Thallium Chalcogenide-Based Wide-Band-Gap Semiconductors: TlGaSe<sub>2</sub> for Radiation Detectors

The wide-band-gap semiconductor thallium gallium selenide (TlGaSe<sub>2</sub>) is promising for X-ray and γ-ray detection. In this study, the synthesis and crystal growth of semiconducting TlGaSe<sub>2</sub> was accomplished using a stoichiometric combination of TlSe, Ga, and Se and a modified Bridgman method. These large detector-grade crystals can be synthesized and cut to dimensions appropriate for a detector. The crystals have mirror-like cleaved surfaces and are transparent red, in agreement with a band gap of 1.95 eV observed in absorption measurements. Single-crystal X-ray diffraction refinements confirm that TlGaSe<sub>2</sub> crystallizes in the monoclinic <i>C</i>2/<i>c</i> space group with a layered crystal structure consisting of planes of GaSe<sub>4</sub> corner-sharing tetrahedra connected by weak Tl–Se bonds. Electronic band structure calculations made using the full-potential linearized augmented plane wave method with the screened-exchange local density approximation, including spin orbit coupling, indicate the unusual characteristic of the hole effective mass being lower than that of the electrons. Photoconductivity measurements on the grown TlGaSe<sub>2</sub> crystals show mobility–lifetime (μτ) products of electrons and holes approaching the values of the state-of-the-art commercial material Cd<sub>0.9</sub>Zn<sub>0.1</sub>Te. The promising properties of this material system are confirmed by the ability of a TlGaSe<sub>2</sub>-based detector to show good signal response to X-rays and resolve Ag K radiation energetically.