Characterization of Pd Impurities and Finite-Sized Defects in Detector Grade CdZnTe
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Characterization of Pd Impurities and Finite-Sized Defects in Detector Grade CdZnTe M.C. Duff1 J.P. Bradley2 Z.R. Dai2 N. Teslich2 A. Burger3 M. Groza3 and V. Buliga3 1
Savannah River National Laboratory, Aiken, SC 29808, U.S.A.
2
Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A.
3
Fisk University, Nashville, TN 37208, U.S.A.
ABSTRACT Synthetic CdZnTe or “CZT” crystals are highly suitable for !-spectrometers operating at the room temperature. Secondary phases (SP) in CZT are known to inhibit detector performance, particularly when they are present in large numbers or dimensions. These SP may exist as voids or composites of non-cubic phase metallic Te layers with bodies of polycrystalline and amorphous CZT material and voids. Defects associated with crystal twining may also influence detector performance in CZT. Using transmission electron microscopy, we identify two types of defects that are on the nano scale. The first defect consists of 40 nm diameter metallic Pd/Te bodies on the grain boundaries of Te-rich composites. Although the nano-Pd/Te bodies around these composites may be unique to the growth source of this CZT material, noble metal impurities like these may contribute to SP formation in CZT. The second defect type consists of atom-scale grain boundary dislocations. Specifically, these involve inclined “finite-sized” planar defects or interfaces between layers of atoms that are associated with twins. Finite-sized twins may be responsible for the subtle but observable striations that can be seen with optical birefringence imaging and synchrotron X-ray topographic imaging. INTRODUCTION Synthetic Cd1-xZnxTe or “CZT” crystals are candidate materials for use as a room temperature-based radiation spectrometer. The ternary alloy with 10% Zn, Cd0.9Zn0.1Te has a high band gap of ~1.6 eV and high resistivity form (~1010 Ω•cm), which facilitates its use as a simplistic semiconductor spectrometer and imager that requires little power and no cryogenic cooling for operation. Over the last decade, the methods for growing high quality CZT have improved the quality of the produced crystals however various defects remain in these materials that can influence their performance as radiation spectrometers. For example, various structural heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity), elemental impurities and secondary phases (SP) can have an impact on the detector performance [1,2,3,4]. Transmission electron microscopy (HR-TEM) has been used to examine the atom scale defects in SP and the bulk of CZT [1,5,6,7]. The morphology and crystalline plane orientation of the SP that consisted of dendritic and Te-rich material in CZT have been investigated in a highly detailed study using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) [8]. We recently characterized SP in a sample of modified vertical Bridgman grown CZT from Yinnel Tech (YT). This material possessed properties that are suitable for its use in !-
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