Local conductivity and the role of vacancies around twin walls of (001)-BiFeO3 thin films

BiFeO3 thin films epitaxially grown on SrRuO3-buffered (001)-oriented SrTiO3 substrates show orthogonal bundles of twin domains, each of which contains parallel and periodic 71 degrees domain walls. A smaller amount of 109 degrees domain walls are also present at the boundaries between two adjacent bundles. All as-grown twin walls display enhanced conductivity with respect to the domains during local probe measurements, due to the selective lowering of the Schottky barrier between the film and the AFM tip [S. Farokhipoor and B. Noheda, Phys. Rev. Lett. 107, 127601 (2011)]. In this paper, we further discuss these results and show why other conduction mechanisms are discarded. In addition, we show the crucial role that oxygen vacancies play in determining the amount of conduction at the walls. This prompts us to propose that the oxygen vacancies migrating to the walls locally lower the Schottky barrier. This mechanism would then be less efficient in non-ferroelastic domain walls where one expects no strain gradients around the walls and thus (assuming that walls are not charged) no driving force for accumulation of defects.