Kenji Iso

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.

Optical polarization characteristics of m-oriented InGaN/GaN light-emitting diodes with various indium compositions in single-quantum-well structure

Spontaneously polarized light emission from m-plane InGaN/GaN light-emitting diodes was studied as a function of In composition in the InGaN single quantum-well layer. Emission wavelength was varied between 394 and 472 nm. A strong correlation was confirmed between optical polarization and In composition; the higher the In composition, the stronger the optical polarization. The photon-energy difference between the emission spectra associated with the two polarizations, ΔM, was evaluated as a function of current. ΔM exhibited a negative monotonic current dependence for the 394 nm emitting sample and the dependence was changed to positive monotonic as the wavelength became longer towards 472 nm. This change was tentatively attributed to the valence band mixing and the crystal momentum conservation that became relevant with the band filling. ΔM and optical polarization exhibited only a moderate correlation; the Fermi–Dirac function has been used to explain the weakened optical polarization under increased current injection.

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.

Compositional Dependence of Nonpolar m-Plane InxGa1-xN/GaN Light Emitting Diodes

Characteristics of m-plane InGaN/GaN light emitting diodes (LEDs) with various indium compositions were investigated. X-ray diffraction revealed that indium compositions in the InGaN multi quantum wells (MQWs) on m-plane substrate were 2?3 times lower than those on c-plane substrate. The optical polarization ratio for m-plane LEDs increased from 0.27 to 0.89 with increasing emission wavelength from 383 to 476 nm due to compressively strained InGaN QWs. The output power of electroluminescence decreased above 400 nm although polarization-related internal electric fields were eliminated.

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.

Effects of piezoelectric fields on optoelectronic properties of InGaN/GaN quantum-well light-emitting diodes prepared on nonpolar (1?0? \bar1 ?0) and semipolar (1?1? \bar{2} ?2) orientations

Optical and electrical characteristics of InGaN/GaN quantum-well (QW) light-emitting diodes (LEDs) are the subjects of this study. Samples were prepared on nonpolar (1 0   0) and semipolar (1 1   2) orientations of bulk GaN substrates. Electrical-bias-applied photoluminescence was employed as a characterization technique. It was confirmed that saturation of reverse photocurrent occurred around 0 V in nonpolar LEDs and at positive voltages in (1 1   2)-oriented LEDs, while our previous study found negative voltages in (0 0 0 1)-oriented LEDs (Masui et al 2008 J. Phys. D: Appl. Phys. 41 165105). These results indicated that (1 1   2)-oriented InGaN/GaN QWs experience piezoelectric fields being in the same direction as the built-in field. Piezoelectric field intensity was estimated to be −0.3 MV cm−1 in the (1 1   2)-oriented QW structure. Spectral comparison between photoluminescence and electroluminescence of the LED samples exhibited a tendency that spectral differences were insignificant in single-QW 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.

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.

Customized Filter Cube in Fluorescence Microscope Measurements of InGaN/GaN Quantum-Well Characterization

Modified optical filter elements were fabricated to access blue and violet spectral ranges of InGaN/GaN light-emitting diodes (LEDs) in characterizing luminescence via a fluorescence microscope. As band gap energies of GaN and InGaN layers were close, it was not trivial to separate excitation and luminescence light, which caused slight detection of excitation light. Nevertheless, the results obtained on c- and m-plane LEDs were compared with our previous work, leading consistent understanding of luminescence characteristics in terms of the quantum-confined Stark effect.