Thallium Chalcogenide-Based Wide-Band-Gap Semiconductors: TlGaSe2 for Radiation Detectors

Abstract

The wide-band-gap semiconductor thallium gallium selenide (TlGaSe2) is promising for X-ray and gamma-ray detection. In this study, the synthesis and crystal growth of semiconducting TlGaSe2 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 TlGaSe2 crystallizes in the monoclinic C2/c space group with a layered crystal structure consisting of planes of GaSe4 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 TlGaSe2 crystals show mobility-lifetime (mu tau) products of electrons and holes approaching the values of the state-of-the-art commercial material Cd0.9Zn0.1Te. The promising properties of this material system are confirmed by the ability of a TlGaSe2-based detector to show good signal response to X-rays and resolve Ag K radiation energetically.