In semiconductors, defects often assist nonradiative relaxation. However, Tl doping can significantly suppress the nonradiative relaxation in alkali halides to increase scintillation efficiency. Without the Tl, it is known that the creation of Frenkel pairs at self-trapped excitons, assisted by excited electron and hole relaxations, is the reason for the nonradiative relaxation. Here we show by first-principles calculation that Tl doping introduces Tl $p$ states inside the band gap to trap the excited electrons. The trapping is highly effective to within several picoseconds, as revealed by time-dependent density functional theory calculations. It alters the nonradiative relaxation process to result in a noticeable increase in the relaxation barrier from 0.3 to 0.63 eV, which reduces the nonradiative relaxation by roughly a factor of 10${}^5$ at room temperature.

• Received 4 September 2012

DOI:https://doi.org/10.1103/PhysRevB.87.205206