Bulk Bridgman growth of cadmium mercury telluride for IR applications

Abstract

Cadmium mercury telluride (CMT, CDxHg1-xTe) is the pre-eminent infrared material, despite the difficulties associated with the production and subsequent 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. The first technique developed was the basic vertical Bridgman process with typical 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 to 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 composition for the two most common regions required (x=0.21 and 0.3 for 8–14 and 3–5 μm atmospheric transmission windows, respectively) is low. The advantage is that both regions of interest are produced in the same crystal. A further advantage is that segregation of impurities also occurs and leads to low background donor levels in Bridgman material. This Bridgman material is used exclusively for photoconductive IR detectors that require n-type material. The main disadvantages of the Bridgman technique are that material is non-uniform in composition in the radial direction, as well as in the growth direction, and there are numerous grain and sub-grain boundaries. An improved process was developed at BAE Systems based on the accelerated crucible rotation technique (ACRT). Here, growth ampoules are subjected to periodic acceleration/deceleration in their rotation, rather than constant rotation 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, normally by a factor of at least ten-fold. The only drawback is that the material 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 considered. We are currently growing 20 mm diameter, 200 mm long crystals of ∼0.5 kg weight with acceptable uniformity of composition and good electrical properties for current photoconductive detector programs. © 2001 Kluwer Academic Publishers