Quantum-Dot Focal Plane Array Has Two-Color Capability
Anne L. Fischer
Mid- (3 to 5 μm) and long-wave (8 to 12 μm) infrared focal plane arrays have found a place in numerous security and military applications that require night-vision capability. Single-color detectors have been common, but monolithic two-color devices offer several advantages, such as the ability to detect the absolute temperature map of a scene.
Two-color detectors have been demonstrated using mercury cadmium telluride (MCT) and quantum-well detectors. Now a group at the University of New Mexico in Albuquerque — in collaboration with researchers from its start-up company, Zia Laser Inc., the University of Texas at Austin and BAE Systems in Nashua, N.H. — has developed a two-color detector based on quantum dots.
The focal plane array made of self-assembled quantum dots acquired this thermal image of a hand at 300K in the background and a soldering iron in the foreground. Courtesy of Sanjay Krishna, University of New Mexico.
One problem in developing MCT focal plane arrays has involved finding large-area homogeneous materials, a result of fundamental variations during crystal growth. Using quantum wells avoids this problem but encounters another: The detectors suffer from a lack of normal-incidence absorption. Nanoscale quantum-dot detectors have been promising because they can be grown on large GaAs wafers with excellent uniformity and offer normal-incidence operation.
In the new work, the research team used self-assembled quantum dots in the two-color, 320 × 256 infrared focal plane array. The approach offers several advantages over quantum-well IR photodetectors, such as lower dark current, normal incidence detection, higher responsivity and improved radiation hardness. The two-color detector uses a voltage-tunable InAs/InGaAs/GaAs dot-in-well structure in which the InAs quantum dots are placed in an InGaAs well, which is then placed in a GaAs matrix.
The investigators tested the detector by taking thermal images at a temperature of 80 K. They used various optical filters to demonstrate operation at 3 to 5 μm and at 8 to 12 μm. The results show promise for quantum-dot focal plane arrays in applications where low registration error is important, such as in the imaging of a quickly changing scene.
They plan to improve performance by working on strain-compensated designs, which will allow for more absorbing layers. They also plan to optimize the process and integration by using resonant cavities to develop detectors with higher operating temperatures.
Applied Physics Letters, May 9, 2005, 193501.
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