Effects of annealing conditions and film thickness on electrical and optical properties of epitaxial Al-doped ZnO films

• High quality Al-doped ZnO films were grown epitaxially on sapphire-R substrates.
• The effect of various annealing conditions on the film properties was investigated.
• The electrical properties of films may be markedly improved or degraded by annealing.
• The presence of oxygen should be avoided in any post-deposition treatment of films at > 400 °C.
• The properties of as-deposited and annealed films depend on the film thickness.

Epitaxial films of Al-doped ZnO were grown at 400 °C on sapphire-R substrates at atmospheric pressure via the aerosol-assisted MOCVD technique. The film microstructure, surface morphology, and electrical and optical properties were investigated in relation to the annealing temperature (300–600 °C), gas atmosphere (argon–hydrogen mixture, argon, nitrogen, vacuum, oxygen, and air) and film thickness (~ 70–1200 nm). The annealing influenced the carrier concentration and mobility in the films, producing an enlarged effect at higher temperatures, whereas the crystalline quality and surface morphology of the films were nearly unaffected by annealing. The electrical properties of the films markedly degraded after annealing in oxygen and air, and improved when annealed in other gas environments. The largest improvement of the electrical properties was obtained by annealing of films in the argon–hydrogen mixture. The rapid evolution of film properties with thickness was observed in the range of small thicknesses (~ 70–170 nm). The film properties were nearly unchanged for further increases in the film thickness up to ~ 800 nm, whereas thicker films exhibited degraded electrical properties due to their partial cracking. The effect of the annealing on film electrical properties increased with the decrease in the film thickness. The gas atmosphere (air or nitrogen) during the films cooling to RT after deposition also influenced the electrical properties of films.