White beam X-ray Diffraction Topography (WBXDT) Studies of Bridgman Grown CdZnTe Crystals
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White beam X-ray Diffraction Topography (WBXDT) Studies of Bridgman Grown CdZnTe Crystals Stephen Babalola1, Samuel Uba1, Anwar Hossain2, Giuseppe Camarda2, Ralph James2, Trent Montgomery1 1 Alabama A&M University, Normal, AL 35811, U.S.A. 2 Brookhaven National Laboratory, Upton, NY 11973. ABSTRACT CZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, we are yet to overcome the technological difficulties in production of large size, defect-free CZT crystals. The most common problem is accumulation of Tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. We employed high energy transmission X-ray diffraction techniques to study the quality of the CdZnTe crystals grown by Bridgman Technique. Crystallinity and defects within two different growth set-ups, i.e. with and without choked seeding, were compared by imaging the crystal orientation topography with white beam X-ray diffraction topography (WBXDT). The X-ray diffraction topography results show high correlation with large-area infrared transmission images of the crystals. Grain boundaries that are highly decorated with Te inclusions are observed. Characteristic Te inclusion arrangements as a result of growth conditions are discussed. We also measured the electronic properties of the detectors fabricated from ingots grown using two Bridgman processes, and observed a reduction in electrical resistivity of choked-seeding-grown CdZnTe crystals. Our results show that although choked seeding technique holds a promise in the realization of high quality mono-crystalline CdZnTe, current growth parameters must be improved to obtain defectfree crystals. These results are helpful to attain optimal seeding process for Bridgman-growth of large single crystals of CdZnTe. INTRODUCTION Cadmium Zinc Telluride (CZT) has emerged as an excellent candidate for room temperature radiation detection applications due to its attractive properties which include high average atomic number, high, tunable band-gap that allows for room temperature device operation and the ruggedness that makes it suitable as hand-held devices. Over the past twenty years, a lot of research has been conducted on the defects in CZT crystals that limits its applicability. Much of the defects have been well characterized [1-4], their effects have been determined and in some cases overcome as a result of the large focus of research and development. CZT has matured to the point of commercial production, however there are challenges in crystal growth and material selection that hinder the large scale production. Grown materials are still very defective, and large, homogeneous, single-crystalline portions need to be harvested for detector grade device fabrications. Thus arise the need for quantitative analysis techniques that are applicable and adequate for large-scale detector screening. An ideal technique must be able to scan large wafers quickly and
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