Assessment of the Modulation Transfer Function in Infrared Focal Plane Arrays

There is a perpetual drive in the infrared (IR) detector community to fabricate detectors with smaller pixel pitches where the ultimate goals are 5 µm for LWIR imaging and 3 µm for MWIR imaging. Such small pixel pitches ideally will result in significantly improving the modulation transfer function (MTF), especially at longer spatial frequencies. The MTF is a key figure of merit (FOM) in all cameras that describes how well an optical system reproduces an objects contrast in the image at different spatial frequencies. However, high performance IR detectors have historically been fabricated out of materials such as InSb or HgCdTe. These semiconductors have material properties that are isotropic, e.g., the hole mobilities along the in-plane and growth directions are equal. However, IR absorbing materials with anisotropic material properties, such as type II superlattice (T2SL) materials, based on InAs/GaInSb or InAs/InAsSb, have become increasingly prevalent. Specifically, the hole mobility is much larger along the in-plane direction than the growth direction. These detectors with anisotropic material properties have been predicted to have higher inter-pixel crosstalk and degraded MTF compared to geometrically identical detectors except with isotropic material properties. The MTF as a function of pixel pitch is analyzed, comparing detectors with isotropic versus anisotropic material properties, to assess the potential of using T2SL detectors with pixel pitches less than 10 µm for imaging.