Computational Models for Crystal Growth of Radiation Detector Materials: Growth of CZT by the EDG Method
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1038-O05-09
Computational Models for Crystal Growth of Radiation Detector Materials: Growth of CZT by the EDG Method Jeffrey J. Derby, and David Gasperino Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave, SE, Minneapolis, MN, 55455-0132 ABSTRACT Crystals are the central materials element of most gamma radiation detection systems, yet there remains surprisingly little fundamental understanding about how these crystals grow, how growth conditions affect crystal properties, and, ultimately, how detector performance is affected. Without this understanding, the prospect for significant materials improvement, i.e., growing larger crystals with superior quality and at a lower cost, remains a difficult and expensive exercise involving exhaustive trial-and-error experimentation in the laboratory. Thus, the overall goal of this research is to develop and apply computational modeling to better understand the processes used to grow bulk crystals employed in radiation detectors. Specifically, the work discussed here aims at understanding the growth of cadmium zinc telluride (CZT), a material of long interest to the detector community. We consider the growth of CZT via gradient freeze processes in electrodynamic multizone furnaces and show how crucible mounting and design are predicted to affect conditions for crystal growth. INTRODUCTION Large, single crystals of cadmium zinc telluride (CZT) form the heart of several advanced gamma detectors, which promise portable, low-cost, and sensitive devices to monitor radioactive materials [1-6]. Decades of development have produced great strides in improved crystal growth processes and better materials for these systems [7,8], and CZT crystals of sufficient quality are now commercially available for simple counting and monitoring applications. However, today’s homeland security needs demand large field-of-view imaging and high-sensitivity, highresolution spectroscopic analysis, which require large, single CZT crystals with spatially uniform charge-transport properties [8]. Affordable material of this size and quality is not yet available. The growth of large CZT crystals is not well understood and surprisingly less mature than semiconductor crystal growth employed for the electronics industry. One reason for this state of affairs is that CZT crystal growth is far more challenging than that of more traditional semiconductor crystals, such as silicon and gallium arsenide. Indeed, the growth of large, single crystals of CdTe or CZT is notoriously difficult; Rudolph [9,10] details the many challenges encountered during growth. The end result for the growth of CZT is that typical yields of useable material from a crystalline boule remain at 10% or lower [11], resulting in very high materials costs. To improve existing radiation detector crystals or to develop new materials, the performance-property-processing loop must be closed. Namely, device performance must be understood in terms of a mechanistic understanding of crystal growth, and this understandi
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