Matching dots and holes to build a better photodetector
Matching quantum dots to properly sized microscopic holes in a gold
film could be the key to building a new type of infrared photodetector, researchers
report. They say it would have sensitivity equal to or better than today’s
detectors, yet offer greater image uniformity and stability over a large sensing
area.
These photodetectors potentially would enable some important medical
applications, team leader Shawn-Yu Lin said. “Such superior and reliable imaging
is essential for imaging of certain cancer cells, such as skin cancer and breast
cancer. It also could be used for infrared circulation diagnosis and evaluation.”
A new infrared photodetector (top), created by integrating quantum dots with a gold
film two-dimensional hole array (bottom), could be useful for medical diagnostics.
Courtesy of Shawn-Yu Lin, Rensselaer Polytechnic Institute.
In fabricating a proof-of-concept device based on the new scheme,
Lin, a professor of physics at Rensselaer Polytechnic Institute, worked with researchers
from the University of New Mexico and the Air Force Research Laboratory, both in
Albuquerque. The investigators outlined their results in a Nano Letters paper published
earlier this year.
In their research, they integrated indium arsenide (InAs) quantum
dots in a gold two-dimensional hole array. This arrangement created a strong interaction
between the quantum dots and the gold-film plasmons, which are electron oscillations.
At the plasmonic resonance, this interaction leads to a 130 percent increase in
the infrared response of the photodetector. Further refinement of the device could
drive that enhancement figure even higher, Lin said.
Their quantum dot infrared photodetector consisted of a stack
of 30 InAs quantum dots embedded inside indium gallium arsenide (InGaAs) quantum
wells, with everything sitting atop a semi-insulating GaAs substrate. The total
thickness of the 30 layers was 2 µm. The stack was optimized for dual-band infrared
absorption, with one peak at ~5 μm and the other at ~9 μm.
On top of the photodetector, they created a 2-D hexagonal hole
array on a 50-nm-thick gold film that measured 300 μm on a side. The array
had a lattice constant – the spacing from center to center along the sides
– that they manufactured at distances ranging from 2.5 to 3.72 μm. They
fabricated the arrays with holes of a diameter from 1.0 to 2.05 μm. In this
way, they produced arrays with various combinations of lattice constant and hole
sizes.
In their research, they illuminated the device with infrared light
and measured the photocurrent it produced. They found that the lattice constant
and hole diameter affected the photodetector enhancement. The right values maximized
the light transmitted through the array and the plasmonic-quantum dot interaction,
leading to the best possible response. What’s more, the increase in signal
came without an increase in noise.
Other parameters that they could have tweaked were the thickness
of the film and the quantum dot size. Those would have changed the photodetector’s
behavior by, for example, adjusting the wavelengths to which it responded.
Lin said he has had many inquiries from industry about the device.
The researchers intend to patent the technique.
For commercial use, the device would need a tenfold, and not 1.5x,
improvement in performance. This is achievable by matching the hole array and film
thickness with quantum dots, Lin said, which would make the device as sensitive
as a conventional detector. But, he added, it will have “better uniformity
and stability over a 2- to 3-in. wafer size for superior imaging over a large area.”
Published: September 2010