Effect of substrate rotation speed on structure and properties of Al-doped ZnO thin films prepared by rf-sputtering

Al-doped ZnO (AZO) thin films were deposited on glass substrates by rf-sputtering at room temperature. The effects of substrate rotation speed (ωS) on the morphological, structural, optical and electrical properties were investigated. SEM transversal images show that the substrate rotation produces dense columnar structures which were found to be better defined under substrate rotation. AFM images show that the surface particles of the samples formed under substrate rotation are smaller and denser than those of a stationary one, leading to smaller grain sizes. XRD results show that all films have hexagonal wurtzite structure and preferred c-axis orientation with a tensile stress along the c-axis. The average optical transmittance was above 90% in UV-Vis region. The lowest resistivity value (8.5×10−3 Ω·cm) was achieved at ωS=0 r/min, with a carrier concentration of 1.8×1020 cm−3, and a Hall mobility of 4.19 cm2/(V·s). For all other samples, the substrate rotation induced changes in the carrier concentration and Hall mobility which resulted in the increasing of electrical resistivity. These results indicate that the morphology, structure, optical and electrical properties of the AZO thin films are strongly affected by the substrate rotation speed.