Yusuke Yoshizumi

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.

High-Power (over 100 mW) Green Laser Diodes on Semipolar

Continuous-wave operation of InGaN green laser diodes (LDs) on semipolar {2021} GaN substrates with output powers of over 100 mW in the spectral region beyond 530 nm is demonstrated. Wall plug efficiencies (WPEs) as high as 7.0–8.9% are realized in the wavelength range of 525–532 nm, which exceed those reported for c-plane LDs. The longest lasing wavelength has reached 536.6 nm under cw operation. These results suggest that the InGaN green LDs on the {2021} plane are better suited as light sources for applications requiring wavelengths over 525 nm.

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Green laser diodes (LDs) on the {2021} plane exhibit lower threshold current densities, nearly half of those on the c-plane in the green region between 520–530 nm. The threshold current of a typical {2021} green LD lasing at 525.5 nm under room temperature cw operation is 51.1 mA, which corresponds to a threshold current density of 4.3 kA/cm2. The threshold voltage is 6.38 V. The characteristics temperature T0 is measured to be 175 K. The perpendicular θ⊥ and parallel θ|| beam divergence angles at half power of the {2021} green LDs are 24 and 11°, respectively. From the viewpoint of the device characteristics, especially the threshold current density, we conclude that the green LDs on the {2021} plane GaN substrates have the essential advantage for obtaining efficient green LDs.

GaN Substrates Operating at Wavelengths beyond 530 nm

The polarization characteristics of InGaN quantum wells on semi-polar {2021} GaN substrates were investigated to reveal the advantageous laser stripe orientation. The polarization ratio exhibited a gradual increase within the range of around 0.2 to 0.3 with an increase in the emission wavelength from 400 to 550 nm under a low current density of 7.4 A/cm2. In addition, the dependence on a current density was quite small up to 0.74 kA/cm2. These results suggest that the laser stripe perpendicular to the a-axis direction is favorable for green laser diodes.

Low Threshold Current Density InGaN Based 520–530 nm Green Laser Diodes on Semi-Polar {2021} Free-Standing 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...

Optical Polarization Characteristics of InGaN Quantum Wells for Green Laser Diodes on Semi-Polar {2021} GaN Substrates

True green GaInN laser diodes with a lasing wavelength above 525 nm under continuous wave operation have been successfully fabricated on semipolar {2021} GaN substrates by improving both the diode structure and epitaxial growth conditions. At a case temperature of 55 °C, their lifetime was estimated to be over 5000 h for an optical output power of 50 mW and over 2000 h at 70 mW.

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

Vertical GaN Schottky barrier diodes (SBDs) were fabricated on free-standing GaN substrates with low dislocation density. Vertical GaN-SBDs with a forward current of 5A and a blocking voltage of 600V exhibit the most superior reverse recovery characteristics among GaN, SiC, and Si diodes. We also confirmed stable forward and reverse aging characteristics for 1000 hours at 150 °C.