Evidence of direct optical transitions in γ-In2Se3

We present an optical study of high crystalline quality gamma-In2Se3 films grown epitaxially on (0001)-oriented sapphire. Well-defined structures are detected at 2.147, 2.240, 2.359, and 2.508 eV in the optical absorption at T = 10 K. On the basis of the selection rules, we associate them to direct optical transitions with excitons built from the upper and spin-orbit splitoff valence bands and the first conduction band at the I' point. The first two lowest excitonic peaks vanish from the absorption spectrum above T = 120 Kat a critical temperature for which the photoluminescence emission fades out as well. It leads us to an estimate of the exciton binding energy of the order of 10 meV in gamma-In2Se3. Last, we interpret additional absorption peaks to LO-phonon replicas of the free exciton with a phonon energy of similar to 14 meV.