Koji Katayama

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\}

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...

GaN Substrates Operating at Wavelengths beyond 530 nm

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

The degradation mechanism responsible for limiting the lifetime of ZnSe-based white light-emitting diodes was studied. A systematic investigation on the effects of pre-existing stacking faults and threading dislocations, along with point defects in the epitaxial layers was conducted by monitoring their behavior during an aging experiment. Our experimental results, in conjunction with numerical calculations performed on degraded device structures, suggest that the deterioration of the internal quantum efficiency owing to the reduction of the net acceptor concentration in the p-ZnMgSSe cladding layer is responsible for the decline of the optical output power. The reduction in the free hole concentration has an effect of increasing the electron leakage current, leading to a decrease in the carrier injection efficiency. It is also proposed that the carrier depletion effect in the p-cladding layer is due to self-compensation resulting from the increase in the concentration of N-related deep donors.

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

The role of N-related point defects in the degradation process of ZnSe-based white light-emitting diodes under operation was investigated. The generation rate of microscopic dark spots, which do not correspond to the original stacking faults or threading dislocations in the epilayer, was found to correspond to the initial concentration of the N-related deep-level H0 (ET−Ev=0.8±0.2eV) in the p-ZnMgSSe layer. Devices with high initial H0 concentrations resulting from thermal treatments, at the same time, showed a drastic reduction in their net acceptor concentrations. The origin of this effect is believed to be due to the deactivation of the N acceptor, rather than the hole-trapping effect of the H0 center.

Degradation mechanism limiting the lifetime of ZnSe-based white light-emitting diodes

531 nm InGaN-based true green LDs on semi-polar {2021} GaN substrates were demonstrated under pulsed operation. 520 nm cw lasing was also achieved by adopting a ridge-waveguide structure. The advantages of this particular semi-polar plane for green LDs will be discussed in the talk.