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Matching dots and holes to build a better photodetector

BioPhotonics
Sep 2010
Hank Hogan, hank.hogan@photonics.com

TROY, N.Y. – 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.”


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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