Improved infrared photodetector technology that uses multiple ultrathin layers of materials should have an impact on critical applications ranging from national defense to medical diagnostics. Arizona State University researchers have discovered how infrared photodetection can be achieved more effectively using certain materials arranged in specific patterns in atomic-scale structures. The scientists formed crystals on multiple nanometer-thick layers of materials, then combined the layered structures to form “superlattices.” Photodetectors made of various crystals absorb different wavelengths of light and convert them into an electrical signal. The conversion efficiency achieved determines a photodectector’s sensitivity and the quality of detection it provides, said electrical engineer Yong-Hang Zhang. Time-resolved photoluminescence measurements on an InAs/InAs0.72Sb0.28 T2SL at 77 K for excess carrier densities ranging from 4.0x1015 to 1.0x1017 cm-3. (Reprinted with permission from E.H. Steenbergen et.al., Appl. Phys. Lett 99, 251110 [2011]. Image copyright 2011, American Institute of Physics) The detection wavelengths of superlattices can be broadly tuned by changing the design and composition of the layered structures. The precise arrangements of the nanoscale materials in superlattice structures help to enhance the sensitivity of infrared detectors, Zhang said. The team combined indium arsenide and indium arsenide antimonide to build the superlattice structures. The combination allows devices to generate photoelectrons necessary to provide infrared signal detection and imaging, said Elizabeth Steenbergen, an electrical engineering doctoral student who performed experiments on the superlattice materials with collaborators at the Army Research Lab. Tranmission electron micrograph of several periods of InAs/InAsSb SL. (Photo copyright David J. Smith, ASU) “In a photodetector, light creates electrons,” Steenbergen said. “Electrons emerge from the photodetector as electrical current. We read the magnitude of this current to measure infrared light intensity.” The scientists’ use of the new materials reduces the loss of optically excited electrons, which increases the electrons’ carrier lifetime by more than 10 times that achieved with other combinations of materials traditionally used in the technology, Zhang said. Carrier lifetime is a key parameter that in the past has limited detector efficiency. The infrared photodetectors made from these superlattice materials are also advantageous because they do not need as much cooling. Such devices are cooled as a way of reducing the amount of unwanted current inside the devices that can “bury” electrical signals, Zhang said. The need for less cooling means that less power is needed to operate the photodetectors, making the devices more reliable and the systems more cost-effective. Night vision camera installed on a car dashboard. The new material developed by ASU has the potential to improve such applications in the future. (Photo copyright Daimler AG) Researchers say that improvements can still be made in the layering designs of the intricate superlattice structures and in developing device designs that will allow the new combinations of materials to work most effectively. The advances could pave the way for improved guided weaponry, sophisticated surveillance systems, industrial and home security systems, infrared detection for medical imaging, and road-safety tools for driving at night or during sandstorms or heavy fog. The research appeared in Applied Physics Letters. For more information, visit: www.engineering.asu.edu