No abstract

We propose a method to determine the current injection efficiency (CIE) and internal quantum efficiency (IQE) of light-emitting diodes (LEDs) during current injection. The method is based on fourth-order polynomial fitting of a modified rate equation to electroluminescence data. Our method can extract the CIE at low injection current densities, unlike conventional methods that generally assume the CIE to be unity. We apply the method to AlGaN-based deep-ultraviolet LEDs. Results show that the CIE was only approximately 51% at low injection current densities and was almost independent of injection current density up to 100 A/cm. The peak IQE was 77%.

We thoroughly explored the physical origin of the efficiency decrease with increasing injection current and current crowding effect in 280 nm AlGaN-based flip-chip deep-ultraviolet (DUV) light-emitting diodes (LEDs). The current spreading length was experimentally determined to be much smaller in DUV LEDs than that in conventional InGaN-based visible LEDs. The severe self-heating caused by the low power conversion efficiency of DUV LEDs should be mainly responsible for the considerable decrease of efficiency when current crowding is present. The wall-plug efficiency of the DUV LEDs was markedly enhanced by using a well-designed p-electrode pattern to improve the current distribution.

Because of its large bandgap of ∼6.0 eV and suitability for high p-type doping, hexagonal boron nitride (h-BN) has become a candidate material that can serve as a p-layer by forming a heterostructure with AlGaN materials with a high Al fraction in deep-ultraviolet optoelectronic devices. The band offsets at the heterojunction are crucial to the device design because they determine the hole and electron transport properties across the heterojunction. In this study, we give the band alignment between h-BN and Al0.7Ga0.3N using the valence and conduction band offsets. The valence band offset of the h-BN/Al0.7Ga0.3N heterojunction is determined via X-ray photoelectron spectroscopy (XPS) to be as small as −0.01 ± 0.09 eV. The small valence band discontinuity that occurs at the h-BN/Al0.7Ga0.3N interface is further confirmed using angle-resolved valence band spectra from the XPS measurements. By combining the bandgap values of Al0.7Ga0.3N and h-BN which were estimated using absorption spectra measurements, the conduction band offset is found to be approximately 0.89 ± 0.09 eV. These results indicate that h-BN is an excellent material for hole injection into Al0.7Ga0.3N. Meanwhile, the electrons can be effectively blocked away from h-BN. These results will be helpful in the design of group-III-nitride-based optoelectronic devices, particularly deep-ultraviolet light-emitting diodes and lasers.

Subwavelength-sized truncated cones satisfying the requirement for evanescent wave coupling effect at the emission wavelength were fabricated on the p-side of a GaN-based blue light-emitting diode (LED). The light-extraction efficiency increased by a factor of approximately 2.2 relative to that of a reference LED with a flat light-extraction surface. This light-extraction enhancement ratio is much larger than that realized by using conventional light-extraction techniques. The evanescent wave coupling effect appearing on the surface of the truncated-cone structure is believed to be the mechanism responsible for the large enhancement of light extraction.