Optical properties upon ZnS film thickness in ZnS/ITO/glass multilayer films by ellipsometric and spectrophotometric investigations for solar cell and optoelectronic applications

ZnS film with various thicknesses (d ≈ 100–350 nm) deposited on the ITO coated glass substrate (ZnS/ITO/glass)
by an electron beam evaporation process was examined structurally and optically. Different techniques were
used to specify and analyze the optical characteristics of the film, such as spectroscopic ellipsometric (SE) and
spectrophotometric (SP). The ZnS/ITO/glass films exhibit a wurtzite hexagonal type structure entrenched in the
crystalline background of ITO film. XRD analysis revealed changes in structural and microstructural parameters,
such as a decrease in lattice parameters, a reduction in microstrain, and a raise in crystallite size. The optical
constants and the optical energy gap were extracted from SE by constructing an ellipsometric optical model,
while the optical constants were calculated from SP by using Murmann’s exact equations. It was seen that the
overall behaviour of the refractive index n of the ZnS/ITO/glass films obtained from SE and SP increases with an
increase in ZnS layer thickness, which is attributed to the increment of the size of the grain. It was additionally
found that, the overall behaviour of the extinction coefficient k of the ZnS/ITO/glass films increases, when the
thickness of the ZnS layer increments. Also, the direct optical transition was observed with energy band gap
diminishes from 3.423 (d = 100 nm) eV to 3.287 eV (d = 350 nm) that is because of the rise in grain size, the
decrease in microstrain, and reduction in lattice constants. Furthermore, it was found that the deposition of ZnS
on ITO coated glass substrate increases the absorption compared with the ZnS/glass film and then reduces the
transmittance and energy band gap. It was concluded that as the ZnS thickness increases, the optical constants of
ZnS/ITO/glass films enhance. Finally, the overall behaviour of the optical constants within the experimental
error range of the ZnS/ITO/glass films with different ZnS layer thicknesses obtained by SE was found to be
consistent with that calculated from SP measurement.