An Effective Purification Process for the Nuclear Radiation Detector Tl₆SeI₄

The semiconductor Tl₆SeI₄ was previously identified as a promising semiconductor for room temperature nuclear radiation detection. As the detection performance and carrier transport strongly depend on the concentration of impurity energy levels acting as scattering centers and carrier trapping, material purification is a crucial prerequisite step to obtain spectroscopic-grade detector performance. In this contribution, we present a highly efficient purification method using a bent ampule for evaporating Se, Tl₂Se, and TlI precursors for Tl₆SeI₄. On the basis of impurity analysis performed by glow discharge mass spectroscopy, the main impurities in Tl₂Se were identified to be Pb, Bi, and Al, while in TlI the main impurities are Al and Sn. The bent-ampule method successfully reduces or removes the Cl, Pb, and Te impurities from the Se precursor, the Pb, Bi, and Al impurities from the Tl₂Se precursor, and removes Sn from TlI. Informed by the analysis results, density functional theory calculations were performed to study the identified impurities and related defects. The calculation results show that Bi and Al act as deep defect levels, which can be detrimental to the detector performance of Tl₆SeI₄. If the growth condition of Tl₆SeI₄ is Tl-rich/Se-poor, impurity of Si can introduce deep donors. However, it becomes electrically benign if growth conditions are Tl-poor/Se-rich, while Sn and Pb impurities are shallow donors. Centimeter-size Tl₆SeI₄ crystals were grown by the two-zone vertical Bridgman method using the purified precursors. The detector made of Tl₆SeI₄ crystal maintains the high resistivity on the order of 10¹¹ Ω·cm after purification, ideal for suppressing leakage current. The detector exhibits both full-energy and Tl escape photopeaks upon 122 keV γ-ray from ⁵⁷Co radiation source. The electron mobility-lifetime product μ_eτ_e for Tl₆SeI₄ detector is 8.1 × 10^–5 cm²·V^–1. On the basis of the carrier rise time measured from output pulses induced by 5.5 MeV α-particles from ²⁴¹Am, the electron and hole mobilities were estimated to be 112 ± 22 and 81 ± 16 cm²·V^–1·s^–1, respectively, comparable to those of the leading detector materials HgI₂ and TlBr. These results validate the potential of this compound for hard radiation detection, and the impurity analysis presented here allows future efforts to focus on reducing the concentration of the identified impurities.