Optical, electrical and structural characteristics of Al:ZnO thin films with various thicknesses deposited by DC sputtering at room temperature and annealed in air or vacuum

Transparent and conductive Al-doped ZnO (AZO) films have been grown with various thicknesses between 0.3 and 1.1 μm by magnetron sputtering at room temperature onto soda lime glass substrates. After deposition, the samples have been annealed at temperatures ranging from 150 to 450 °C in air or vacuum. The optical, electrical, and structural characteristics of the AZO coatings have been analyzed as a function of the film thickness and the annealing parameters by spectrophotometry, Hall effect measurements, and X-ray diffraction. As-grown layers are found polycrystalline, with hexagonal structure that shows some elongation of the unit cells along the c-axis, having visible transmittance ∼85–90% and resistivity ∼1.6–2.0 mΩ cm, both parameters slightly decreasing when the film thickness increases. Heating in air or vacuum produces further elongation of the crystalline lattice together with some increase of the visible transmittance and a decrease of the electrical resistance that depends on the heating temperature and atmosphere. The best characteristics have been obtained after treatment in vacuum at 350 °C, where the highest carrier concentrations are achieved, giving visible transmittance ∼90–95% and resistivity ∼0.8–0.9 mΩ cm for the AZO layers with various thicknesses. Some relationships between the analyzed properties have been established, showing the dependence of the lattice distortion, the band gap energy and the mobility on the carrier concentration.