Atomic layer deposition of Al-doped ZnO films: Effect of grain orientation on conductivity

Al-doped ZnO (AZO) films were deposited by atomic layer deposition (ALD) on borosilicate glass and sapphire(0001) substrates. The Al composition of the films was varied from 1% to 4% by controlling the ratio of Zn:Al pulses. Film resistivity was measured as a function of Al content and the substrate temperature used for ALD deposition. X-ray diffraction (XRD) was performed on the films, showing a reduction in lattice parameter, as a function of Al concentration, indicating that Al³⁺ ions occupy substitutional sites in the ZnO lattice. The resistivity of films deposited on sapphire substrates (7.7 × 10^-4 Ωcm) was lower than that on glass (3.0 × 10^-3 Ωcm), because of the formation of textured grains with the c-axis aligned with respect to the sapphire surface, as confirmed by XRD. The surface morphology of the films on glass and sapphire was compared using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), which showed similar grain sizes on each substrate, suggesting that the difference in conductivity was due to grain orientation rather than microstructural differences. Optical transparency was measured to be >80% for wavelengths of 370-1600 nm.