The effect of an electron blocking layer (EBL) on the efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) is investigated. At low current density, the LEDs with a p-AlGaN EBL show a higher external quantum efficiency (EQE) than LEDs without an EBL. However, the EQE of LEDs without an EBL is higher than LEDs with an EBL as injection current density is increased. The improved EQE of LEDs without an EBL at high current density is attributed to the increased hole injection efficiency.

Photonic crystal (PC) structures on green light emitting diodes (LED) were successfully fabricated using nanoimprint lithography. The stamp, with a two-dimensional pillar structure, was fabricated using laser interference lithography through a double exposure technique. To achieve PC structures with a precise dimension, thermal treatment of the photoresist was employed during stamp fabrication; this proved to be effective for the control of the diameter and shape of the hole. The two-dimensional PC structures, with a period of 295 nm, diameter of 180 nm and depth of 100 nm, on the green LEDs resulted in nine-fold enhancement of the photoluminescence intensity compared to the as-deposited samples without PC structures.

We demonstrate the surface plasmon-enhanced blue light-emitting diodes (LEDs) using Ag nanoparticles embedded in p-GaN. A large increase in optical output power of 38% is achieved at an injection current of 20 mA due to an improved internal quantum efficiency of the LEDs. The enhancement of optical output power is dependent on the density of the Ag nanoparticles. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Ag nanoparticles embedded in p-GaN.

Flexible pulse sensors that can detect subtle skin surface deformation caused by arterial pulses are key components for developing non‐invasive continuous pulse waveform monitoring systems that provide vital health status parameters. Piezoelectric pulse sensors (PPSs) offer a promising solution for flexible pulse sensors due to their relatively high sensitivity and stability, and low power consumption, when compared with conventional active pulse sensors. However, the reported high‐performance PPSs contain toxic lead, which limits their practical applications. In this study, a highly sensitive and flexible PPS that detects surface deflections on the micrometer scale is fabricated with single‐crystalline group III‐nitride thin film. This biocompatible flexible PPS is sensitive enough to detect pulse waveform with detailed characteristic peaks from most arterial pulse sites when attached to the skin surface without applying external pressure. Useful physiological parameters such as the pulse rate, artery augmentation index, and pulse wave velocity can be drawn from the as‐acquired pulse waveforms. The flexible PPS can also be used to continuously monitor the arterial pulse waveform.

This letter reports the properties of GaN-based green light-emitting diodes ͑LEDs͒ having a p-GaN photonic crystal layer with a photonic bandgap ͑PCWG͒ and without a photonic bandgap ͑PCOG͒. With decreasing the photoluminescence ͑PL͒ detection angle from 140°to 60°, the enhancement of PL intensity of LED with PCWG was largely increased from 9 to 25 times, compared to that of LEDs without a patterned structure, while the PL intensity of LED with PCOG was increased from 4.6 to 5.6 times. The electroluminescence output power of green LEDs with a PCWG was enhanced about two times compared to LEDs with a PCOG. These results suggest that the light extraction of green LEDs can be greatly increased by using PCWG instead of PCOG.