HEMT) with high Al composition of 0.51 has been developed. The epitaxial layers were grown on a free-standing AlN substrate to improve crystalline quality. The fabricated device exhibited a maximum drain current (Idsmax) of 25.2 mA/mm with a maximum transconductance (gmmax) of 4.7 mS/mm. The characteristic features of the device were a high source-to-drain breakdown voltage of 1800 V and a high applicable gate-to-source voltage of 4 V in the forward direction. Temperature dependence of DC characteristics demonstrated that the drain current degradation at elevated temperatures for the AlGaN-channel HEMT was appreciably small as compared with the conventional AlGaN/GaN HEMT. This is the first report showing successful DC operation of AlGaN-channel HEMT with high Al composition of over 0.5.

High-Efficiency 352 nm Quaternary InAlGaN-Based Ultraviolet Light-Emitting Diodes Grown on GaN Substrates

We demonstrated high-efficiency ultraviolet (UV) light-emitting diodes (LEDs) with an emission wavelength of 352 nm using a quaternary InAlGaN multiple-quantum-well (MQW) emitting region. GaN substrates were used in order to eliminate the effects of threading dislocations. Emission fluctuation due to In segregation was clearly observed even in a dislocation-free area by a cathode luminescence (CL) image of a quaternary InAlGaN MQW, which is considered to contribute to the achievement of a high-efficiency UV emission. The maximum UV output power obtained was as high as 7.4 mW under room temperature (RT) CW operation. The maximum external quantum efficiency (EQE) was 1.1% with an injection current of 50 mA, which is the highest EQE ever obtained for 350-nm-band UV LEDs with a top-emission geometry.

Effects of In composition on ultraviolet emission efficiency in quaternary InAlGaN light-emitting diodes on freestanding GaN substrates and sapphire substrates

350-nm-band quaternary InAlGaN quantum-well (QW) structures have been fabricated on low-dislocation-density GaN substrates and sapphire substrates, and the impact of the In composition on ultraviolet emission efficiency has been investigated. Photoluminescence, cathodoluminescence, and atomic force microscopy measurements have revealed that with increasing In concentration, band-gap inhomogeneity in InAlGaN layers is promoted and quantum efficiency becomes less sensitive to dislocation densities. However, this does not necessarily result in the enhancement of QW emission efficiency, because both a decrease in growth temperature and an increase in Al concentration degrade the InAlGaN crystalline quality. It is found that improving the InAlGaN crystalline quality by increasing the growth temperature while maintaining an In fluctuation effect with minimal In composition is essential for highly efficient InAlGaN QWs. The utilization of the optimized InAlGaN growth condition significantly improves the output p...

Influence of residual oxygen impurity in quaternary InAlGaN multiple-quantum-well active layers on emission efficiency of ultraviolet light-emitting diodes on GaN substrates

The influence of residual impurities in quaternary InAlGaN active layers on the emission efficiency of 350nm band ultraviolet light-emitting diodes on GaN substrates has been investigated. Secondary ion mass spectrometry and capacitance-voltage measurements have revealed that a large amount of oxygen is incorporated into the InAlGaN active layers owing to relatively low growth temperatures for Al-content epitaxial films. The increase of the InAlGaN growth temperature from 780to830°C results in both the residual oxygen level decrease from 1×1018to4×1017cm−3 and the output power improvement from 0.6to2.6mW at 100mA. It is also found that devices containing lower oxygen concentration in the InAlGaN active layers demonstrate a higher electroluminescence intensity ratio of the band-edge emission from the well layers to the donor-acceptor-pair emission from the p-type layers at low temperatures. These experimental results, in conjunction with numerical calculations, suggest that the reduction in the oxygen impu...

Advantages of gan substrates in InAlGan quaternary ultraviolet-light-emitting diodes

We fabricated InAlGaN quaternary ultraviolet (UV)-light-emitting diodes (LEDs) on GaN substrates with a low dislocation density and on GaN templates consisting of n-GaN on sapphire substrates, and compared the characteristics of these LEDs. A UV LED on a GaN substrate showed a considerably higher output power than that on a GaN template and no saturation even at an injection current of 500 mA. Cathodeluminescence images indicated the advantages of using GaN substrates in InAlGaN UV LEDs.

MOVPE grown InGaAs/GaAsSb type II quantum well photodiode for SWIR focal plane array

Infrared sensors with type II quantum well structure have gained great attention and have shown advanced progress. InGaAs/GaAsSb type II quantum well structures are considered as an attractive material system for realizing low dark current PDs owing to lattice-matching to InP substrate. In this report, we describe successful operation of PIN-PDs with InGaAs/GaAsSb quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). MOVPE method is well-known to have good uniformity which leads to mass-production of focal plane array. Planer type pin-PDs were adopted. The p-n junction was formed in the absorption layer by the selective diffusion of zinc. Electrical and optical characteristics of pin-PDs such as well number dependence of responsivity, were investigated. Dark current was 9.0 μA/cm2 at 233 K, which has better uniformity compared to those of MBE sample, and responsivity of 0.8 A/W in SWIR region were obtained. This result indicates that planer photodiode using MOVPE grown InGaAs/GaAsSb type II quantum wells is a promising candidate for consumer applications.

Low-Resistive Ohmic Contacts for AlGaN Channel High-Electron-Mobility Transistors Using Zr/Al/Mo/Au Metal Stack

An AlxGa1-xN/AlyGa1-yN high-electron-mobility transistor (HEMT) with AlGaN as a channel layer has been fabricated on a sapphire substrate for high-output-power and high-frequency electronic applications. One of the key process steps for the AlGaN-channel HEMT is to ensure low resistivity for source/drain ohmic contacts. In this work, the electrical characteristics of Zr/Al/Mo/Au ohmic contacts for AlGaN-channel HEMTs were investigated at annealing temperatures from 850 to 1000 °C. An AlGaN-channel HEMT was fabricated with Al contents of 0.3 and 0.55 for the channel and barrier layer, respectively. A minimum ohmic contact resistivity of 2.6×10-4 Ω cm2 was achieved for the Al0.55Ga0.45N/Al0.3Ga0.7N heterostructure after annealing at 950 °C.