Evidence of direct optical transitions inγ−In2Se3

We present an optical study of high crystalline quality $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ 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 $\mathrm{\ensuremath{\Gamma}}$ point. The first two lowest excitonic peaks vanish from the absorption spectrum above $T=120$ K at 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 $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$. Last, we interpret additional absorption peaks to LO-phonon replicas of the free exciton with a phonon energy of $\ensuremath{\sim}14$ meV.