Micro-LEDs show promise for biology

Michael A. Greenwood

As LEDs of varying shapes and sizes find their way into myriad applications, researchers are seeking to devise a high-performance micro-LED that could be integrated effectively into biological research.

Investigators at Tyndall National Institute in Cork, Ireland, report that they have developed a micro-LED fashioned from quantum dots and housed in a geometrically designed shell that better directs the light that is created.

Micro-LEDs created with quantum dots and housed within parabolic sidewalls exhibited a fourfold increase in the substrate-emitted power density. Courtesy of Brian Corbett.

Team member Brian Corbett described traditional LEDs as “massive” in comparison with the device designed by his group. The tiny LEDs — measuring 17 μm in diameter — could be used in a number of diverse applications, including microfluidics and cytometry. Additionally, their broad bandwidth and small size, which could allow them to be coupled to an optical fiber, also might enable their use in miniature OCT applications; however, experiments with this have not yet been performed.

The micro-LEDs were housed in parabolic sidewalls, an architecture that improved extraction efficiency by directing the emitted light through the LEDs’ transparent substrate. Although other LED designs generate a high quantum yield, much of the light is totally internally reflected and absorbed before it is emitted, resulting in only about 2 percent of the generated light escaping.

Arrays of the micro-LEDs were fabricated on n-doped GaAs substrates. The active layer of the micro-LED consisted of stacks of InAs quantum dots placed inside InGaAs quantum wells. The design was completed with a p-doped GaAs cap layer.

During experiments, the light-current characteristics of a 16 × 16 micro-LED array were compared with those of an LED with a traditional planar design. The investigators found that the micro-LED had a fourfold increase in the emitted power density at an operating current density of 20 A/cm². The micro-LED also exhibited a broad emission spectrum of 225 nm with a current injection of 5 mA. At low current levels, the LEDs’ peak emission was around 1265 nm.

Corbett said that he and his colleagues are looking at a number of other potential applications for the device, with the eventual goal of preparing it for commercial release. In the meantime, they also are seeking to develop micro-LEDs that can operate at other wavelengths and are looking at ways to improve the height-to-diameter ratio and the LEDs’ housing.

Applied Physics Letters, March 24, 2008, Vol. 92, 123501.