Dual band LWIR/VLWIR type-II superlattice photodiodes.

Multiband detection capability is a critical attribute of practical infrared (IR) sensing systems for use in missile defense detect-and-track applications. This capability, already demonstrated in mercury-cadmium telluride (MCT) photodiodes and quantum well infrared photodetectors (QWIPs), has not previously been explored in type II-superlattices (T2SLs), a newer system which is under consideration to meet next-generation sensor needs. Like QWIPs, T2SLs are composed of layers of III-V compound semiconductors grown by molecular beam epitaxy (MBE), and have an infrared gap that is determined primarily by the layer thicknesses. With the exceptional control of MBE over layer thicknesses and the ability to grow multiple bandgap structures under compatible growth conditions, T2SL-based multiband IR focal plane arrays (FPAs) are expected to have advantages in spectral control and pixel-to-pixel uniformity over MCT. Additionally, T2SLs have intrinsically higher quantum efficiency than QWIPs, in which the optical selection rules for intersubband transitions forbid the absorption of normally incident light. Here we describe the first results for a T2SL dual band detector with independent long-wave and very-long-wave infrared responsivity bands, with cutoffs of 11.4 and 17 mu m respectively. The p-n-p device contains "W"-structured T2SL (WSL) active regions for enhanced band selectivity, owing to the quasi-two-dimensional density of states for WSLs. Photodetector results are demonstrated using a maskset designed to fabricate single-band diodes, 3-terminal dual band devices, and 2-terminal band selectable devices to comply with different dual band FPA read-out architectures.