Low Temperature Crystal Growth and Characterization of Cd 0.9 Zn 0.1 Te for Radiation Detection Applications
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Low Temperature Crystal Growth and Characterization of Cd0.9Zn0.1Te for Radiation Detection Applications Ramesh M. Krishna1, Timothy C. Hayes1, Peter G. Muzykov1, and Krishna C. Mandal1 1 Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA ABSTRACT Cd0.9Zn0.1Te (CZT) detector grade crystals were grown from zone refined Cd, Zn, and Te (7N) precursor materials, using the tellurium solvent method. These crystals were grown using a high temperature vertical furnace designed and installed in our laboratory. The furnace is capable of growing up to 8” diameter crystals, and custom pulling and ampoule rotation functions using custom electronics were furnished for this setup. CZT crystals were grown using excess Te as a solvent with growth temperatures lower than the melting temperatures of CZT (1092°C). Tellurium inclusions were characterized through IR transmittance maps for the grown CZT ingots. The crystals from the grown ingots were processed and characterized using I-V measurements for electrical resistivity, thermally stimulated current (TSC), and electron beam induced current (EBIC). Pulse height spectra (PHS) measurements were carried out using a 241 Am (59.6 keV) radiation source, and an energy resolution of ~4.2% FWHM was obtained. Our investigation demonstrates high quality detector grade CZT crystals growth using this low temperature solvent method. INTRODUCTION Among the wide variety of semiconductor materials available today, CdZnTe (CZT) is one of the most promising materials for producing nuclear radiation detectors. Unlike other popular options on the market, CZT operates at room temperature (300K), has a high average atomic number (Z = ~50), a wide band-gap ( 1.5 eV at 300K), and has a high density (5.8 g/cm3) [1]. Applications for nuclear radiation detectors, such as national security, medical imaging, and high-energy astrophysics have increased the need for spectrometer-grade detectors, which has driven the commercial development of CZT in recent years. However, one major impediment to CZT’s widespread use is the high cost of producing large defect-free bulk single crystals of CZT. This is due to the presence of Te inclusions in the bulk crystals, Zn segregation, and defects in the crystal structure caused by impurities and mechanical stress. Furthermore, the high melting temperature of CZT increases the risk of contamination from impurities present in the fused silica ampoules [2]. A low temperature growth method has been developed and implemented for producing large volume single crystals of CZT, using Te-solvent method. Growth temperatures are kept below the melting point of CZT, reducing the risk of contamination from the quartz ampoules. Details of the crystal growth, characterization, detector fabrication, and detector performance are presented in the following sections.
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EXPERIMENT Crystal Growth of CZT (Cd0.9Zn0.1Te:In; 5 to 25 ppm In) was performed using in-house zone refined (~7N) Cd, Zn, and Te precursor materials. The zone refined precursors were p
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