Growth of ZnO thin films at low temperature by plasma-enhanced atomic layer deposition using H2O and O2 plasma oxidants

Zinc oxide (ZnO) thin films were grown at 70 degrees C by plasma-enhanced atomic layer deposition using H2O and O-2 plasmas. Plasma oxidants were used in order to improve the ZnO crystallinity and optoelectronic properties, avoiding high-temperature synthesis. The deposition parameters were optimized to achieve saturation in each reaction step. X-ray photoelectron spectroscopy (XPS) reveals high purity of the obtained ZnO films. X-ray diffraction (XRD) measurements indicate that the grown layers are polycrystalline and that the H2O plasma synthesis leads to better crystallinity than the O-2 plasma as inferred from the intensity of the (100) and (002) peaks. The films are with high optical transmission, similar to 90%, as inferred from UV-visible (UV-Vis) transmittance measurements, and optical band gaps of 3.22 and 3.23 eV for H2O and O-2 plasma, respectively. Atomic force microscopy (AFM) indicates that the films are smooth, with an average roughness of similar to 0.22 nm. The growth rate was found to be in the range of 1.2-1.4 angstrom/cycle. The XPS, XRD, UV-Vis, and AFM results prove the possibility to obtain high-quality ZnO films by O-2 and H2O plasma processes at 70 degrees C with chemical, structural, and optical properties promising for flexible electronics. ZnO films were successfully deposited on polyethylene terephthalate substrates using the optimal conditions for H2O plasma process. No damage of the film surface or substrate was observed.