Leading-edge longwave-infrared (LWIR) photodetectors are typically based on mercury cadmium telluride (HgCdTe, or MCT), leading to high sensitivity and detection speeds, especially in the form of avalanche photodiodes (APDs). But MCT is a tough material to grow consistently; in addition, MCT APDs have low gain, necessitating high bias voltages and leading to excess spectral noise. Two-dimensional materials such as graphene, black phosphorous, and black arsenic phosphorous show some promise, but also have various disadvantages. Manijeh Razeghi and her group at Northwestern University (Evanston, IL) are taking another approach to LWIR device design that has produced the first gain-based LWIR photodetector using band-structure engineering based on a type-II superlattice material.
This new design, which, cooled to 77 K, demonstrated LWIR photodetection of 1284 A/W and a specific detectivity of 2.34 × 1013 cm Hz1/2/W at a peak detection wavelength of 6.8 μm during testing and a 1/e cutoff wavelength of 8 μm, could lead to new levels of sensitivity for next-generation LWIR photodetectors and focal-plane-array (FPA) imagers. The work could have applications in earth science and astronomy, remote sensing, night vision, optical communication, and thermal and medical imaging.
The type-II superlattice is a quantum-mechanics-based material system known for its outstanding growth uniformity and exceptional band-structure engineering (the ability to control the material’s bandgap). Razeghi’s team used the new material in a heterojunction phototransistor device structure, a detection system known for its high stability, but one previously limited to shortwave and near-infrared detection. During testing, the type-II superlattice allowed each part of the photodetector to be carefully tuned to use the phototransistor to achieve high optical gain, low noise, and high detectivity. Reference: A. Dehzani et al., Light Sci. Appl. (2021); https://doi.org/10.1038/s41377-020-00453-x.
