Mid Infrared Focal Plane Arrays with Nanoscale Quantum Dots and Superlattices

Presently, the state of the art photon detectors for the mid wave infrared (MWIR, 3-5 mum) and long wave infrared (LWIR, 8-12 mum) are based on interband transitions in bulk indium antimonide (InSb) and HgCdTe alloys respectively. Two emerging technologies for next generation sensors that offer enhanced functionality include (i) intersubband detectors based on nanoscale quantum dots and (ii) interband transitions in the Type II InAs/GaSb strain layer superlattice system. Infrared detectors based on InAs/GaSb strain layer superlattices (SLSs) appear as a promising alternative to the present-day infrared detection technologies. SLSs offer numerous advantages over existing detector technologies, including better uniformity, reduced tunneling currents, normal incidence absorption and suppressed Auger recombination. SLSs are characterized by the broken-gap type-II alignment. Presently all SLSs detectors are based on a photodiode (p-i-n or n-i-p) design. During the conventional fabrication process of photodiodes, the deep etch through the absorbing region is utilized in order to define the optical area of the detector. Electronic surface states within the energy band gap of SLS are generated, resulting in large surface leakage currents. The suppression of these currents is the most demanding challenge for the SLS technology. We have recently fabricated a high performance InAs/GaSb SLS detector with a P on N polarity and a 320 times 256 MWIR FPA with a noise equivalent temperature difference of 24 mK at 77K. In the quantum dots in a well (DWELL) heterostructure, InAs quantum dots are placed in a thin InGaAs quantum well that is in turn placed in a GaAs matrix. Three-color DWELL detectors, operating at 78K, with spectral response in the MWIR (lambda
_p1
~ 4 mum), LWIR (lambda
_p2
~ 8 mum) and VLWIR (lambda
_p3
~ 23 mum) regime have been fabricated in our group. Recently, we have fabricated the first long wave infrared and two-color quantum dot focal plane array (320times256 pixels).