The Influence of Thermal Treatment on Monocrystalline CZT and Tellurium Inclusions
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The Influence of Thermal Treatment on Monocrystalline CZT and Tellurium Inclusions Jonathan Lassiter1, Charles Payton1, Maxx Jackson1, Samuel Uba1, Claudiu Muntele2, Stephen Babalola1. 1
Alabama A&M University, Normal, AL
2
Cygnus Scientific Services, Huntsville, AL
ABSTRACT Cadmium Zinc Telluride (CZT), considered as a viable material for use in room temperature radiation detectors, has an undesired presence of tellurium inclusions in the bulk. Thermal treatment, in the form of annealing, has been utilized to test the viability of refining CZT into better detector material, either by the elimination of the tellurium inclusions or by the migration of the inclusions under a temperature gradient, but usually with a deterioration of electrical properties. We took infrared micrographs and current voltage (IV) characteristics of CZT samples prior to thermal treatment. We carried out 24-hour thermal treatments with a range of temperature from 100oC to 700oC to determine an optimal annealing temperature and to verify changes in the sizes, morphologies, and locations of the tellurium inclusions on the surfaces and within the crystal bulk of the CZT. The IV curves and resistivities prior to and after thermal treatments were compared, as were the infrared micrographs before and after annealing. Also, the changes in electrical properties of the samples with annealing conditions were compared against structural changes monitored at the same steps during the annealing process, in order to understand the effects of the thermal annealing to the radiation detector properties of the material. Correlations between the shape, size and position of inclusions and electrical properties of the material were attempted. INTRODUCTION Cadmium Zinc Telluride (CZT) is a compound semiconductor material which has been utilized for room temperature radiation detection [1]. CZT is growth through use of vertical Bridgman method, and only about 25% of each CZT ingot grown is considered to be useable, detector grade CZT while the remaining 75% are unusable due to grain boundaries, twins, and other defects [2]. One of the problems encountered is a defect in the form of tellurium inclusions in the bulk of the crystal [3 – 5, 7]. Tellurium inclusions are created from morphological instabilities in the growth interface where the melt droplets, rich in tellurium, are captured from the layer preceding the growth interface [6, 7]. These tellurium inclusions have been experimentally demonstrated to cause charge trapping, changing the internal electrical fields of CZT operating under operating voltage bias. This charge trapping reduces the performance of the material as a detector [3,7]. The logic underlying this research effort is to attempt reducing the presence of the tellurium inclusions, and to find a mechanism to explain the changes in the electrical properties of the CZT detectors as a function of temperature and inclusion sizes.
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