Hisashi Yamada

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.

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.

Impact of Substrate Miscut on the Characteristic of m-plane InGaN/GaN Light Emitting Diodes

Effect of m-plane GaN substrate miscut was investigated for InGaN/GaN multi quantum wells (MQWs) light emitting diodes (LEDs). The n-GaN and InGaN/GaN LEDs were grown by metal organic chemical vapor deposition on m-plane (1100)GaN substrates with miscut angles toward the [0001] direction of 0.01 (nominally on-axis m-plane), 0.45, 0.75, 5.4, and 9.6°. The surface morphology improved with increasing the miscut angles toward the [0001] direction. The peak emission wavelength of the electroluminescence grown on the on-axis m-plane and the miscut angle of 0.45° was 391–396 nm, while the miscut angle of 5.4 and 9.6° showed 440–454 nm. These results demonstrate that the surface morphology of GaN and In incorporation in the MQWs are strongly impacted by the miscut angle of GaN substrate.

Optical polarization characteristics of InGaN∕GaN light-emitting diodes fabricated on GaN substrates oriented between (101¯0) and (101¯1¯) planes

Optical polarization characteristics of InGaN∕GaN light-emitting diodes (LEDs) were studied. Light-emitting diode samples were fabricated on four types of GaN substrates near (101¯0) orientation with intentional off-axis cuts of 0°, 5°, 10°, and 27° towards [0001¯]. A confocal microscope was used to characterize the optical polarization of electroluminescence at various currents. The highest polarization ratio of 0.91 was measured on samples fabricated on a 5° off-cut substrate. First moments were calculated on emission spectra to assess emission peak shifts of two polarization components. We drew a conclusion that substrate off-axis cut is a technique to improve optical polarization characteristics of nonpolar-oriented InGaN∕GaN LEDs.

Non‐polar‐oriented InGaN light‐emitting diodes for liquid‐crystal‐display backlighting

— This article addresses spontaneously polarized light emission from GaN-based light-emitting diodes (LEDs) fabricated on electrically non-polar crystallographic orientations and application of spontaneously polarized emission for backlighting of liquid-crystal displays (LCDs). The first half of the article describes polarized light emission from GaN-based LEDs and its role in solid-state lighting technology. The second half reports on our experimental work to explore the potential of non-polar LEDs for LCD backlighting applications. Optical transmission of non-polar LED emission was characterized through a liquid-crystal layer. Extinction ratios of 0.21 were measured between zero and an applied bias voltage to the liquid-crystal cells. These extinction ratios are not particularly high yet; nevertheless, the experiment has demonstrated the potential of such non-polar LEDs for LCD backlighting.

Thin metal intracavity contact and lateral current-distribution scheme for GaN-based vertical-cavity lasers

The authors report an effective lateral current-distribution scheme to achieve uniform hole injection in GaN-based vertical-cavity lasers. A thin (∼5nm) intracavity Pd∕Au layer is used to simultaneously achieve a low-resistance Ohmic contact and effective lateral current distribution across a circular injection aperture. Precise placement of a thin metal layer in a vertical-cavity laser is shown to yield negligible single-pass optical loss. Light-emitting diodes utilizing this intracavity contact and lateral current-distribution scheme are demonstrated, with effective lateral current distribution observed for aperture diameters up to 36μm. Continuous-wave operation at current densities exceeding 10kA∕cm2 is demonstrated.

Evaluation of GaN substrates grown in supercritical basic ammonia

GaN crystals grown by the basic ammonothermal method were investigated for their use as substrates for device regrowth. X-ray diffraction analysis indicated that the substrates contained multiple grains while secondary ion mass spectroscopy (SIMS) revealed a high concentration of hydrogen, oxygen, and sodium. Despite these drawbacks, the emission from the light emitting diode structures grown by metal organic chemical vapor deposition on both the c-plane and m-plane epitaxial wafers was demonstrated. The SIMS depth profiles showed that the diffusion of the alkali metal from the substrate into the epitaxial film was small, especially in the m-direction.