Enhancement in the crystallinity of ZnO nanostructures induced by rapid thermal annealing

ZnO is a multifunctional semiconductor material, with a wide band gap (~3.37eV), high coupling energy (60 meV), excellent chemical and thermal stability, and enhanced optical, electronic and structural properties, enabling its wide range of applications. In this study, crystalline ZnO films were deposited on silicon substrates, obtained by a chemical method, using Zn(NO₃)₂ as the Zn precursor, NaOH as the precipitating agent, and SDS as the surfactant. The deposited ZnO films were initially treated at 150°C, followed by RTA at 550 and 900°C, for 300 sec. Morphological and structural analyses were compared with conventionally treated samples to highlight the changes induced by RTA. SEM analysis showed that temperature influenced the morphology of ZnO NPs, obtaining particles with spherical shapes and sizes between 15 and 60 nm in a short time. In addition, the tendency agglomeration increases with increasing temperature. Structural analysis indicated a high degree of crystallinity and a hexagonal wurtzite structure; the size of the surface boundaries decreased as the temperature increased from 550°C to 900°C. The FTIR spectra show absorption bands that can be associated with the vibration mode of the Zn-O bonds, and confirm the improvement of crystallinity by enhancing the intensity of the peaks. The surface properties exhibit high wetting capacity, through contact angles with an average value of 45°, regardless of the RTA applied. The results obtained highlight the potential of the RTA process as an efficient alternative to classical thermal methods for improving the quality of crystalline ZnO films, increasing their potential for various technological applications, by controlling the thermal cycle parameters.

Acknowledgments: This work was supported from a grant of the Ministry of Research, Innovation and Digitization, CNCS-UEFISCDI, project number PN-IV-P2-2.1-TE-2023-0417, within PNCDI IV, and by the Core Program within the National Research Development and Innovation Plan 2022-2027, project no. 2307.