Very long wave infrared detectors with improved low temperature performance

Very long wave infrared (VLWIR) detectors with targeted cut-offs (50% of peak response) in excess of 14μm at 70K will be described. Historically these detectors would achieve better than Rule 07 at higher operating temperatures (greater than about 80K) but can deviate significantly from Rule 07 with deeper cooling due to trap assisted tunnelling currents becoming dominant. In the current work, bandgap engineering and controlled doping profiles are used to control the electric field profile and bandgap to reduce tunnelling probability. The advantages of Leonardo’s MOVPE and mesa technology have been described previously. The ability to grow heterostructures allows bandgap engineering to enhance VLWIR performance. Diffusion currents can be limited by restricting the volume of the absorbing region with higher Cd mole fraction cladding layers. Junctions can be placed in the higher bandgap material to reduce both generation via traps within the depletion layer and tunnelling, the latter by keeping the peak field away from the absorber. The placement of the junction and the control of doping levels is such that the minority carrier barrier created is depleted through at low voltage. Within the absorber itself, the ability to have controlled low doping limits Auger generation. Mesas provide a route for optical concentration. Reducing the junction area further reduces leakage currents, while allowing the full optical area determined by the pixel pitch. Ray tracing predicts an optical concentration of 5x is achievable for a pixel pitch of 16 μm, with path lengths through the absorber enhanced by slanted passes and multiple reflections. This allows a reduced absorber thickness compared to planar designs and a further reduction in the generation volume for diffusion currents. These innovations have enabled improved performance at lower operating temperatures. Quantum efficiency, spectral response and dark current including comparison with Rule07 is reported.