Aluminum-doped zinc oxide thin films deposited by electrospray method

Aluminum-doped ZnO (AZO) thin films are deposited by economically-versatile fabrication method of electro-spraying onto two types of substrates glass and silicon wafer, at substrate temperature of 300 degrees C, voltage of 18 kV and emitter to collector distance of 6 cm, Zinc acetate dehydrate dissolved in a mixture of deionized water, ethanol and acetic acid is used for deposition of undoped ZnO film, while for Al-doping one two precursor materials are chosen - aluminum chloride and aluminum nitrate dissolved in ethanol. The doping levels are 5 and 10 wt% and the concentration dependence of films properties is investigated. All films exhibit polycrystalline structure with decreasing crystallites size from 21 nm for undoped films to 19 nm and 17 nm for AZO films doped with 5 and 10 wt% Al, respectively. The shift of the XRD peaks of doped samples towards higher diffraction angles reveals successful incorporation of Al into ZnO lattice. The optical properties are investigated by ellipsometric and photoluminescence measurements. A decrease of refractive index from 1.89 for undoped sample to 1.84 and 1.83 for 5 and 10% AZO films, respectively and an increase of optical band gap from 3.30 eV to 3.40 eV are observed. A substantial decrease of sheet resistance is measured for AZO films: 86 Omega/square and 61 Omega/square for 5 wt % and 10% doped samples, respectively. Simultaneously, the transmittance of the films is kept high. The average values in the visible range for films with thickness of 200 nm are 86% for undoped ones and 84% and 80% for 5 and 10% Al-doped films, respectively. Thus, the applicability of the studied films as transparent conductive electrodes is demonstrated.