Effects of deposition time on structural and optical properties of ZnS and ZnS/Au thin films grown by thermal evaporation

Abstract

In this work, by exploiting the thermal evaporation technique, Zinc sulfide (ZnS) thin films are grown on glass substrates at 473 K. The effects of the deposition time on the thickness, structural, morphological, and optical properties of the thin films are investigated. The samples were deposited at 4 × 10−6 mbar with different deposition times (10, 20, and 30 min) whereas the thin films are characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), and ultraviolet–visible spectrophotometers (UV–Vis). XRD analysis indicates that all fabricated films have a cubic structure of β-ZnS phase. Moreover, crystallite size, lattice constant, dislocation density, and micro-strain are calculated from XRD data. According to the results, crystallite size shows an increasing trend from 7.05 to 12.28 nm as deposition time increases. The dependency of surface roughness on deposition time is also observed. Optical parameters of the films such as transmittance spectra, direct optical band gap energy, refractive index, extinction coefficient, complex dielectric constant, dispersion energy parameters, static refractive index, and static high-frequency dielectric constant are carefully determined. The films exhibit high transparency and zero absorption in the visible and infrared regions. The refractive index values can be well fitted to Cauchy's relations for extrapolation to near strong absorption region. The prepared ZnS thin film with low surface roughness, high transparency, and large optical band gap may find its application as optical interfacing and CIGS solar cells. Moreover, a thin layer of Au ∼20 nm was deposited by the thermal evaporation method on the prepared ZnS films and the optical transmittance spectra show that the bilayers possess a good transmittance ratio while the disappearance of spectral interference was observed as a result of Au deposition.