Photonic Crystal Improves IR Sensor Performance
Demonstration device employs dots-in-a-well architecture.
Daniel S. Burgess
Scientists at the University of New Mexico in Albuquerque and at California Institute of Technology in Pasadena have reported that the addition of a two-dimensional hexagonal photonic crystal to a quantum dots-in-a-well infrared photodetector improves its performance by more than an order of magnitude. They note that the method should be suitable for use with a variety of IR sensors with applications in reconnaissance and rescue, medical diagnostics and firefighting.
Sanjay Krishna, an assistant professor of electrical and computer engineering at the university’s Center for High Tech Materials, said that the technique is particularly promising for use with long-wavelength IR sensors. The holes of a crystal optimized for operation with an 8- to 10-μm device, for example, are on the order of 2 to 3 μm in diameter, making fabrication straightforward using standard optical lithography techniques. It also may be possible to introduce defects into particular elements of the structure to favor the response to photons of specific energies or to obtain polarization information, effectively creating a multi- or hyperspectral detector from a single focal plane array.
In the proof-of-concept experiment, the researchers employed a dots-in-a-well structure comprising N-doped InAs quantum dots in InGaAs quantum wells sandwiched between GaAs, which they grew by solid-source molecular beam epitaxy. They employed electron-beam lithography to etch the photonic crystal region through the top face of the device into the active region, choosing a lattice spacing of 2.4 μm for optimum performance at 8.1 μm.
Using a Thermo Nicolet 870 Fourier transform IR spectrometer and a Stanford Research Systems SR760 fast Fourier transform spectrum analyzer with a calibrated blackbody at 800 K, they compared the spectral response, photocurrent density, noise and conversion efficiency of the sensor with those of a similar device without the modification. They found that the response of the sensor with the photonic crystal was up to a factor of five higher in the range of 6 to 10 μm, and that the photocurrent density was more than an order of magnitude higher with no change in dark current.
The signal-to-noise ratio and the conversion efficiency also were more than an order of magnitude higher, with the conversion efficiency at a bias voltage of –2.6 V increasing from 7.5 percent in the unmodified device to 95 percent in the sensor with the photonic crystal region. Krishna noted that it may be possible to further improve the performance by exciting additional modes of the crystal.
Optimizing the modified sensors thus will require a better understanding of how to couple the incoming radiation into the photonic crystal as desired. For example, Krishna said, the scientists are investigating coupling into band-edge or defect modes for broadband and polarization-sensitive or narrowband imaging applications, respectively. Nevertheless, he is confident that the group will be cooperating with industry to demonstrate a thermal camera based on the technology.
Applied Physics Letters, April 10, 2006, 151104.
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