Yohei Enya

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.

\{20\bar{2}1\}

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

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.

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

Carrier/exciton localization in InGaN quantum wells (QWs) for green laser diodes fabricated on semi-polar {2021} GaN substrates is assessed using time-resolved photoluminescence (TRPL) spectroscopy. The estimated characteristic energy, which represents the localization depth in a {2021} InGaN QW, is 15.1 meV. This value is much smaller than that reported for c-plane green InGaN QWs, indicating a high compositional homogeneity of {2021} InGaN QWs and consequently suggesting that the GaN semi-polar {2021} plane is suitable for fabricating green laser diodes.

Long-Lifetime True Green Laser Diodes with Output Power over 50 mW above 525 nm Grown on Semipolar \{20\bar{2}1\} GaN Substrates

Nanoscopic photoluminescence (PL) properties of a green-emitting {2021} InGaN single quantum well (SQW) are investigated by scanning near-field optical microscopy (SNOM). Carrier/exciton diffusion outside the probe aperture of 150 nm is demonstrated by a multimode SNOM technique. The estimated diffusion lengths are ~70 nm along the [1014] direction and ~50 nm along the [1210] direction, and are in between those of (1122) and (0001) InGaN QWs. This finding is well accounted for by the difference in carrier/exciton lifetimes. Furthermore, atomic force microscopy (AFM) reveals ridge structures along the [1210] direction. Superimposing the SNOM-PL image with an AFM image, we find a clear correlation between the spatial distributions of PL peak wavelength and surface morphology.