Defect engineering and dopant activation of room temperature grown aluminium-doped zinc oxide thin films

The impact of structural defects on the electrical properties of aluminium doped zinc oxide (AZO) thin films is investigated by varying sputter deposition and post deposition annealing conditions. Results demonstrate sputtered species of high kinetic energy at high radio frequency power (or low sputtering pressure) facilitate the production of AZO films with enhanced crystallinity, grain growth, and compactness with reduced trap defects at grain boundaries. A resistivity of 1.11 × 10^–3 Ω.cm is achieved for the optimised as-deposited samples at room temperature (RT), using 3.95 W/cm² sputtering at 2 mTorr of Ar. Post deposition annealing via pulsed Krypton Fluoride (KrF λ = 248 nm) excimer laser annealing (ELA) and rapid thermal annealing (RTA), provided a functional means to further manipulate the defects in terms of density and distribution. ELA (5 pulses at 125 mJ/cm² in air) and RTA (300 °C/20 s in nitrogen) resulted in a ∼50 % resistivity reduction to ∼5.20 × 10^–4 Ω.cm due to an increase of both free electron density and Hall mobility. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Hall Effect measurements demonstrated a reduction of structural, adsorbed, and morphological defects, with an enhancement of compactness and the effective incorporation of Al into the ZnO lattice. ELA increased the visible transparency from 82 % to 86 %, and the bandgap (Eg) from 3.69 eV to 3.80 eV. RTA also increased the bandgap to 3.80 eV, with a slightly larger increase in the visible transparency to 88 %. The optimised ELA and RTA procedures present a roadmap of rapid annealing conditions following low temperature deposition suitable for the optoelectronics industry and transparent electrodes applications.