Electronic structure and defect properties of Tl6SeI4: Density functional calculations

We report density functional calculations of electronic structure, phase diagram, and dielectric, optical, and defect properties of Tl6SeI4. We discuss how electronic structure and defect properties affect resistivity and carrier mobility-lifetime (mu tau) products in Tl6SeI4. We find large Born effective charges due to covalency involving Tl-6p states. High Born charges generally enhance the static dielectric constant. This provides a mechanism for effective screening of charged defects and impurities. We find that high resistivity can be obtained under near-stoichiometric growth conditions via Fermi level pinning near the middle of the band gap by shallow donors and acceptors, as opposed to deep traps that can give high resistivity, but at the expense of short carrier drift lengths. Defect calculations also reveal the presence of deep native donors that may cause electron trapping. The experimentally observed good mu tau products may be explained by a combination of small effective masses and effective screening of charged defects. High resistivity and good mu tau products make Tl6SeI4 a promising room-temperature radiation detector material. We also show the calculated defect diffusion barriers, which affect defect migration under external bias in a detector.