Temperature dependent photoluminescence studies in CsI:Tl films with varying thicknesses

Abstract

Films of Tl doped CsI in the 10 nm to 3 µm thickness range were deposited on silicon substrates using the physical vapor deposition technique under vacuum conditions. The films exhibited a columnar growth behavior with a preferential orientation along 〈200〉. The photoluminescence (PL) properties of the deposited films were studied in the 77–300 K temperature range and found to be significantly different compared to the bulk CsI:Tl single crystal. In the excitation process, the unperturbed self trapped exciton (STE) excitation of the host was found to be dominant in films of lower thicknesses and direct excitation in “A” band of Tl+ and STE perturbed by the Tl+ gradually evolved with increasing film thickness. At low temperatures, the PL from a 1 µm thick CsI:Tl film revealed an unperturbed STE excitation at 5.9 eV with a corresponding emission around 3.7 eV that vanishes near room temperature. This suggests that the energy transfer from the host excitons to the activator site (Tl+) is weak at low temperatures in the CsI:Tl films.