Effect of Impurities on Properties of Tl₆SeI₄ Semiconductor for Hard Radiation Detection

The semiconductor Tl₆SeI₄ is a promising candidate material for room temperature X-ray and γ-ray detection. The carrier transport and detection performance strongly depend on concentration of impurity levels acting as carrier trapping and scattering centers. The main impurities in Tl₆SeI₄ were identified as O, C, Si, Sn, Al, Pb, Bi, Cl, Br, S and Te. To assess the effect of impurities on detector performance, theoretical calculations were performed based on first-principles density functional theory (DFT) including nonlocal exchange-correlation functionals. The results show that Al and Bi introduce deep defect levels in the band gap and can be detrimental to the detector performance of Tl₆SeI₄. Si impurities can act as deep donors if Tl₆SeI₄ is grown in Tl-rich/Se-poor conditions, but become electrically benign in Tl-poor/Se-rich conditions, Pb and Sn are shallow donors, and Cl, Br, S, Te impurities are inactive. Interstitial O introduces trapping levels near the middle of the gap, but will become electrically inactive if its concentration is reduced below the level of intrinsic Se vacancies, in which case O atoms will occupy Se sites. The effect of various chemical environments on defect properties was examined and the optimal conditions for material synthesis were suggested.