Abstract
Three-dimensional (3D) backside reflector, compared with flat reflectors, can improve the probability of finding the escape cone for reflecting lights and thus enhance the light-extraction efficiency (LEE) for GaN-based light-emitting diode (LED) chips. A triangle-lattice of microscale SiO2 cone array followed by a 16-pair Ti3O5/SiO2 distributed Bragg reflector (16-DBR) was proposed to be attached on the backside of sapphire substrate, and the light-output enhancement was demonstrated by numerical simulation and experiments. The LED chips with flat reflectors or 3D reflectors were simulated using Monte Carlo ray tracing method. It is shown that the LEE increases as the reflectivity of backside reflector increases, and the light-output can be significantly improved by 3D reflectors compared to flat counterparts. It can also be observed that the LEE decreases as the refractive index of the cone material increases. The 3D 16-DBR patterned by microscale SiO2 cone array benefits large enhancement of LEE. This microscale pattern was prepared by standard photolithography and wet-etching technique. Measurement results show that the 3D 16-DBR can provide 12.1% enhancement of wall-plug efficiency, which is consistent with the simulated value of 11.73% for the enhancement of LEE.
Highlights
Low light-output efficiency is one of the biggest obstacles for the extensive use of GaN-based light-emitting diodes (LEDs) in general lighting
The extensively used reflector always consists of multiple thinfilms, with which the LED chip makes up planar optical waveguides
In order to reduce the computation resource, this 3D model only consists of three layers, including the sapphire substrate, the n-type GaN, and the p-type GaN (p-GaN)
Summary
Low light-output efficiency is one of the biggest obstacles for the extensive use of GaN-based light-emitting diodes (LEDs) in general lighting. To enhance the light-output of the top side in LED chips, a backside reflector is used [1,2,3]. The extensively used reflector always consists of multiple thinfilms, with which the LED chip makes up planar optical waveguides. Lights outside the critical angle would be confined within the device and be repeatedly reflected by the GaN/air interface. These lights would be absorbed by semiconductor materials, multiple quantum well, and metal electrodes, which would convert to heat. In order to break the planar structure, three-dimensional (3D) reflectors can be used
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