Deposition and characterization of ZnO:Al thin films by ultrasonic spray pyrolysis

Al-doped ZnO (AZO) thin films were prepared using simple, flexible and cost-effective ultrasonic spray pyrolysis (USP) technique at different substrate temperatures. Zinc acetate dehydrate (Zn (CH₃COO)₂.2H₂O) and Aluminum acetylacetonate (C₁₅H₂₁AlO₆) were used as precursors and the solvent was a mixture of de-ionized water, methanol and acetic acid. Substrate temperatures are varied for 3 at% Al-doped film between 450°C to 500°C. The film´s structural, optical and electrical properties were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-VIS transmittance spectroscopy, photoluminescence (PL) and Hall measurements. The obtained films were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. Grain sizes varied from 21.3 to 25.3 nm based on substrate temperature. FESEM images revealed that the film morphology is strongly affected by the substrate temperature. Transmission measurements showed that for visible wavelength (400-700 nm), the AZO films have an average transmission of 75%. Optical band gap of AZO films is varied from 3.26 to 3.29 eV with the increase in substrate temperature. PL spectra showed ZnO:Al films with a low density of native defects. Resistivity of the films varied from 0.7 Ohm-cm to 2 × 10^-2 Ohm-cm. Minimum electrical resistivity was obtained for film deposited at 475°C with film thickness of 602 nm.