Modeling the Crystal Growth of Cadmium Zinc Telluride: Accomplishments and Future Challenges
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Modeling the Crystal Growth of Cadmium Zinc Telluride: Accomplishments and Future Challenges Jeffrey J. Derby, David Gasperino, Nan Zhang, and Andrew Yeckel Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455-0132, U.S.A. ABSTRACT The availability of large, single crystals of cadmium zinc telluride (CZT) with uniform properties would lead to improved performance of gamma radiation detectors fabricated from them. However, even though CZT crystals are the central element of these systems, there remains relatively little fundamental understanding about how these crystals grow and, especially, how crystal growth conditions affect the properties of grown crystals. This paper discusses the many challenges of growing better CZT crystals and how modeling may favorably impact these challenges. Our thesis is that crystal growth modeling is a powerful tool to complement experiments and characterization. It provides an important approach to close the loop between materials discovery, device research, systems performance, and producibility. Specifically, we discuss our efforts to model gradient freeze furnaces used to grow large CZT crystals at Pacific Northwest National Laboratories and Washington State University. Model results are compared with experimental measurements, and the insight gained from modeling is discussed. 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 growt
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