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 degrees 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 degrees C as well as the transformation to a polycrystalline phase at deposition temperatures >200 degrees C. Atomic force microscopy (AFM) and X-ray diffraction (XRD) reveal that as-deposited films exhibit low surface roughness (rms approximate to 2.9 nm at 500 degrees C) and a crystal size that is monotonously increasing from 8 to 32 nm for deposition temperatures in the range of 200-500 degrees 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 degrees C yielding values on the order of 25 cm(2) V(-1)s(-1) and 10(6), respectively.