Efficiency Of Deep Ultraviolet Light-emitting Diodes Research Articles

Enhancement of Light Efficiency of Deep-Ultraviolet Light-Emitting Diodes by Encapsulation with a 3D Photonic Crystal Reflecting Layer.

Recently, UVC LEDs, which emit deep ultraviolet light, have found extensive applications across various fields. This study demonstrates the design and implementation of thin films of three-dimensional photonic crystals (3D PhCs) as reflectors to enhance the light output power (LOP) of UVC LEDs. The 3D PhC reflectors were prepared using the self-assembly of silica nanospheres on a UVC LED lead frame substrate via the evaporation-induced method (side) and the gravitational sedimentation method (bottom), respectively. These PhCs with the (111) crystallographic plane were deposited on the side wall and bottom of the UVC LED lead frame, acting as functional materials to reflect UVC light. The LOP of UVC LEDs with 3D PhC reflectors at a driving current of 100 mA reached 19.6 mW. This represented a 30% enhancement compared to commercial UVC LEDs with Au-plated reflectors, due to the UVC light reflection by the photonic band gaps of 3D PhCs in the (111) crystallographic plane. Furthermore, after aging tests at 60 °C and 60% relative humidity for 1000 h, the relative LOP of UVC LEDs with 3D PhC reflectors decreased by 7%, which is better than that of commercial UVC LEDs. Thus, this study offers potential methods for enhancing the light output efficiency of commercial UVC light-emitting devices.

Analysis of the Light-Extraction Efficiency of Flip-Chip Deep-Ultraviolet Light-Emitting Diodes with Embedded Photonic Crystals

In the present study, embedded photonic crystals were introduced into the p-GaN contact layer of light-emitting diodes to increase the light-extraction efficiency of flip-chip deep-ultraviolet light-emitting diodes. As demonstrated by performing three-dimensional finite-difference time domain simulation, the embedded photonic crystals could be designed to increase the total reflectivity on the p-GaN contact layer, thereby increasing the light-extraction efficiency. In addition, the effects of the embedded photonic crystals configurations on the light-extraction efficiency of deep-ultraviolet light-emitting diodes were examined. Light-extraction efficiency over 21% for transverse magnetic polarized emission could be generally expected by rigorously optimizing the active layer position, as well as the depth, period, and filling factor of photonic crystals. Moreover, an investigation was conducted on the light-extraction efficiency over the whole ultraviolet spectrum, and more light-extraction efficiency harvest on a broadband ultraviolet spectrum has been achieved as compared with flip-chip planar deep-ultraviolet light-emitting diodes. The optimized structure will be promising for increasing the light-extraction efficiency of AlGaN-based flip-chip deep-ultraviolet light-emitting diodes.

Thermal and nonthermal factors affecting the quantum efficiency of deep-ultraviolet light-emitting diodes

The optical characteristics of AlGaN-based multiple-quantum-well (MQW) light-emitting diodes (LEDs) with peak wavelengths ranging from 265–340 nm were characterized over a wide current range. It was found that thermal effects due to self-heating can be largely eliminated by pulsing the LEDs with a duty cycle below 0.2%. The current-induced energy shift up to 850 A/cm2 was negligible in the deep-UV LEDs, indicating the lack of localization effects. The quantum efficiency of the LEDs increased monotonically at low currents and attained a low saturated value at high currents. This is in contrast to the efficiency rolloff behavior of typical InGaN-based LEDs resulting from carrier delocalization at high injection levels. The efficiency saturation of the deep-UV LEDs suggests that defects play an important role in both carrier injection and recombination processes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)