Progress in bulk cadmium mercury telluride over the last 25 years

It is ~50 years now since the first development of the ternary compound cadmium mercury telluride (MCT) in bulk crystal form for infrared (IR) applications. MCT (Cd _x Hg_1–x Te) is still the pre-eminent infrared material, and by varying the x value the material can be made to cover all the important IR ranges of interest (i.e. 1–3, 3–5 and 8–14 μm). The first half of its history really concerned the use of bulk-grown slices to produce a wide range of photoconductive (PC) IR detectors for various applications in the longwave (8–12 μm), midwave (3–5 μm) and shortwave (1–3 μm) regions. The second half of its history has seen its main function supplanted by various forms of epitaxial growth, particularly liquid phase epitaxy, metal-organic vapor phase epitaxy and molecular beam epitaxy, to produce the more versatile photovoltaic focal plane arrays (FPA) currently being manufactured. However, there have been small pockets of activity across the world during the last 25 years studying defects, growth issues, some residual PC detector work and lately its use in large-area optical components that give protection to MCT FPAs in niche applications. It is this later period that forms the focus of this paper.