UV LEDs reach shorter wavelengths

29 Sep 2004

LEDs emitting at 250 nm and 255 nm could prove useful in biological and chemical sensors.

Researchers at the University of South Carolina, US, have fabricated ultraviolet LEDs emitting 0.57 mW at 255 nm and 0.16 mW at 250 nm. Currently unpackaged, the team expects these power levels to increase by a factor of 3 to 5 with flip-chip bonding. (Applied Physics Letters 85 2175)

Ultradeep ultraviolet (UDUV) sources emitting between 250 nm and 260 nm could have many potential uses including a replacement for mercury lamps emitting at 253.7 nm and as an excitation source in biological and chemical sensors.

Asif Khan and his colleagues have grown high quality AlGaN layers containing up to 72% aluminium. Using these layers as cladding in LED structures allows the team to produce its UDUV devices.

Khan's team grows its LEDs on a sapphire substrate. The first layer to be deposited is an AlN buffer layer followed by a ten-period AlN/AlGaN superlattice. Next is the Al_0.72Ga_0.28N cladding layer which also serves as the n-type contact layer.

"The Al composition of 72% was chosen to provide transparency at 250 nm since our devices emit through the substrate. This layer is critical for determining device performance," say Khan and his co-workers. "We reduced the thickness of the AlGaN cladding layer to 1.4 microns to mitigate cracking."

The active region consists of three quantum wells, where the aluminium composition tunes the emission wavelength from 250 nm to 255 nm.

The team tested a 200 micron square device emitting at 255 nm and a 150 micron square device emitting at 250 nm. To avoid self-heating, both LEDs were measured under pulsed biased conditions.

Peak output powers were measured for pump currents of 1000 mA and 300 mA for the 255 nm and 250 nm devices respectively. The team reports external quantum efficiencies of 0.015% for the 255 nm emitter and 0.01% for the 250 nm emitter.

Author
Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.