Reducing Defect Density of Bridgman-Grown Tl₆SeI₄ Semiconductor Crystals for Room-Temperature γ-ray Detection

The thallium-based halide semiconductor, Tl 6 SeI 4, is a promising material for room-temperature X-ray and γ-ray detection; however, its performance has been limited by deep-level defects and thallium oxide impurities, which degrade charge transport and detection capabilities. In this study, we present an enhanced purification process for precursor materials (Tl, TlI, and Se) using zone refinement, vapor transport, and bent-tube techniques. This approach significantly reduces deep-level defects and impurities, enabling the growth of high-quality centimeter-sized Tl 6 SeI 4 single crystals using the two-zone vertical Bridgman method. Thermally stimulated current spectroscopy showed a reduction in defect density from ∼1.61 × 10 15 to ∼2.46 × 10 13 cm –3 . The fabricated detectors exhibited high room-temperature resistivity (∼10 10 Ω·cm), effectively suppressing the leakage current. Improved photoresponses were observed for 122 keV γ-rays from 57 Co, 662 keV γ-rays from 137 Cs, and 1.173 MeV γ-rays from 60 Co, with particularly notable enhancements in detection efficiency for 137 Cs and 60 Co sources, compared to earlier studies with the same material. The mobility-lifetime (μτ) products were estimated to be 1.63 × 10 –5 cm 2 ·V –1 for electrons and 2.6 × 10 –5 cm 2 ·V –1 for holes, while time-of-flight measurements showed electron and hole drift mobilities of 26 and 18 cm 2 ·V –1 ·s –1, respectively. These results highlight the critical importance of material purification in optimizing the Tl 6 SeI 4 detector performance.