Exciton Transitions in Indium Halides

Abstract

Reflection, absorption and emission spectra of InBr, InI and InBr 1- x I 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- x I 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.