Underlying mechanism of Al incorporation in sol-gel-based dip-coated ZnO:Al thin films

Al-doped ZnO films were applied to glass substrates using a sol-gel-based dip-coating method. The chemical composition, surface morphology, crystal structure, and optical and electrical properties were examined to elucidate the mechanism underlying doping efficiency. The optimum Al-doping was 1.00 at%. Al doping induced an increase in the crystallite size, light transmission, optical band gap energy, and carrier concentration but decreased the resistivity of the films. Excessive doping level beyond 1.00 at% degraded all the characteristics of the films. The underlying mechanism of the Al dopant atoms in the ZnO lattice is related to the growth units. The addition of aluminum nitrate promoted the formation of Zn(OH)(2) growth units in the solution, whereas its further addition resulted in the formation of additional Zn(OH)(2), along with the Zn(OH)(4)(2-). The deterioration in the characteristics of the films was attributed to the Zn(OH)(4)(2-) formation. Understanding the relation between the film characteristics and growth-unit formation is a base tool to increase the optimum Al-doping level, improving film application towards high performance of optoelectronic devices.