New type of ultraviolet diode could lead to compact anthrax detector
29 June 2008
A new class of ultraviolet photodiode could make possible a compact,
reliable and cost-effective sensor to detect anthrax and other
bioterrorism agents in the air.
Research conducted at the Georgia Institute of Technology, shows that
ultraviolet avalanche photodiodes offer the high gain, reliability and
robustness needed to detect these agents and help authorities rapidly
contain an incident like the 2001 US anthrax attacks. The fabrication
methods and device characteristics were described at the 50th Electronic
Materials Conference in Santa Barbara on June 25.
Details of the photodiodes were also published in the February 14
issue of the journal Electronics Letters and the November 2007
issue of the journal IEEE Photonics Technology Letters.
“The military is currently using photomultiplier tubes, which are
bulky, fragile and require a lot of power to run them, or silicon
photodiodes that require a complex filter so that they only detect the
desired ultraviolet light,” said Russell Dupuis, Steve W Chaddick
Endowed Chair in Electro-Optics in Georgia Tech’s School of Electrical
and Computer Engineering (ECE) and a Georgia Research Alliance Eminent
ECE associate professor Douglas Yoder, assistant professor Shyh-Chiang
Shen and senior research engineer Jae-Hyun Ryou collaborated on this
research, which is funded by the Defense Advanced Research Projects
Agency (DARPA) and the Georgia Research Alliance.
The team chose to develop avalanche photodiodes for this bioterrorism
application because the devices can detect the signature fluorescence of
biological molecules in a sample of air. Since most of the molecules of
interest to the researchers emit ultraviolet light, they designed
special photodiodes that detect the fluorescence in the ultraviolet
region, but have no response to visible light.
“We built our photodiodes with gallium nitride, which is a
semiconductor that can be used to create photodiodes that require no
filters because this material has an inherent response to ultraviolet,
but no response to visible light or solar flux,” explained Dupuis.
To improve the sensitivity at ultraviolet wavelengths, the
researchers designed the gallium nitride photodiodes to operate in a
mode that employs avalanche multiplication. The avalanche multiplication
phenomenon is used to multiply normally tiny currents by factors of up
to one million, thus dramatically increasing the device gain.
Avalanche photodiodes can create much larger currents for each photon
compared to normal photodiodes. Once the necessary electric field
strength has been achieved inside the device, the avalanche effect
starts with just one free electron. Since the illuminated photodiode
will contain many free electrons, an avalanche will always occur if the
electric field is large enough.
“One electron-hole pair that is produced by a photon absorption event
creates a million other electron-hole pairs and the current becomes a
pulse of current that you can detect with special electronics,” added
The researchers fabricated high-performance gallium nitride
ultraviolet avalanche photodiodes on bulk gallium nitride substrates
that demonstrate optical gains of 100,000 at ultraviolet wavelengths
from 280 to 360 nanometers.
The gallium nitride device structures were grown by metalorganic
chemical vapour deposition, a technique for depositing thin layers of
atoms onto a semiconductor wafer. Many layers can be built up, each of a
precisely controlled thickness and composition, to create a material
which has specific optical and electrical properties. This is the first
time gallium nitride was successfully used in the fabrication of
photodiodes having ultraviolet optical gains greater than 10,000.
Since demonstrating the feasibility of the photodiodes to exhibit the
avalanche effect, the research team has been developing a more advanced
structure capable of operating as a Geiger-mode detector, so that the
photodiodes are sensitive enough to detect only one photon at a time.
When the Geiger-mode detector is connected to the avalanche circuitry, a
single electron-hole pair can trigger a strong avalanche current to flow
from just one photon.
Yoder, who works on Georgia Tech’s Savannah, Ga. campus, is
developing computer models of the new photodiodes to calculate the
detailed electronic and optical transport. Yoder’s goal is to optimize
the materials and design of the Geiger-mode avalanche detector to assure
optimal, reproducible performance of the avalanche photodiodes.
“Doug’s work is pivotal because these applications don’t require one
working detector, they might require thousands of uniform detectors in
the same chip that all function the same way, so our ability to
manufacture identical photodiodes and detectors is important,” said
With proper manufacturing, these avalanche photodiodes can be used
for more than detecting bioterrorism agents. They can also be used
detect fires, gun muzzle flashes, missile propulsion flames and maybe
even cancer cells, according to Dupuis.