Structural and electrical characterization of ZnO films grown by spray pyrolysis and their application in thin-film transistors

The role of the substrate temperature on the structural, optical, and electronic properties of ZnO thin films deposited by spray pyrolysis using a zinc acetate precursor solution is reported. Analysis of the precursor compound using thermogravimentry and differential scanning calorimetry indicates complete decomposition of the precursor at around 350 °C. Film characterization using Fourier Transform Infrared Spectroscopy (FTIR), photoluminescence spectroscopy (PL), and ultraviolet-visible (UV-Vis) optical transmission spectroscopy suggests the onset of ZnO growth at temperatures as low as 100 °C as well as the transformation to a polycrystalline phase at deposition temperatures >200 °C. Atomic force microscopy (AFM) and X-ray diffraction (XRD) reveal that as-deposited films exhibit low surface roughness (rms ≈ 2.9 nm at 500 °C) and a crystal size that is monotonously increasing from 8 to 32 nm for deposition temperatures in the range of 200-500 °C. The latter appears to have a direct impact on the field-effect electron mobility, which is found to increase with increasing ZnO crystal size. The maximum mobility and current on/off ratio is obtained from thin-film transistors fabricated using ZnO films deposited at >400 °C yielding values on the order of 25 cm ² V^-1s^-1 and 10⁶, respectively. The evolution of the structural and electronic properties of ZnO films grown by spray pyrolysis as a function of deposition temperature is reported. It is established that higher deposition temperatures lead to the formation of larger ZnO crystalline domains and the fabrication of n-channel thin-film transistors with significantly improved operating characteristics.