Abstract: The invention provides a method of depositing a buffer layer or film onto a surface of a substrate. The method includes providing the substrate in a controlled atmosphere and exposing the substrate to a vapour comprising a film forming species. While the substrate is exposed to the vapour, two or more ion beams are provided incident upon the surface of the substrate to assist formation of the film. The respective axes of incidence of the two or more ion beams are distinct and are selected and controlled in order to maintain the arrival rate ratio, maximise the deposition rate, and maximise the biaxial alignment of the layer so formed.