Optical constants of wurtzite ZnS thin films determined by spectroscopic ellipsometry

The complex dielectric functions of wurtzite ZnS thin films grown on (0001) Al₂O₃ have been determined by using spectroscopic ellipsometry over the spectral range of 1.33-4.7 eV. Below the band gap, the refractive index n is found to follow the first-order Sellmeir dispersion relationship n²(λ)=1+2.22λ²/(λ ²-0.038²). Strong and well-defined free excitonic features located above the band edge are clearly observed at room temperature. The intrinsic optical parameters of wurtzite ZnS such as band gaps and excitonic binding energies have been determined by fitting the absorption spectrum using a modified Elliott expression together with Lorentizan broadening. Both parameters are found to be larger than their zinc blende counterparts.