Large volume single crystal growth of Cadmium Zinc Telluride with minimal secondary phases for room temperature radiation detector application
Swain, Santosh Kumar
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Two major aspects of Cadmium Zinc Telluride (CdZnTe) crystal growth for room temperature radiation detection application namely tellurium rich second phase defects and single crystal yield have been addressed. Various approaches were considered towards the minimization of these defects both during growth and post growth thermal treatment in cadmium environment. Since the issue of retrograde solubility in CdZnTe ternary alloy system causes tellurium precipitation, different cooling mechanisms were also devised to achieve minimal secondary phases. Some important and encouraging results were obtained relative to the size and distribution of secondary phases upon differently cooling the crystals after growth. Thermomigration of tellurium were also observed while post processing samples in a temperature gradient, in (Cd,Zn) atmosphere. Results indicated orders of magnitude reduction on secondary phases at the expense of sample resistivity. Apart from the issue of secondary phases, CdZnTe also suffers from low single crystal yield. Since grain boundaries are known to hinder the transport properties of charge carriers, it is necessary to have large single crystal volumes with good uniformity for better charge collection efficiency. In our crystal growth facility, single crystal volumes up to 25x25x20 mm3 have been grown using unseeded vertical Bridgman technique. However reproducibility has been a major issue as far as single crystal yield is concerned. In order to consistently produce large volume detector grade material, seeding was attempted in vertical Bridgman set up. Certain important requirements for successful seeded growth are partial melting of the seed from top, proper melt mixing before starting growth and maintaining a convex interface shape to promote outward grain growth. To achieve these conditions simultaneously, we have taken extreme care in designing our experimental set up. Seed crystals with various orientations have been used in different growths. Preliminary results on these growths indicate success in achieving partial seed melting and proper control on the thermal environment. However the grown crystals exhibit large number of twins suggesting that the use of seed crystal with proper orientation is critical.