Optical properties of Sub-30 nm-thick ZnS films studied by spectroscopic ellipsometry

The sub-30 nm-thick ZnS thin films with different thicknesses were deposited on Si substrates by electron beam evaporation (EBE). The thickness and optical constants of ZnS films were obtained by fitting spectroscopic ellipsometry (SE) data using Tauc-Lorentz Model. The variation of refractive index, extinction coefficient, optical band gap, the complex dielectric function of the samples with film thickness was analyzed. Due to the increase of film density, the refractive index increases with the thickness, and the extinction coefficient has the same trend. Due to the quantum size effect, the optical band gap decreases from 3.56 eV to 3.23 eV with increasing film thickness. Two peaks (E0 and E1) are obtained from the complex dielectric function spectrum, which most likely corresponds to the central energy of the exciton optical transition. Three critical points (CP) energies (EA, EB, and EC) and the types of interband optical transitions were extracted from the standard linear analysis of the second-order derivatives of the dielectric function. The three CPs energies have a significant thickness dependence, which is mainly attributed to quantum size effects. This result is beneficial for the design and application of photonic and optoelectronic devices with sub-30 nm-thick ZnS films.