Diffusion-induced doping effects of Ga in ZnO/Ga/ZnO and AZO/Ga/AZO multilayer thin films

In this work, transparent conductive oxide (TCO) film structures were designed with thin gallium (Ga) interlayers added in between zinc oxide (ZnO) and aluminium (1-at% Al)-doped zinc oxide (AZO)-based multilayers. The ZnO/Ga/ZnO and AZO/Ga/AZO films were grown on glass substrates and annealed in ambient argon (Ar). The lowest sheet resistance of 131.71 Ω/□ was obtained from the AZO/Ga/AZO films with 10 mg of Ga interlayer after annealing at 400 °C for 90 min in Ar. The average transmittance was approximately 80% in the visible region. The XRD showed a change of lattice parameters. It suggested that Ga3+ ions partially diffused into AZO-based layers similar to doping. The XPS survey spectra and the XPS depth profile showed the Ga2p3/2 peak at the position of metallic Ga and the diffusion of Ga atoms into the grain boundary of AZO. This means the annealed Ga in the middle layer moves via the lattice sites or between the lattice sites forming both substitution and interstices. These imply that the architected AZO/Ga/AZO multilayer system leads to increase of charge mobility as well as of carrier concentration.