Atsushi Nishikawa

Room-Temperature Red Emission from a p-Type/Europium-Doped/n-Type Gallium Nitride Light-Emitting Diode under Current Injection

We have succeeded in the growth of europium (Eu)-doped GaN layer grown by organometallic vapor-phase epitaxy (OMVPE) and demonstrated the first low-voltage operation of current-injected red emission from a p-type/Eu-doped/n-type GaN light-emitting diode (LED) at room temperature. The bright red emission was obtained with an applied voltage as low as 3 V under normal lighting conditions. At a dc current of 20 mA, the output power, integrated over the 5D0–7F2 transition in Eu3+ ions (around 621 nm), was 1.3 µW. This result suggests a novel way to realize GaN-based red LEDs and monolithic devices comprising red, green and blue GaN-based LEDs.

Improved luminescence properties of Eu-doped GaN light-emitting diodes grown by atmospheric-pressure organometallic vapor phase epitaxy

We investigated the luminescence properties of Eu-doped GaN (GaN:Eu) grown by atmospheric-pressure organometallic vapor phase epitaxy. The GaN:Eu exhibited radiant red emission due to the intra-4f shell transition of Eu3+ ions at room temperature. The intensity of the dominant peak was about 4 times higher than that in the sample grown at 10 kPa, even though the Eu concentration was only half that of the 10 kPa sample. This was mainly caused by the enhancement of the energy transfer from the GaN host to Eu ions. The enhanced energy transfer resulted in improved luminescence properties of a GaN:Eu light-emitting diode.

Excitation of Eu3+ in gallium nitride epitaxial layers: Majority versus trap defect center

We report studies of the excitation mechanism of Eu ions in situ doped during organometallic vapor-phase epitaxy (OMVPE) of GaN. We find that the bright red emission under above-band gap excitation originates primarily from an incorporation site that exhibits high excitation efficiency but occurs in low relative abundance ( 97%).

Highly Uniform Electroluminescence from 150 and 200 mm GaN-on-Si-Based Blue Light-Emitting Diode Wafers

We report on the on-wafer device characteristics of 150 and 200 mm GaN-on-Si-based blue LED wafers grown by metalorganic chemical vapor deposition on Si(111) substrates with electroluminescence at 447 nm. Excellent uniformity was achieved with standard deviations of 3.9% for the electroluminescence intensity, 0.6–0.8% for the peak wavelength and 1.3% for the forward voltage. The high uniformity confirms the viability of the GaN-on-Si technology on large-diameter substrates for next-generation LED manufacturing. The reverse bias current leakage mechanism is also investigated to provide an insight into improving device reliability.

Structural phase transitions and equations of state of Se and Te under high pressure

Selenium and tellurium are known to undergo several structural phase transitions under pressure. We have investigated the band-structures and the total energies of β-P 0 type (rhombohedral) structure and body-centered cubic structure of Se and Te within the local density-functional formalism using the norm-conserving pseudopotential method. The calculated equations of state are in good agreement with the experimental ones. We also discuss the structural stability of the high pressure phases

Improved Eu Luminescence Properties in Eu-Doped GaN Grown on GaN Substrates by Organometallic Vapor Phase Epitaxy

We have grown Eu-doped GaN on a freestanding GaN substrate by organometallic vapor phase epitaxy and investigated its red luminescence that is due to intra-4f shell transitions of Eu3+ ions. The optimum growth temperature for Eu luminescence was 50 °C higher than that of Eu-doped GaN on a sapphire substrate. The highest emission intensity was more intense than that on the sapphire substrate, while the Eu luminescence lifetime was identical for both substrates. These results indicate that energy transfer from the GaN host to the Eu3+ ions occurs more efficiently in Eu-doped GaN on the GaN substrate because of a low dislocation density.

Luminescence properties of Eu-implanted GaN-based semiconductors

We investigated the relationship between optical properties and crystal quality of Eu-implanted GaN-based semiconductors. Emission peaks that are due to intra-4f shell transitions in Eu3+ ions were observed in Eu-implanted GaN, InGaN, and AlGaN. Emission wavelengths from the Eu ions were constant regardless of the matrix materials. The emission peak intensity in the Eu-implanted AlGaN was higher than that in the Eu-implanted InGaN or GaN because of less matrix damage by ion implantation.

Optical properties of Eu-implanted GaN and related-alloy semiconductors

We investigated the optical properties of Eu-implanted GaN and related-alloy semiconductors by photoluminescence (PL) and time resolved PL measurements at room temperature (RT). After thermal annealing, the red emission was observed in each sample due to the intra-4f transitions in Eu3+ ions. The PL intensity increases with Al incorporation into GaN while it decreases with In incorporation into GaN, indicating that the emission efficiency depends on the host materials while the emission wavelength is independent of the host materials. Since the decay times of Eu emission of the samples are found to be similar by time resolved PL measurements, the origin of the Eu emission of each sample is supposed to be same. Therefore, the difference in the PL intensity is attributed to the difference in the energy transfer efficiency because of the ion implantation damage and/or bandgap energy of the host materials.