The LED industry is hunting for innovative technologies and application fields, as lighting and backlight market growth wane. UV LEDs over the past year has caught many LED manufacturers attention, the budding UV-A LED market application sector in nail curing, air purification, and printing applications is attracting top LED players including Nichia and Osram to invest in the sector. UV-C LED or DUV LED that has a wavelength ranging from 200 nm to 280 nm remains a market sector that barely exists at the moment, due to certain technical challenges that call for new solutions.
UV LED market is gaining traction, as industry insiders note the Minamata convention to phase out all mercury based lamps by 2020 will offer large growth potential, since most UV lights on the market are still mercury-vapor lights. However, it has been difficult for companies relying on visible LED chip structures to make efficient DUV LEDs in the 260 nm wavelength, which is the most effective range for destroying DNA in virus and bacteria in sterilization applications.
|Jason Electric presentation shows challenges in current DUV LED designs using visible LED chip structures. (All photos courtesy of LEDinside)|
Current challenges in DUV LED chip designs
Low internal quantum efficiency (IQE) caused by high lattice mismatch between sapphire substrate and aluminium nitride (AlN) template used to make UV LEDs, lack of good encapsulent materials, improving thermal dissipation and UVC LED lifetimes are some of the issues manufacturers still need to overcome, said James Wu, Vice President, Qingdao Jason Electric during IR+UV Summit organized by SEMI on Thursday.
Using sapphire substrate based blue LED manufacturing methods, the IQE is usually below 20% internal quantum efficiency (IQE) in DUV LED applications compared to 50% to 60% in blue LEDs, while the external quantum efficiency (EQE) is about 8%, said Wu.
The main cause of the low IQE is high lattice mismatch between sapphire and AlN, where the defect rate is much larger compared to blue LEDs. One solution to get around the loss of light from lattice defects is growing thicker AlN buffer layers, which at best brings defect density to 109 cm2.Although, the large defect density can be significantly lowered to 105 cm2 by switching to bulk AlN substrate, the material is simply too costly at the moment, explained Wu.