The effects of oxygen vacancies in space stable aluminum-doped zinc oxide thin films

Polycrystalline ZnO:Al (AZO) thin films are routinely deposited by RF magnetron sputtering. The figures of merit, resistivity (rho) and transmittance (T), of AZO films deposited using an oxide target are heavily dependent upon residual water vapor within the growth chamber Zinc oxide is an inherently n-type material in which the oxygen vacancy (VO) is the main donor defect. The substitution of Al onto the Zn sublattice has been shown to enhance the conductivity of AZO; however, this work shows that VO donors play a critical role in the degenerate doping of AZO. Additional oxygen supplied by residual water vapor during growth was found to produce transparent (T>80%) and resistive (rho∼14 O cm) films due to the reduced density of donor species through the filling of VO and the oxidation of Al. Films grown without the presence of additional oxygen exhibited average resistivities on the order of 3x10-3 O cm and optical transmittance values of -60%. An increased density of VO and active Al 3+ donors contribute to the reduced resistivity, whereas metallic Al segregated at grain boundaries negatively affected the film optical properties. Post-deposition anneals in N2 were found to generate additional VO and incorporate of Al into the ZnO lattice. X-ray photoelectron spectroscopy (XPS) measurements of the asymmetric O 1s core level were used to determine the degree of oxygen deficiency within the layers as a function of residual water vapor and post-growth annealing.