Assessment of the Modulation Transfer Function in Infrared Detectors With Anisotropic Material Properties: Type-II Superlattices

Historically, high-performance infrared (IR) detectors have 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, which directly impacts detector performance. The impact of utilizing T2SL materials with anisotropic material properties is assessed by simulating the crosstalk and modulation transfer function (MTF) in sequential two-color MW/LW T2SL focal plane arrays. The MTF is a key figure of merit (FOM) in all cameras which describes how well an optical system reproduces an object's contrast in the image at different spatial frequencies. The detector MTF depends on numerous parameters, especially material transport parameters such as the mobility, and as such the MTF is sensitive to the mobility anisotropy. This dependence, and the MTF already being a valuable FOM, makes the MTF a natural metric to assess the impacts of adopting an absorbing material with anisotropic material properties.