Purification and Improved Nuclear Radiation Detection of Tl₆SI₄ Semiconductor

The wide-band-gap semiconductor Tl₆SI₄ (2.14 eV) has high photon stopping power and is a promising material for detecting X-rays. In order to improve its photoresponse to low-flux γ-rays, material purification prior to crystal growth is crucial. In this contribution, we report effective purification protocols, impurity analysis, followed by synthesis and crystal growth, charge transport, and detector performance of large-sized Tl₆SI₄ crystals. Purification methods of evaporation and zone refining were developed, and their high effectiveness was confirmed by impurity analysis via glow discharge mass spectrometry. Centimeter-sized single crystals were grown using the Bridgman method. The improved properties after material purification were confirmed by photoluminescence measurements. The energy of the valence band maximum of a Tl₆SI₄, measured with photoemission spectroscopy in air (PESA), is ∼5.34 ± 0.05 eV. Detector devices fabricated from the single crystal exhibit a high resistivity of 5 × 10¹² ω·cm. The detector shows promising photoresponse under 22.4 keV Ag Kα X-rays and 122 keV γ-rays from ⁵⁷Co. Spectroscopic energy resolution was achieved for 5.5 MeV α-particles from a ²⁴¹Am radiation source with a full width at half-maximum of 27% at an electric field intensity of 2500 V·cm^-1. On the basis of its spectral response to ⁵⁷Co γ-rays, the electron mobility-lifetime product μ_eτ_e was estimated as 1.4 × 10^-5 cm²·V^-1. Drift mobility measurements via a time-of-flight technique using spectral photoresponse induced by α-particles reveal a high electron mobility of 35 ± 7 cm²·V^-1·s^-1