Exciton Transitions in Indium Halides

Abstract

Reflection, absorption and emission spectra of InBr, InI and InBr_1−xI_x mixed crystals have been measured at 2 K by using polarized light. In InBr, a prominent exciton peak is found at 2.48 eV for the polarization E⁄⁄b, which is on the high energy side of the lowest direct exciton peak at 2.33 eV allowed for E⁄⁄c. Three square root steps due to indirect excitons have been observed for E⁄⁄b in contrast to four steps for E⁄⁄c. Six momentum conserving phonons are confirmed. Luminescence from free and shallow bound indirect excitons is identified in InBr. The first exciton peak in InBr_1−xI_x mixed crystals shifts continuously from InI to InBr. Indirect-direct crossover at x=0.5 leads to the conclusion that the lowest energy band gap of InI is of direct type. The indirect transition in InBr is assigned to the transition from S to T point in the Brillouin zone.