Optical Properties of Bismuth Tri-Iodide Single Crystals. II. Intrinsic Absorption Edge

Abstract

The optical absorption spectra near the fundamental edge have been measured on high quality single crystals of BiI 3 . Above 50 K, the absorption coefficient near the edge is well described by the Urbach's rule. At lower temperatures, the corresponding transitions are of the allowed indirect type involving the creation of free excitons, and absorption components associated with three phonon energies have been resolved. The energies of the phonons are found to be 2.6±0.1, 7.1±0.2 and 13.4±0.4 meV. The absorption cross-section of the indirect transition is very large compared with that observed in indirect semiconductors and ionic crystals. These results are discussed in view of band nature of BiI 3 . Preliminary results on sharp absorption lines near the edge are also given.