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 γ-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 (μτ) products of electrons and holes approaching the values of the state-of-the-art commercial material Cd_0.9Zn_0.1Te. 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.