Incoherent ellipsometry below energy gap of TlInS2

The layered material TlInS2 was studied by spectroscopic phase modulated ellipsometry in the energy range 0.75-2.00 eV at room temperature. By using an incoherent reflection model and assuming that optic axis (c*) of TlInS2 is normal to the layer plane, the refractive indices in E // c* and E ⊥ c* orientations of the electrical vector, E, of the incident light were obtained for a region of photon energies below the energy gap of this material. A remarkable increase of the birefringence at photon energies approaching energy gap (2.4 eV) was observed to be a good illustration of the fact that band gap exciton transitions in TlInS2 at room temperature are allowed in E // c* and forbidden in E ⊥ c* orientation, respectively. It is shown that biaxial effects in TlInS2 are small and sample-dependent.