Investigation of Semi-Insulating Cs 2 Hg 6 S 7 and Cs 2 Hg 6‑x Cd x S 7 Alloy for Hard Radiation Detection Hao Li, † Christos D. Malliakas, † Zhifu Liu, § John A. Peters, § Maria Sebastian, †,§ Lidong Zhao, † Duck Young Chung, ‡ Bruce W. Wessels, §,∥ and Mercouri G. Kanatzidis* ,†,‡ † Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡ Material Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ∥ Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States * S Supporting Information ABSTRACT: A new method is described to synthesize the semiconductor Cs 2 Hg 6 S 7 and its alloy with Cd. Using the as- synthesized material, large single crystals have been grown by the Bridgman method under an improved set of crystal growth parameters. In addition, Cd alloying in the form of Cs 2 Hg 6−x Cd x S 7 (x = 0.25, 0.5, 0.75, etc.) as well as doping with In, Cl was investigated and the influence on the electronic properties was studied. Cd alloying increases the band gap of Cs 2 Hg 6 S 7 from 1.63 to 1.84 eV. Doping with In and Cl however creates electron carriers and changes p-type samples of Cs 2 Hg 6 S 7 into n type. A 30-fold increase in the resistivity of the single crystals from 2 × 10 6 to 0.65 × 10 8 Ω cm has been achieved. The carrier mobility-lifetime product of the Cs 2 Hg 6 S 7 crystals has been increased to 1.7 × 10 −3 cm 2 /V for electrons (μτ) e and 2.4 × 10 −3 cm 2 /V for holes (μτ) h (HgCl 2 doped). The measured (μτ) e value is comparable to the commercial CdZnTe crystal while the (μτ) h is 10 times higher. Detection of Ag X-ray radiation is demonstrated using the as-grown Cs 2 Hg 6 S 7 crystals. ■ INTRODUCTION Cs 2 Hg 6 S 7 is a novel compound that is promising for X-ray and γ-ray detection. 1−4 It crystallizes in the tetragonal space group P4 2 nm and meets the key requirements necessary to provide good X-ray and γ-ray response at room temperature. These are (a) an optical band gap of 1.63 eV, which is large enough for potentially high dark electrical resistivity but small enough for small electron−hole ionization energy; (b) a high mass density of 6.896 g/cm 3 and elements with high atomic number (Z Cs = 55, Z Hg = 80) for high stopping power of hard radiation; 5 (c) high carrier mobility-lifetime (μτ) products measured in previously grown Cs 2 Hg 6 S 7 single crystals of (μτ) e = ∼0.9− 2.0 × 10 −3 cm 2 /V and (μτ) h = ∼1.0−3.4 × 10 −4 cm 2 /V. 2−4,6 These (μτ) values are comparable or superior to the benchmark detector material Cd 1−x Zn x Te (CZT) [(μτ) e = ∼8 × 10 −4 − 4.2 × 10 −2 cm 2 /V, (μτ) h = ∼1.0 × 10 −4 cm 2 /V), 7−12 which motivates our attention to this material. The electrical resistivity of Cs 2 Hg 6 S 7 samples however is relatively low at ∼10 6 Ω cm compared to 10 10 Ω cm for CZT. Nevertheless, because of the many positive qualities, it is worth pursuing the optimization of the growth of Cs 2 Hg 6 S 7 crystals to obtain higher quality and higher resistivity samples. Previously, we have synthesized Cs 2 Hg 6 S 7 using the following reactions: 1,3 + → Cs S 6HgS Cs Hg S 2 2 6 7 (1) + → Cs S 6Hg Cs Hg S 2 7 2 6 7 (2) Though large quantity of raw material for crystal growth can be obtained using reaction 2, the resistivity of the as-grown crystals is in the range of 0.8−8.3 × 10 6 Ω cm, which is too low for practical X-ray and γ -ray detection applications. In this paper, we employed a new method to synthesize Cs 2 Hg 6 S 7 raw material with higher purity for crystal growth and obtained specimens with resistivity of the order of 10 8 Ω cm. The low-energy native defects in Cs 2 Hg 6 S 7 have been theoretically calculated using density functional calculations, 4 which suggested that the most stable ones are likely Cs and Hg vacancies. If this prediction is correct, the undoped samples are likely to be p-type. Under this premise and based on the experimental Seebeck measurements which indicate p-type character for undoped stoichiometric samples, we chose to explore n-type dopants to compensate the majority carrier and realize high resistivity. In order to control the carrier concentration, n-type In/Cl doping was tried on Cs 2 Hg 6 S 7 . We also investigated the e ffects of Cd alloying in Cs 2 Hg 6−x Cd x S 7 to understand how the properties vary with composition. Here we present the characterization of the grown Received: July 31, 2014 Revised: September 14, 2014 Published: September 23, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 5949 dx.doi.org/10.1021/cg501151r | Cryst. Growth Des. 2014, 14, 5949−5956