Ill. --- After the anthrax attacks in the United
States in 2001 the threat of a larger and more deadly
bioterrorism attack -- perhaps from smallpox, plague
or tularemia -- became very real. But the ability
to detect such biological agents and rapidly contain
an attack is still being developed.
In a significant finding, researchers at Northwestern
University's Center for Quantum Devices have demonstrated
solar-blind avalanche photodiodes (APDs) that hold
promise for universal biological agent detection.
Once optimized, these sensitive detectors could be
combined with the ultraviolet light-emitting diodes
(LEDs) already pioneered by the Center for Quantum
Devices to create an inexpensive detection system
capable of identifying the unique spectral fingerprints
of a biological agent attack.
The Northwestern team, led by center director Manijeh
Razeghi, became the first to demonstrate 280 nanometer
APDs. These devices, based on aluminum gallium nitride
(AlGaN) compound semiconductors, have a photocurrent
gain of more than 700.
The tiny-sized APDs should be capable of efficient
detection of light with near single photon precision.
Previously, photomultiplier tubes (PMTs) were the
only available technology in the short wavelength
UV portion of the spectrum capable of this sensitivity.
These fragile vacuum tube devices are expensive and
bulky, hindering true systems miniaturization.
The APD technology may see further use in the deployment
of systems for secure battlefield communication.
Wavelengths around 280 nanometers are referred to
as the solar-blind region; in this region, the UV
light is filtered out by the ozone layer providing
for a naturally low background signal. Solar-blind
APDs are intrinsically able to take advantage of
this low background level, while PMTs must use external
filters to become solar-blind. This makes secure
battlefield communication possible utilizing a combination
of compact, inexpensive UV LEDs and UV APDs both
developed at the Center for Quantum Devices.
The technology for the realization of solar-blind
APDs is based on wide bandgap AlGaN compound semiconductors.
To date, no semiconductor-based solar-blind APDs
have been reported. This is due to numerous difficulties
pertaining to the crystal growth of AlGaN compound
The major obstacle in demonstrating high performance
solar-blind APDs is the high number of crystalline
defects present in the AlGaN semiconductor material.
However, researchers at the Center for Quantum Devices
have been able to realize high-quality AlGaN so as
to demonstrate avalanche gain in the solar-blind
Northwestern's results were presented recently by
Razeghi at the APD workshop organized by Henryk Temkin,
a new program manager at the Defense Advanced Research
Projects Agency (DARPA).
Contact: Megan Fellman