Size-dependent optical and dielectric properties of nanocrystalline ZnS thin films synthesized via rf-magnetron sputtering technique

Nanocrystalline ZnS thin films have been synthesized by radio frequency magnetron sputtering technique on glass and Si substrates at a substrate temperature 300 K. X-ray diffraction and selected area electron diffraction studies confirmed the formation of nanocrystalline cubic phase of ZnS in the films, although the target material was hexagonal ZnS. The particle size, calculated from the XRD patterns of the thin films was found in the range 2.06–4.86 nm. TEM micrographs of the thin films revealed the manifestation of ZnS nanoparticles with sizes in the range 3.00–5.83 nm. UV–vis–NIR spectrophotometric measurements showed that the films were highly transparent (∼90%) in the wavelength range 400–2600 nm with a blue shift of the absorption edge. The direct allowed bandgaps have been calculated and they lie in the range 3.89–4.44 eV. The particle size, calculated from the shift of direct bandgap, due to quantum confinement effect lying in the range 3.23–5.60 nm, well support the TEM results. The room temperature photoluminescence spectra of the films showed two peaks centered around 315 and 450 nm. We assigned the first peak due to bandgap transitions while the latter was due to sulfur vacancy in the films. The composition analysis by energy dispersive X-rays also supported the existence of sulfur deficiency in the films. The dielectric property study showed high dielectric constant (85–100) at a higher frequency (>5 kHz).