Doping strategies for highly conductive Al-doped ZnO films grown from aqueous solution

Aluminum-doped zinc oxide (AZO) thin films are prepared by a low temperature (100 °C) aqueous solution deposition method with a subsequent UV post-deposition treatment at 140 °C. Film growth is governed by the retrograde solubility of zinc–ammine complexes at basic conditions (pH 11.4). Aluminum was introduced into the film as a dopant by co-precipitation, either using an aluminum metal foil or aluminum nitrate as a precursor. As the presence of Al ions in the solution influences the film morphology as well as the opto-electronic properties, different profiles of the dopant supply were examined and optimized with respect to the optical and electrical properties of the AZO layer. It was found that independent of the chosen aluminum precursor a linearly increasing dopant concentration in the solution is favorable for achieving dense, highly transparent (<8% absorption between 400 and 900 nm) AZO thin films with a lowest resistivity of 3.4 × 10−3 Ω cm.