Infrared Detectors Based on New Types of Devices and Materials

Infrared detectors working at photon wavelengths beyond the visible are an essential component of any modern day defense and security installation. Not only do they provide a night vision capability, but they can also be designed to operate at wavelengths that are less sensitive to atmospheric absorption, turbulence and particulates, providing a clear image that cannot be attained using visible light detectors.
For the best high-end devices, the traditional method of fabrication has been based on cooled Mercury Cadmium Telluride (MCT) photodiodes. MCT is an expensive II-VI material and its photodiodes are often limited by non-uniformities and poor stability. At SCD we have therefore adopted an alternative approach based on more manufacturable III-V materials. Using advanced III-V heterojunction architectures, it is possible to achieve performance comparable with the MCT alternative, both with respect to operating temperature and wavelength tunability. The principles of a barrier device will be described which enable high operating temperatures to be reached, because the barrier suppresses the major source of dark current noise due to Shockley-Reed-Hall (SRH) traps. This barrier architecture can be implemented using multi-layer superlattice materials with a nanometer scale period that allow the operating wavelength of the detector to be adjusted over virtually the whole of the atmospheric infrared transparency range. A quantum mechanical model will be described, which is based on an advanced implementation of kp theory. This model can simulate both the band alignments of the absorber and barrier materials for correct device design, and the full spectral response and dark current of the complete detector, for matching its performance to the system requirements. A novel design has also been developed for a next generation "dual band" architecture operating in two different wavelength ranges at essentially the same time. A simulation of the expected performance will be presented.