Kenji Fujito

Optical properties of yellow light-emitting diodes grown on semipolar (112¯2) bulk GaN substrates

We demonstrate high power yellow InGaN single-quantum-well light-emitting diodes (LEDs) with a peak emission wavelength of 562.7nm grown on low extended defect density semipolar (112¯2) bulk GaN substrates by metal organic chemical vapor deposition. The output power and external quantum efficiency at drive currents of 20 and 200mA under pulsed operation (10% duty cycle) were 5.9mW, 13.4% and 29.2mW, 6.4%, respectively. It was observed that the temperature dependence of the output power of InGaN LEDs was significantly smaller than that of AlInGaP LEDs.

Demonstration of Nonpolar m-Plane InGaN/GaN Laser Diodes

The first nonpolar m-plane (1-100) nitride laser diodes (LDs) have been realized on low extended defect bulk m-plane GaN substrates. The LDs were grown by metal organic chemical vapor deposition (MOCVD) using conditions similar to that of c-plane device growth. Broad area lasers were fabricated and tested under pulsed conditions. Lasing was observed at duty cycles as high as 10%. These laser diodes had threshold current densities (Jth) as low as 7.5 kA/cm2. Stimulated emission was observed at 405.5 nm, with a spectral line-width of 1 nm.

Defect reduction in (11¯00) m-plane gallium nitride via lateral epitaxial overgrowth by hydride vapor phase epitaxy

This letter reports on extended defect density reduction in m-plane (11¯00) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase epitaxy. Several dielectric mask patterns were used to produce 10 to 100 μm-thick, partially and fully coalesced nonpolar GaN films. X-ray rocking curves indicated the films were free of wing tilt. Transmission electron microscopy showed that basal plane stacking fault (SF) and threading dislocation (TD) densities decreased from 105cm−1 and 109cm−2, respectively, less than 3×103cm−1 and ∼5×106cm−2, respectively, in the Ga-face (0001) wing of the LEO films. SFs persisted in ⟨0001⟩-oriented stripe LEO films, though TD reduction was observed in the windows and wings. Band-edge cathodoluminescence intensity increased 2 to 5 times in the wings compared to the windows depending on the stripe orientation. SFs in the low TD density wings of ⟨0001⟩-stripe films did not appear to act as nonradiative recombination centers.

High Brightness Blue InGaN/GaN Light Emitting Diode on Nonpolar m-plane Bulk GaN Substrate

Improved nonpolar m-plane (1100) light emitting diode (LED) with a thick InGaN active layer of 8 nm and a thick GaN barrier layer of 37.5 nm for multi-quantum-well (MQW) structure have been fabricated on low extended defect bulk m-plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence (EL) emission from the packaged LED was 468 nm. The output power and external quantum efficiency (EQE) were 8.9 mW and 16.8%, respectively, at a DC driving current of 20 mA.

High-Power Blue-Violet Semipolar (2021) InGaN/GaN Light-Emitting Diodes with Low Efficiency Droop at 200 A/cm2

We report a high-power blue light-emitting diode (LED) with a high external quantum efficiency and low droop on a free-standing (2021) GaN substrate. At a forward current of 20 mA, the LED showed a peak external quantum efficiency of 52% and an output power of 30.6 mW. In higher current density regions, the LED also showed outstanding performance, with droop ratios of 0.7% at 35 A/cm2, 4.3% at 50 A/cm2, 8.5% at 100 A/cm2, and 14.3% at 200 A/cm2. The output power and external quantum efficiency at 200 A/cm2 were 266.5 mW and 45.3%, respectively.

Partial strain relaxation via misfit dislocation generation at heterointerfaces in (Al,In)GaN epitaxial layers grown on semipolar (112¯2) GaN free standing substrates

Misfit strain relaxation via misfit dislocation (MD) generation was observed in heteroepitaxially grown (Al,In)GaN layers on free-standing semipolar (112¯2) GaN substrates. Cross-section transmission electron microscope images revealed MD arrays at alloy heterointerfaces, with the MD line direction and Burgers vector parallel to [11¯00] and [112¯0], respectively. The MD structure is consistent with plastic relaxation by dislocation glide on the (0001) plane. Since (0001) is the only slip plane, the plastic relaxation is associated with tilt of the epitaxial (Al,In)GaN layers. The tilt, measured via high-resolution x-ray diffraction, can be used to quantify the relaxation.

Indium incorporation and emission properties of nonpolar and semipolar InGaN quantum wells

We report indium incorporation properties on various nonpolar and semipolar free-standing GaN substrates. Electroluminescence characterization and x-ray diffraction (XRD) analysis indicate that the semipolar (202¯1¯) and (112¯2) planes have the highest indium incorporation rate among the studied planes. We also show that both indium composition and polarization-related electric fields impact the emission wavelength of the quantum wells (QWs). The different magnitudes and directions of the polarization-related electric fields for each orientation result in different potential profiles for the various semipolar and nonpolar QWs, leading to different emission wavelengths at a given indium composition.

High-Efficiency Single-Quantum-Well Green and Yellow-Green Light-Emitting Diodes on Semipolar (2021) GaN Substrates

We demonstrate high-efficiency green and yellow-green single-quantum-well light-emitting diodes (LEDs) grown on semipolar (2021) GaN substrates by metal organic chemical vapor deposition. The output power and external quantum efficiency at a driving current of 20 mA under a pulsed condition with a 10% duty cycle are 9.9 mW and 20.4% for the green LED and 5.7 mW and 12.6% for the yellow-green LED, respectively. The electroluminescence linewidth narrowing, which is related to the band-filling effect caused by potential fluctuations, is not observed.

Semipolar (1011) InGaN/GaN Laser Diodes on Bulk GaN Substrates

The first semipolar nitride laser diodes (LDs) have been realized on low extended defect density semipolar (1011) GaN bulk substrates. The LDs were grown by conventional metal organic chemical vapor deposition (MOCVD). Broad area lasers were fabricated and tested under pulsed conditions. Lasing was observed at a duty cycle of 0.025% with a threshold current density (Jth) of 18 kA/cm2. Stimulated emission was observed at 405.9 nm with a full width at half maximum (FWHM) of less than 0.3 nm.

Continuous-wave Operation of AlGaN-cladding-free Nonpolar m-Plane InGaN/GaN Laser Diodes

We demonstrate continuous-wave (CW) operation of nonpolar m-plane InGaN/GaN laser diodes without Al-containing waveguide cladding layers. Thick InGaN quantum wells (QWs) are used to generate effective transverse optical mode confinement, eliminating the need for Al-containing waveguide cladding layers. Peak output powers of more than 25 mW are demonstrated with threshold current densities and voltages of 6.8 kA/cm2 and 5.6 V, respectively. The unpackaged and uncoated laser diodes operated under CW conditions for more than 15 h.