Optical properties of gallium iodide GaI3 have been studied for the first time in the energy range of 2 - 8 eV. The fundamental absorption edge is located at 3.1 eV at 15 K, and it shows anomalously large red shift with increasing temperature. It is found that a logarithmic plot of the absorption coefficient gives a straight line for all temperatures investigated, indicating that the absorption edge of GaI3 is well described by the Urbach rule. The exciton-phonon interaction and the exciton relaxation process are discussed on the basis of these results as well as the luminescence result.
"Optical properties of galium iodide", Proc. SPIE 2362, International Conference on Excitonic Processes in Condensed Matter, (9 February 1995); doi: 10.1117/12.200951; https://doi.org/10.1117/12.200951