Junichi Sonoda

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.

30-mW-Class High-Power and High-Efficiency Blue Semipolar (1011) InGaN/GaN Light-Emitting Diodes Obtained by Backside Roughening Technique

The first 30-mW-class semipolar blue light-emitting diode (LED) on a free-standing (1011) GaN substrate has been demonstrated by using microscale periodic backside structures. The light extraction efficiency and corresponding output power were greatly enhanced, by up to 2.8-fold (bare chip) compare with conventional devices. At a driving current of 20 mA, the LED showed an output power of 31.1 mW and an external quantum efficiency of 54.7%. Semipolar GaN LED technology is now comparable to commercial c-plane blue LED technology, not only in terms of internal material properties but also in terms of chip processing techniques.

Quantum-confined Stark effect on photoluminescence and electroluminescence characteristics of InGaN-based light-emitting diodes

The quantum-confined Stark effect (QCSE) on InGaN-based light-emitting diodes (LEDs) was investigated as a part of the continuing study of exploring differences between photoluminescence (PL) and electroluminescence (EL) characteristics. The luminescence characteristics were related to electrical characteristics of green and amber LEDs by employing the electrical-bias-applied PL technique. By inspecting the band diagram, it has been found that the separation of quasi-Fermi levels, which strongly affects the QCSE, can be quantified and related to the luminescence. In order to compare PL and EL characteristics, attention was paid to the QCSE during the PL and EL measurements. Despite the control of the QCSE, differences were still confirmed between PL and EL characteristics, which have led us to the conclusion to that there are other unrevealed origins for the differences.

GaN-Based Integrated Lateral Thermoelectric Device for Micro-Power Generation

Lateral thermoelectric devices were fabricated using c-plane GaN thin films grown on sapphire by MOCVD. The device design is appropriate for on-chip integration for power generation in the 1 V and tens of µA range. The fabricated devices were measured to have a maximum open circuit voltage of 0.3 V with a maximum output power of 2.1 µW (=0.15 V×14 µA) at a relatively small temperature difference (ΔT) of 30 K and an average temperature (Tavg) of 508 K. In addition, the suitability of GaN for high temperature thermoelectric applications was confirmed by measurements at 825 K.

Substitution of oxygen by fluorine in the GdSr 2 AlO 5 :Ce 3+ phosphors: Gd 1-x Sr 2+x AlO 5-x Fx solid solutions for solid state white lighting

Solid solutions between two isotypic host compounds: GdSr(2)AlO(5) and Sr(3)AlO(4)F; Gd(1-x)Sr(2+x)AlO(5-x)F(x):Ce(3+) (GSAF:Ce(3+)), have been prepared across the complete solid solution range x. Depending on x, the series display considerable optical tunability of emission wavelengths in the range 574 nm to 474 nm, which is attributed to the decreased crystal field splitting arising from increased host ionicity with fluorine addition. Applying the GSAF:Ce(3+) phosphors on InGaN LEDs (lambda (max) = 405 nm and 450 nm) permits white lighting sources to be prepared. The characteristics of these are reported.

Chip Shaping for Light Extraction Enhancement of Bulk c-Plane Light-Emitting Diodes

The enhancement of light extraction from light-emitting diodes (LEDs) grown on bulk c-plane gallium nitride (GaN) through chip shaping was investigated. Photolithography and diamond scribing determined the individual LED chip geometries (triangles, parallelograms, and rectangles). A light-absorbing material was applied to the sidewalls of individual LED dies systematically and measured in an integrating sphere to determine the amount of photon extraction from the sidewalls. The systematic use of the absorber was repeated in commercial ray tracing software yielding almost identical results and allowed the determination of the extraction efficiency for each chip geometry. The total extraction efficiencies were 47.7, 52.6, and 53.1% for the rectangle, parallelogram, and triangle, respectively.

Vertical Stand Transparent Light-Emitting Diode Architecture for High-Efficiency and High-Power Light-Emitting Diodes

Using a transparent ZnO vertical stand as a submount, a novel Light-emitting diode architecture, which is similar to conventional lighting bulbs, was proposed. The emission power of a blue LED based on c-plane (0001) bulk GaN was increased by 14.2 and 5.1% compared with those of conventional and suspended die packages, respectively. The output power and external quantum efficiency of LEDs respectively reached 31.7 mW and 57.1% at a forward current of 20 mA under direct current conditions. The high thermal conductivity and refractive index of the transparent submount simultaneously resulted in high current operation and high external efficiency.

High Power and High Efficiency Blue InGaN Light Emitting Diodes on Free-Standing Semipolar (3031) Bulk GaN Substrate

High power and high efficiency semipolar (3031) nitride light emitting diodes (LEDs), fabricated on low extended defect bulk GaN substrates, are reported for the first time. The LEDs were grown by metal organic chemical vapor deposition (MOCVD) at atmospheric pressure. The peak wavelength was 452 nm, and a minimal redshift of <1 nm was observed between 5–100 mA, in comparison to large blueshifts in c-plane LEDs. The output power and external quantum efficiency (EQE) of the packaged 200 ×500 µm2 was 14.48 mW and 26.5%, respectively, at 20 mA.

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.