Study of Structural Defects in CdZnTe Crystals by High Resolution Electron Microscopy
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Study of Structural Defects in CdZnTe Crystals by High Resolution Electron Microscopy A. Hossain, A. E. Bolotnikov, G. S. Camarda, Y. Cui, R. Gul, K-H. Kim, K. Kisslinger, D. Su, G. Yang, L. H. Zhang, and R. B. James Brookhaven National Laboratory, Upton, NY 11973, USA ABSTRACT We investigated defects in CdZnTe crystals produced from various conditions and their impact on fabricated devices. In this study, we employed transmission and scanning transmission electron microscope (TEM and STEM), because defects at the nano-scale are not observed readily under an optical or infrared microscope, or by most other techniques. Our approach revealed several types of defects in the crystals, such as low-angle boundaries, dislocations and precipitates, which likely are major causes in degrading the electrical properties of CdZnTe devices, and eventually limiting their performance.
INTRODUCTION Today’s CdZnTe (CZT) radiation detectors suffer from several problems related to material uniformity and hampered charge transport that degrade the devices’ overall performance [1-4], and hence restrict their widespread deployment for applications in national security and medical imaging. Such deterioration in the detector’s performance results from various point- and extended defects, like vacancies, impurities, dislocations, and precipitates. Unstable conditions during crystal growth and post-growth treatment are likely responsible for these defects. To characterize them, and better understand the challenges they pose for detectors, conventional techniques, such as visualizing dislocation-etched pits, IR transmission microscopy, and white X-ray diffraction topography have been used; however, they provide information on bulk properties only. Therefore, as new technologies become available there is an opportunity and a need to use them to observe directly the nano-structural and other related defects in CZT to clarify in-depth the origins of the formation of extended and point defects, and the crystals’ compositional distribution. Electron microscopy offers a variety of techniques that give the ability to characterize the microstructure and the chemical compositional distribution of materials down to the nanoscale. High-resolution electron microscopy (HREM) can afford information about the crystal’s lattice and its atomic structure that is far more detailed than is currently known; such comprehensive data is essential to identifying the sources of the defects. We have been systematically characterized such structural defects in CZT crystals grown by different methods. Our objective is to identify and to quantify various types of defects, their concentrations, and origins. In this study, we detailed the structural features and local chemical composition of CZT crystals grown via different growth methods, using high-resolution transmission electron microscopes (TEM/STEM). Our data on the nano-scale demonstrated distinct structural defects in the crystals, such as interstitial atoms, low-angle boundaries, dislocations, and precipitations th
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