11. Infrared Detectors

Tellurium-based compounds such as cadmium telluride (CdTe) and mercury cadmium telluride (HgCdTe) have been used as infrared (IR) detectors for over half a century. These versatile narrow gap semiconducting materials are characterized by a direct energy gap and have the ability to obtain both high and low carrier concentrations, high electron mobility of electrons, and low dielectric constant. Nanophotosensors with cadmium chalcogenide (Te, Se, and S) semiconductor nanocrystals are considered to be best candidates to detect spacecraft cracks without increasing payload or changing the thermal properties of heat-shielding of spacecraft. Hg_1−x Cd _x Te (MCT) is the most widely used infrared (IR) detector material in military applications, compared to other IR detector materials, primarily because of two key features: it is a direct energy band gap semiconductor and its band gap can be engineered by varying the Cd composition to cover a broad range of wavelengths. A small change of lattice constant with composition makes it possible to grow high-quality layers and heterostructures. These can thus be used for detectors operated at various modes, and can be optimized for operation spanning the wide range of the IR spectrum (short-wave infrared (SWIR): 1–3 μm, middle wavelength IR (MWIR: 3–5 μm; long-wavelength IR: 8–14 μm) to very long-wave infrared (VLWIR): 14–30 μm, and at temperatures ranging from that of liquid helium to room temperature. Other specific advantages include a direct energy gap, ability to obtain both low and high carrier concentrations, high mobility of electrons, and low dielectric constant. However, in spite of the various advantages, the material suffers from technological disadvantages partly due to the presence of a weak Hg–Cd bond, which results in bulk, surface, and interface instabilities. Uniformity and yield are still issues especially in the long-wavelength infrared (LWIR) region. Nevertheless, these are leading candidates for IR photoconductive and photovoltaic detector materials in particular for military and space applications. This chapter reviews the development and applications of these materials and competitive technologies for IR detection.