A comparative study of physical properties of pure and In-doped nanostructured ZnO polycrystalline thin film for optoelectronic applications

In this research work a comparative study of pure and In-doped ZnO polycrystalline thin films was made successfully deposited onto fused silica by reactive e-beam thermal evaporation at 300 °C. The structural and optical properties were assessed by employing X-ray diffraction (XRD), Raman spectroscopy, photoluminescence atomic force microscopy and spectroscopic ellipsometry (SE). XRD pattern, EDS and the principal Raman phonon band at 438 cm⁻¹ confirmed purely polycrystalline wurtzite structured ZnO and incorporation of In at the Zn lattice sites. In studying the structural properties, the characteristic (002) plane was used as the focal point. Structural analysis showed that with In incorporation, the crystallites exhibited a preferential orientation along (002) c-plane perpendicular to the substrate. With In-doping (3.9 at.%), the optical band-gap increased and compressive strains were developed within the film. The prominent optical phonon mode at 587 cm⁻¹ presented a low Raman intensity for the sample prepared in the oxygen environment and was assigned to oxygen vacancies. The film thickness and optical constants [refractive index (n), extinction coefficient (k)] were determined by SE study using Cauchy curve fitting model. PL emission spectra showed strong UV emission at 370–373 nm and a feeble visible (green) emission at 512–520 nm. The UV emission showed Stoke’s shift with incorporation of In at the lattice sites as the emitted energy is lower that the band-gap energy of ZnO. The observed properties showed that ZnO can be made significantly important an electronic and optical material for various optoelectronic applications by incorporating In as the dopant material.