Determination of the direct bandgap value in In4Se3 thin films

The value and the nature of the bandgap of In4Se3 are still not well defined, with a large spread of the experimental data between 0.42 and 1.68 eV and an uncertain nature, predicted to be indirect by ab initio band structure calculations. Here we report on the optical transmission and photoluminescence (PL) performed in In4Se3 thin films grown by coevaporation on (0001)-oriented sapphire wafers. The quality of the polycrystalline layers allows the first detection of the excitonic-like transition in the optical absorption of this compound at low temperature. The PL detected under weak laser excitation shows a bound exciton emission at 0.75 eV. Strong laser irradiation reveals a quadratic dependence of the PL intensity on the optical excitation, which demonstrates a stimulated emission at 0.79 eV in relation with an exciton-exciton scattering process. On the basis of a reasonable estimate of the exciton energy, equal to 10 - 15 meV, we evaluate the direct bandgap of In4Se3 to 0.82 +/- 0.01 eV at low temperature.