at the University of Southern California and the University
of Texas at Austin have built and tested a device
based on nanostructures called quantum dots that can
sensitively detect infrared radiation in a crucial
wavelength range. Quantum dot IR receptor unit.
The atmosphere is opaque to most infrared, but it
is transparent for a narrow "window" between
8 and 12 microns. Night vision goggles, military target
tracking devices and environmental monitors utilize
this range of wavelengths.
Anupam Madhukar, holder of the Kenneth T. Norris Chair
in the USC Viterbi School of Engineering with appointments
in the departments of materials science, biomedical
engineering and physics, says "a class of existing
infrared detectors are based on what is called 'quantum
well' technology. But we have created a detector based
on different physics--quantum dot physics--that works
at least as well and has the potential to perform
Madhukar worked with Joe C. Campbell, who holds the
Cockrell Family Regents Chair in the UT Austin College
of Engineering's department of electrical and computer
engineering. The two engineers described the device
in the April 24 issue of Applied Physics Letters.
The device uses self-assembled "quantum dots,"
island-like pyramidal structures made of semiconductors.
Each dot has a core of indium arsenide surrounded
by gallium arsenide and an indium-gallium arsenide
alloy. A single dot is approximately 20 nanometers
(2 millionths of a centimeter) in base size and about
4 nanometers in height.
The three-dimensional confinement of electrons within
these structures creates unique, characteristic behavior.
By using varying proportions of the materials and
changing synthesis procedures, engineers can tailor
quantum dots for use in lasers, detectors, optical
amplifiers, transistors, tunneling diodes, and other
"Quantum dots are emerging as the most viable
semiconductor nanotechnology for future higher performance
communication systems, biomedical imaging, environmental
sensors, and infrared detection," said Madhukar.
Unlike their alternatives, quantum dot infrared detectors
strongly absorb radiation shining perpendicular to
the plane of an array of quantum dots.
By contrast, the alternate quantum well detectors
don't pick up radiation that shines straight down
on them. To achieve this "necessitates additional
processing steps," Madhukar said. This increases
the cost of the well detectors.
When the engineers benchmarked the new device using
standard tests, its detectivity was nearly 100 times
higher than the previously reported peak for quantum
dot systems. The new range is competitive with the
corresponding values for the well-established quantum
well infrared photo detectors.
"It is about an order of magnitude lower than
a third technology, mercury-cadmium-telluride material
based infrared detectors. These now provide the best
available performance, but suffer from materials uniformity
and long-term stability issues," said Campbell.
The researchers expect that placing the dot arrays
in a configuration called a "resonant cavity,"
which traps the radiation and bounces it back and
forth between mirroring walls, will make them more
The U.S. Air Force Office of Scientific Research supported
the research under the U.S. Department of Defense
sponsored Multidisciplinary University Research Initiatives
(MURI) Program in Nanoscience.
Contacts: A. Madhukar, USC, 213-740-4323; Joe C. Campbell,
UT Austin, 512-471-9669