Epitaxial oxide growth on polar (111) surfaces

Obtaining atomically smooth surfaces and interfaces of perovskite oxide materials on polar (111) surfaces presents a particular challenge as these surfaces and interfaces will reconstruct. Here, the effect of the use of screening buffer layers on the epitaxial growth on such polar surfaces is investigated. Both transmission electron microscopy and in situ reflective high energy electron diffraction data imply that the buffer layers, SrRuO3 or LaAlO3, restore a near bulk-like termination at growth temperature, allowing for coherent growth of BiFeO3 and CaTiO3 for all deposited unit cell layers of the film material. An important crystalline orientation in the perovskite system, in terms of the ability to use strain engineering and to fabricate superlattices, is the (111) direction (A schematic drawing of a (111) oriented perovskite is shown in Figure 1(a)). For example, if superlattices of perovskites are grown in the (111) orientation, it is possible to build artificial double perovskites.1,2 Another example is work done by Xiao et al.,3 which shows that it is possible to build a topological insulator by growing in the (111) direction. A lot of knowledge within the field of oxide thin films has been gained by the many heteroepitaxial systems involving cubic (001) substrates like SrTiO3 in which case the surface and interface with the film perovskite structure has been well studied. From these studies, it has become clear that a good perovskite substrate surface fulfills three important criteria: (1) single termination; (2) a stable interface with air/vacuum without major reconstructions; and (3) a stable interface with the film material.