Cheng proposes infrared detector enhancements

ECE Keh-Yung Cheng has developed a new method to enhance infrared detectors through a modification of their nanostructures.

Cheng said people typically use mercury cadmium telluride alloy for detectors, which can potentially be very hard to make into two-dimensional arrays because of the material’s non-uniformity. A two-dimensional photodetector array, called a focal plane array (FPA), is able to see two-dimensional structures, he said. Researchers are essentially trying to find another material system to make FPAs.

According to Cheng, one way to achieve this improved detector array is through the use of the quantum well structure. The quantum well infrared photocathode (QWIP) is a good device, with only one problem, said Cheng: the quantum well, governed by quantum mechanical principles, cannot detect infrared light coming into the detector unless it is shining at an angle. “That makes it very difficult when you have a detector array which can’t detect normal incident infrared radiation,” said Cheng. “So they have to bend the light in the detector, which cuts down the efficiency.”

To overcome this issue, Cheng and other research groups have investigated the use of the quantum dot infrared photodetector. Quantum dot detectors, which can potentially merge speed and freedom from cooling, still have some weaknesses in terms of the strain which causes the material to deteriorate with the addition of multiple layers of quantum dots.

Cheng consequently has used a different approach through a strain-induced lateral-layer ordering (SILO) process. “We found that we can fabricate quantum wires in the active region of the detector,” said Cheng. “People have tried to do this before, but we’re the first ones to successfully apply this to [a] quantum wire infrared photodetector.” With this approach, the detector is able to identify direct incident infrared radiation, or see head-on images, and has good responsivity over the mid-infrared range.

In addition, Cheng said the technique may be used to make lasers and other devices—essentially anything in which you can apply the quantum wire technology. Lasers and transistors have utilized the quantum wire nature of the material in the past, and the third application may now translate into detectors.

Cheng said it is also possible to adjust the quantum wire dimensions to detect multiple wavelengths instead of a single wavelength. This could lead to significant improvements in military applications relating to night vision and gas detection for environmental monitoring. Devices would allow night vision to differentiate between colors to better identify targets. Anything emitting heat could also be detected: longer wavelengths expose incidences of thermal radiation and can map out temperature differences to locate tumors, Cheng said.

Such findings by Cheng and his students were presented at the North American Molecular Beam Epitaxy Conference this past fall. The technology that makes this improvement possible was actually invented around 15 years ago, Cheng said, but this is the first time it has been applied to the detector. A group from the Academia Sinica in Taiwan has been helping Cheng and his research team take measurements as well.

The first demonstrations of the device have not been perfect, which Cheng said is to be expected. Yet they have been able to show the principle and potential of the technology’s application, he noted. Cheng wants to improve the device design in the future. “We do see some problems associated with the material designs,” he said. “We have pretty much figured out how to improve it, it’s just a matter of if we can get additional funding to continue our research.”

Thus far, his research has been supported by the National Reconnaissance Office, the Army Research Office, and DARPA.