Alkali Metal Chalcogenides for Radiation Detection
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Alkali Metal Chalcogenides for Radiation Detection J. A. Peters1, Zhifu Liu1, and B. W. Wesselsa), 1,2 I. Androulakis3, C. P. Sebastian3, Hao Li3, and M. G. Kanatzidis3 1 Materials Research Center, Northwestern University, Evanston, IL 60208 2 Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208 3 Department of Chemistry, Northwestern University, Evanston, IL 60208
ABSTRACT We report on the optical and charge transport properties of novel alkali metal chalcogenides, Cs2Hg6S7 and Cs2Cd3Te4, pertaining to their use in radiation detection. Optical absorption, photoconductivity, and gamma ray response measurements for undoped crystals were measured. The band gap energies of the Cs2Hg6S7 and Cs2Cd3Te4 compounds are 1.63 eV and 2.45 eV, respectively. The mobility-lifetime products for charge carriers are of the order of ~10-3 cm2/V for electrons and ~10-4 cm2/V for holes. Detectors fabricated from the ternary compound Cs2Hg6S7 shows well-resolved spectroscopic features at room temperature in response to γ-rays at 122 keV from a 57Co source, indicating its potential as a radiation detector. INTRODUCTION The need for nuclear radiation detectors with good spectroscopic performance that operate at room temperature has led to a widening search for new wide gap semiconductors. To date, several semiconductors have had significant impact on radiation detection that include Ge, HgI2, CdTe, and Cd1-xZnxTe1,2,. However, there are a number of limitations in the use of these semiconductors. For example, Ge requires cryogenic cooling to function as a radiation detector. It also lacks sufficient sensitivity to gamma rays despite its impressive performance because of its low density and low atomic number3. Furthermore, while Cd1-xZnxTe is currently a leading material for room temperature radiation detectors, growth of large homogeneous crystals remains a challenge. Thus it is evident that new materials are needed for advanced radiation detectors that operate at room temperature. The choice of suitable semiconductor materials for radiation detection is mainly determined by the energy range of interest. The selection criteria for these materials relevant for x-ray and γ-ray detection include high atomic number, high density, and wide energy band gaps in the range 1.6 < Eg < 2.6 eV. Moreover, these materials must work at room temperature. However, compound semiconductors with a high atomic number typically have band gaps of less than 1 eV. It has been previously shown that dimensional reduction can be exploited to generate new ternary compounds with desired energy band gaps. These compounds include A2Hg6Q7 (A=Cs, Rb, K; Q=S, Se), A2Hg3Q4 (A=Cs, K; Q=S, Se), and A2Cd3Q4 (A=Cs, K; Q=S, Se, Te) 4,5,6. Here, we present the optical and charge transport properties of two alkali heavy metal chalcogenides Cs2Hg6S7 and Cs2Cd3Te4 grown using a modified Bridgman method. These chalcogenides are promising candidate materials for radiation detection because of their high atomic number elements (ZCs = 55,
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