Naruhito Iwasa

High-Brightness InGaN Blue, Green and Yellow Light-Emitting Diodes with Quantum Well Structures.

High-brightness blue, green and yellow light-emitting diodes (LEDs) with quantum well structures based on III-V nitrides were grown by metalorganic chemical vapor deposition on sapphire substrates. The typical green LEDs had a peak wavelength of 525 nm and full width at half-maximum (FWHM) of 45 nm. The output power, the external quantum efficiency and the luminous intensity of green LEDs at a forward current of 20 mA were 1 mW, 2.1% and 4 cd, respectively. The luminous intensity of green LEDs (4 cd) was about 40 times higher than that of conventional green GaP LEDs (0.1 cd). Typical yellow LEDs had a peak wavelength of 590 nm and FWHM of 90 nm. The output power of yellow LEDs was 0.5 mW at 20 mA. When the emission wavelength of III-V nitride LEDs with quantum well structures increased from the region of blue to yellow, the output power decreased dramatically.

Hole Compensation Mechanism of P-Type GaN Films

Low-resistivity p-type GaN films, which were obtained by N2-ambient thermal annealing or low-energy electron-beam irradiation (LEEBI) treatment, showed a resistivity as high as 1×106 Ωcm after NH3-ambient thermal annealing at temperatures above 600°C. In the case of N2-ambient thermal annealing at temperatures between room temperature and 1000°C, the low-resistivity p-type GaN films showed no change in resistivity, which was almost constant between 2 Ωcm and 8 Ωcm. These results indicate that atomic hydrogen produced by NH3 dissociation at temperatures above 400°C is related to the hole compensation mechanism. A hydrogenation process whereby acceptor-H neutral complexes are formed in p-type GaN films was proposed. The formation of acceptor-H neutral complexes causes hole compensation, and deep-level and weak blue emissions in photoluminescence.

Superbright Green InGaN Single-Quantum-Well-Structure Light-Emitting Diodes

Superbright green InGaN single quantum well (SQW) structure light-emitting diodes (LEDs) with a luminous intensity of 12 cd were fabricated. The luminous intensity of these green InGaN SQW LEDs (12 cd) was about 100 times higher than that of conventional green GaP LEDs (0.1 cd). The output power, the external quantum efficiency, the peak wavelength and the full width at half-maximum of green SQW LEDs were 3 mW, 6.3%, 520 nm and 30 nm, respectively, at a forward current of 20 mA. The p-AlGaN/InGaN/n-GaN structure of green InGaN SQW LEDs were grown by metalorganic chemical vapor deposition on sapphire subsutrates.

Thermal Annealing Effects on P-Type Mg-Doped GaN Films

Low-resistivity p-type GaN films were obtained by N2-ambient thermal annealing at temperatures above 700°C for the first time. Before thermal annealing, the resistivity of Mg-doped GaN films was approximately 1×106 Ωcm. After thermal annealing at temperatures above 700°C, the resistivity, hole carrier concentration and hole mobility became 2 Ωcm, 3×1017/cm3 and 10 cm2/Vs, respectively. In photoluminescence measurements, the intensity of 750-nm deep-level emissions (DL emissions) sharply decreased upon thermal annealing at temperatures above 700°C, as did the change in resistivity, and 450-nm blue emissions showed maximum intensity at approximately 700°C of thermal annealing.

InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate

InGaN multi-quantum-well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN (ELOG) substrate was demonstrated to have a lifetime of more than 1150 h under room-temperature continuous-wave operation. After 4 μm etching of the ELOG substrate, the etch pit density was about 2×108 cm2 in the region of the 4-μm-wide stripe window, but almost zero in the region of the 7-μm-wide SiO2 stripe.

High‐power InGaN single‐quantum‐well‐structure blue and violet light‐emitting diodes

High‐power blue and violet light‐emitting diodes(LEDs) based on III–V nitrides were grown by metalorganic chemical vapor deposition on sapphire substrates. As an active layer, the InGaN single‐quantum‐well‐structure was used. The violet LEDs produced 5.6 mW at 20 mA, with a sharp peak of light output at 405 nm, and exhibited an external quantum efficiency of 9.2%. The blue LEDs produced 4.8 mW at 20 mA and sharply peaked at 450 nm, corresponding to an external quantum efficiency of 8.7%. These values of the output power and the quantum efficiencies are the highest ever reported for violet and blue LEDs.

Room‐temperature continuous‐wave operation of InGaN multi‐quantum‐well structure laser diodes

Continuous‐wave (cw) operation of InGaN multi‐quantum‐well structure laser diodes (LDs) was demonstrated at room temperature (RT). The threshold current and voltage of the LD were 130 mA and 8 V, respectively. The threshold carrier density was 9 kA/cm2. The lifetime of the LDs under RT cw operation was 1 s due to large heat generation. Mode hopping of the emission wavelength of the LDs was observed. The average wavelength drift due to temperature increase was 0.066 nm/K between 20 and 70 °C, because of the temperature dependence of the gain profile due to band‐gap narrowing of the InGaN active layer.

InGaN/GaN/AlGaN-Based Laser Diodes with Modulation-Doped Strained-Layer Superlattices

InGaN multi-quantum-well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10000 h under continuous-wave operation at 20° C. Under operation at a high temperature of 50° C, the lifetime was longer than 1000 h. With the operating current increasing to above the threshold, a self-pulsation with a high frequency of 3.5 GHz was observed. The carrier lifetime was estimated to be 1.8 ns from the pulsed modulation of the LDs.

Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates

InGaN multi-quantum-well-structure laser diodes (LDs) grown on GaN substrates were demonstrated. The LDs showed a small thermal resistance of 30 °C/W and a lifetime longer than 780 h despite a large threshold current density of 7 kA/cm2. In contrast, the LDs grown on a sapphire substrate exhibited a high thermal resistance of 60 °C/W and a short lifetime of 200 h under room-temperature continuous-wave operation.

ROOM-TEMPERATURE CONTINUOUS-WAVE OPERATION OF INGAN MULTI-QUANTUM-WELL STRUCTURE LASER DIODES WITH A LIFETIME OF 27 HOURS

The continuous-wave (cw) operation of InGaN multi-quantum-well structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 27 h. The threshold current and the voltage of the LDs were 80 mA and 5.5 V, respectively. The threshold current density was 3.6 kA/cm2. Longitudinal modes with a mode separation of 0.042 nm were observed under cw operation at RT. When the temperature of the LDs was varied, large mode hopping of the emission wavelength was observed between adjacent quantum well or quantum dot subbands. The carrier lifetime and the threshold carrier density were estimated to be 10 ns and 2×1020/cm3, respectively.