Technical Insight
Efficiency increase for deep UV LEDs
Better material quality and chip encapsulation drive up the power and efficiency of deep UV LEDs
A US team claims to have raised the bar for the output power of UV LEDs with a peak emission wavelength of 365 nm or less. The partnership between Sensor Electronic Technology, Rensselaer Polytechnic Institute and the US Army Research Laboratory has fabricated a 278 nm LED delivering 9.3 mW at 20 mA and 30 mW at 100 mA.
This powerful chip, which features high-quality encapsulation and high internal efficiency, will help the development of UV LEDs for homeland security, medical sensing, curing and sterilization.
Traditionally, UV LED efficiency has been held back by a high density of crystal defects and low light extraction. In most UV LEDs, light extraction is hampered by absorption in the p-type contact and a lack of die encapsulation. Many polymers are unsuitable for encapsulation, because they absorb the light and degrade.
Last year this US team built a UV LED that could address some of these weaknesses. Devices were modified through the insertion of transparent p-type cladding and contact layers, and the introduction of a novel p-type reflective stack with a reflectivity of 70 percent at 275 nm. Now, UV LED performance has been further improved. A cut in the dislocation density in the active region has boosted internal quantum efficiency, and the introduction of a robust polymer that encapsulates the device has increased light extraction.
The team’s latest UV LEDs are grown by migration-enhanced MOCVD on sapphire and feature a p-type short period superlattice. “This is transparent to UV emission making possible multiple pass light extraction,” explains Tim Bettles, Director of Business Development at Sensor Electronic Technology. UV LEDs were formed by dicing wafers into 350 µm by 350 µm chips, processing them with an approach that ensured uniform current spreading and flip-chip mounting them in TO-39 packages to collect as much emission as possible through the sapphire side. Encapsulation with a UV-transparent, stable material that was shaped and chosen for its particular refractive index boosted light extraction from the packaged LED. When driven at 20 mA, encapsulation boosted output power from 6.9 mW to 9.3 mW.
The team will not disclose its latest goals. However, it is possible that they will try and reduce the forward voltage of the device – a drive current of 20 mA requires biasing at 8.4 V.
M. Shatalov et al. Appl. Phys.
Express 5 082101 (2012)
