Bulk growth of near-IR cadmium mercury telluride (CMT)

Abstract

Cadmium mercury telluride (CMT, CdxHg1−xTe) is still the pre-eminent infrared material, despite the difficulties associated with its production and subsequent processing. By varying the x value, the system can be made to cover all the important infrared (IR) ranges of interest. The two most common regions required are x∼0.21 and 0.3 for 8–14 and 3–5 μm atmospheric transmission windows, that is, long wave, LW, and mid wave, MW, respectively. Recently we have extended the growth process to produce both very long wavelength and near-IR material for various applications. This paper focuses on the work undertaken to produce near-IR material, where higher starting x values are used. Growth takes place in simple 2-zone furnaces with the pure elements contained in thick-walled high-purity silica ampoules. The thick ampoule walls are needed to contain the high (up to ∼70 atm) mercury vapor pressures within the ampoules. An improved ampoule seal-off procedure was developed to enable us to grow at the higher temperatures (hence higher pressures) needed for these higher x start crystals. The accelerated crucible rotation technique (ACRT) modification to the basic Bridgman process is used to grow the crystals. Here, ampoules are subjected to periodic acceleration/deceleration in their rotation, rather than constant rotation as in the Bridgman process, which stirs the melt during growth and produces flatter solid/liquid interfaces. This, in turn, improves the radial and axial compositional uniformity of the material. An additional advantage of ACRT is that the improved radial compositional uniformity enables larger diameter material to be considered. We are currently growing 20 mm diameter, 200 mm long crystals of ∼0.5 kg weight with good uniformity of composition. The assessment of the near-IR material has included wavelength mapping of both radially cut slices and axially cut planks. The latter gives useful information on the shape and change in the solid/liquid interface as growth proceeds. Quenching experiments reveal actual solid/liquid interfaces that confirm the findings of the wavelength mapping. Images taken with an IR camera reveal features in slices, for example, cracks, inclusions of second phase and swirl patterns, the origin of the latter is unknown.