Advances In (Hg,Cd)Te Materials Technology

Abstract Mercury cadmium telluride, Hgi_xCdxTe, a direct band-gap semiconducting alloy with the band-gap adjustable by varying the alloy composition, has become the material-of-choice for innumerable infrared-detection appli­ cations over the 2 to 20 micrometer spectral range. N-type, p-type and intrinsic material can be readily pre­ pared, either by doping or by adjustment of alloy stoichiometry, greatly enhancing the versatility of this alloy system.Material requirements for the next generation of infrared detectors, large area two-dimensional arrays, have placed unprecedented demands on Hgi_xCdxTe materials technology. Major research efforts are being con­ ducted which address this need including epitaxial and bulk crystal growth methods, both on earth and in space.The difficulties encountered in preparing detector grade Hgi_xCdxTe will be discussed, together with the present state-of-the-art. Emphasis will be on bulk crystal growth.IntroductionIntrinsic quantum detectors highly sensitive to infrared radiation with wavelengths ranging from 2 to 20 micrometers can be prepared from Hgi_xCdxTe alloys. This versatility of the (Hg,Cd)Te (Hgi_xCdxTe where x determines alloy composition) alloy system results primarily from two (Hg,Cd)Te material properties. First, (Hg,Cd)Te is a direct band-gap alloy for which the energy gap is adjustable from 0 to 1.6 eV, depending on alloy composition (x). Consequently, a desired energy gap can be precisely specified. Second, (Hg,Cd)Te alloys can be prepared either p-type, intrinsic, or n-type. No other alloy system is known to have these com­ bined materials properties. Therefore, considerable impetus exists to prepare high-quality (Hg,Cd)Te crystals of uniform composition.As device technology has advanced from single-element to linear multielement arrays, and finally to large two-dimensional multielement arrays, requirements for detector quality (Hg,Cd)Te materials have expanded to larger physical dimensions. The (Hg,Cd)Te materials technology has been particularly strained to meet the material requirements for the large two-dimensional multielement arrays.The (Hg,Cd)Te alloys can be prepared as thin films or as bulk crystals. Research and development involving each of these preparation methods have been conducted by many investigators in an effort to meet the advancing materials requirements.In the following sections, different growth methods with their advantages and limitations will be described briefly. Emphasis will be on highly developed growth methods where recent technological advances have occurred. It will be shown that both thin-film and bulk-crystal growth methods show considerable promise for satisfying materials requirements for the next generation of large-area (Hg,Cd)Te infrared detectors.Thin film growth of (Hg,Cd)TeThin-film preparation methods are attractive because of their useful geometric form, because of the possi­ bility of producing heterostructures tailored for a specific application, and because growth of p-n junctions is possible. Numerous techniques for film preparation including sputtering 1»2, evaporation^, vapor-phase epitaxy4>5j liquid-phase epitaxy^*7>&, current controlled epitaxy^, and others have been attempted. Only growth methods that yield large mono-crystalline films will be discussed here.Extreme difficulty is encountered in maintaining alloy stoichiometry when using the evaporation and sputtering methods of preparation. Films are generally Hg-deficient because of Hg loss to the vapor. Little development of these growth methods has occurred.Vapor-phase epitaxy and current-controlled epitaxy have significant potential. However, both are in early stages of development. Current-controlled epitaxy is particularly attractive because of the low temperatures involved ( « 300° C), the possibility of direct growth of p-n junctions, and the virtual elimination of substrate-film interdiffusion.LPE from Te-rich solutionsThe most highly developed film preparation method is liquid-phase epitaxy from tellurium-rich solutions. Inherent advantages of growth from tellurium-rich solutions are derived from the relatively low temperatures and pressures necessary for crystal growth. Interaction of the prepared film with the substrate is reduced, minimizing impurity contamination and compositional variations resulting from interdiffusion. In addition,