Researchers at Northwestern University have developed a new infrared imaging system that delivers a 16-fold increase in resolution over long wavelength infrared radiation (LWIR) cameras currently used in industrial, security and nighttime surveillance applications. Based on a type of semiconductor called a Type-II InAs/GaSb superlattice, the IR camera is mercury-free, more robust, cheaper to produce and can collect 78 percent of the light showing temperature differences as small as 0.02° C.
Superlattices, or quantum well structures as they are also known, have been known about for many years, but it has taken until now for the materials to be better understood and utilized. Superlattices are a layered periodic structure of two or more semiconducting elements each with a thickness of only a couple of nanometers. They are thermal conductors that use photon diffusion and scattering to determine heat variations. Superlattices are broken into three types according to their chemical composition with type-II formed from layers of Indium (In), Arsenic (As), Gallium (Ga) and Antinomy (Sb).
"Type-II [superlattice] is a very interesting and promising new material for infrared detection," Professor Manijeh Razeghi from the Center for Quantum Devices in the McCormick School of Engineering and Applied Science said. "Everything is there to support its future: the beautiful physics, the practicality of experimental realization of the material. It has just taken time to prove itself, but now, the time has come."
Using the Type-II InAs/GaSb superlattices as an alternative to existing LWIR cameras the researchers have been able to achieve a 16-fold increase in the number of pixels in an IR image. Current LWIR cameras are based on mercury cadmium telluride (MCT) materials, but the Type-II superlattice is mercury-free, more durable, and stands to reduce camera cost once the technology is commercially manufactured.
"Not only does it prove Type-II superlattices as a viable alternative to MCT, but also it widens the field of applications for infrared cameras," Razeghi said. "The importance of this work is similar to that of the realization of mega-pixel visible cameras in the last decade, which shaped the world's favor for digital cameras."
Their results were recently published in the journal Applied Physics Letters.