Bulk Bridgman growth of cadmium mercury telluride for IR applications

Cadmium mercury telluride (CMT, CdxHg1-xTe) is the pre-eminent infrared mat erial, despite the difficulties associated with the production and subseque nt processing of this ternary compound. By varying the x value the material can be made to cover all the important infrared (IR) ranges of interest. T he first technique developed was the basic vertical Bridgman process with t ypical crystal dimensions of 13 mm diameter and 150 mm length. We found it necessary to purify both the mercury and the tellurium on-site before use t o obtain the required electrical properties. There is marked segregation of the matrix elements in Bridgman growth that is both a disadvantage and an advantage. Its disadvantage is that the yield of material in terms of compo sition for the two most common regions required (x=0.21 and 0.3 for 8-14 an d 3-5 mum atmospheric transmission windows, respectively) is low. The advan tage is that both regions of interest are produced in the same crystal. A f urther advantage is that segregation of impurities also occurs and leads to low background donor levels in Bridgman material. This Bridgman material i s used exclusively for photoconductive IR detectors that require n-type mat erial. The main disadvantages of the Bridgman technique are that material i s non-uniform in composition in the radial direction, as well as in the gro wth direction, and there are numerous grain and sub-grain boundaries. An im proved process was developed at BAE Systems based on the accelerated crucib le rotation technique (ACRT). Here, growth ampoules are subjected to period ic acceleration/deceleration in their rotation, rather than constant rotati on as in the Bridgman process. The major effect of this is to stir the melt during growth and produce flatter solid/liquid interfaces. This, in turn, improves the radial and axial compositional uniformity of the material, nor mally by a factor of at least ten-fold. The only drawback is that the mater ial is now p-type as grown and must be annealed in mercury vapor to convert it to n-type. An additional marked advantage of ACRT is that the improved radial compositional uniformity enables larger diameter material to be cons idered. We are currently growing 20 mm diameter, 200 mm long crystals of si milar to0.5 kg weight with acceptable uniformity of composition and good el ectrical properties for current photoconductive detector programs. (C) 2001 Kluwer Academic Publishers.