Microstructural, optical and electrical properties of annealed ZnO:Al thin films

The effects of post-deposition annealing atmosphere (under vacuum, in 5% H₂ in N₂ atmosphere, or in pure N₂ atmosphere) and temperature (400, 450, or 500 C) on the microstructural, optical and electrical properties of RF-sputtered ZnO:Al thin films have been investigated. A detailed analysis by X-ray diffraction complemented by Raman spectroscopy was performed. The electrical transport properties have been then studied by Hall effect measurements of temperature-dependent charge carrier concentration and conductivity. Under vacuum and in N₂/H₂ atmosphere, the annealing process clearly improves the electrical properties of ZnO:Al films. Annealing in a N₂/H₂ ambient leads to a high increase of the carrier concentration, as well as an increase in mobility. The increase of the carrier concentration may limit the increase of the mobility by ionized impurity and phonon scattering. In the opposite, annealing in pure N₂ introduces acceptor centers in the bulk and at the grain boundaries of the thin films, which degrades the electrical properties by a grain barrier limited transport.