Al, Mg Co-doped ZnO thin films: Effect of the annealing temperature on the resistivity and ultraviolet photoconductivity

Abstract

Interesting electrical and optical properties can be obtained from sol-gel doped Zinc Oxide (ZnO) materials by controlled introduction of dopants in the precursor solution. In this work we investigate the effects of annealing temperature on Al and Mg co-doped ZnO thin films. All samples were dip coated from the same solution, using identical techniques and pre-heating temperature, but were annealed in air at various temperatures ranging from 500°C to 650°C. A three-order magnitude reduction in electrical resistivity was achieved for an increase of 100°C annealing temperature. The structural properties and the energy band-gap remained relatively unchanged during this process. Photocurrent measurements carried out using an ultraviolet excitation source indicate a 5x increase in magnitude, along with increase in persistence behavior. The mechanism behind these effects has been linked to temperature-induced variation in Mg to Al ratio in the films, measured using energy dispersive spectroscopy. Our results indicate that strong in-plane variation of resistivity in ZnO thin films can be carried out using localized heating, opening the way to simpler fabrication processes.