A comprehensive study of structural and optical properties of ZnO bulk crystals and polycrystalline films grown by sol-gel method

Abstract

The presented article concerns the comparison between two different zinc-oxide structures - bulk crystals and polycrystalline thin films. Bulk crystals were grown by a Bridgman method. For thin-film production, a sol-gel spin-coated method was chosen. A part of thin layers samples was annealed in 600 oC to induce recrystallization. The morphological and structural properties of all samples were investigated using various microscopy techniques, X-ray diffraction, and Raman spectroscopy. Confocal and scanning electron microscopy, as well as XRD, was used to estimate the influence of the recrystallization process on the morphology of the samples. The Raman vibrations in different scattering geometries were determined using polarized Raman spectra. What is more, in the case of the non-annealed sol-gel layer, the localized reorientation of crystallites was observed, using Raman microscopy. The morphology of the samples was compared to their optical properties, which were investigated by exploiting UV-Vis absorption and photoluminescence spectroscopy. Absorption spectroscopy allowed us to estimate the energy bandgap for different types of ZnO layers and to compare the values obtained for the ZnO crystal structure obtained by the Bridgman method. The photoluminescence and Raman spectroscopy were used to determine the possible defects correlated with the growth conditions.