Overcoming atomic structural defects in oxide film synthesis with low temperature

An ongoing challenge in synthesizing oxide thin films and heterostructures is separating the oxide from the substrate. In 2016, researchers from California and New York proposed growing crystals with a water-soluble layer, made of Sr₃Al₂O₆, between oxides and substrates, which can then be dissolved to peel off the oxide thin films. While promising, the Sr₃Al₂O₆ buffer layer method introduces structural defects in the atomic lattice, resulting in undesirable cation movements. Recently, the researchers have demonstrated that lowering the growth temperature results in high-quality oxide superlattices on top of the water-soluble buffer layer. They report their finding in APL Materials.

Principal investigator Lena Kourkoutis says that together with the group of Harold Hwang they have spent years optimizing the growth conditions for films grown directly on the substrate but when introducing Sr₃Al₂O₆ they had not known the diffusion of cations to be the cause of unintended chemistry changes in the film. The atomic-scale examination of the synthesis process that drove the hypothesis verified in the study came about only after recent advances in electron microscopy made it possible.

The researchers discovered that the Sr₃Al₂O₆ buffer layer method can cause unavoidable structural changes in the atomic lattices that, in turn, induce cation diffusion. They found that by lowering the thermodynamic driving force for diffusion during the growth of crystals, cation movements are suppressed, reducing unintended chemistry changes and thereby improving the quality of oxide thin films produced.

So far, the researchers have tested their new method on La_0.7Sr_0.3MnO₃/SrTiO₃ superlattices. The coauthors indicate they intend to refine the method to extend the reach of this water-soluble buffer layer to produce freestanding oxide thin films, while also making the films thinner.

Source: “Mapping cation diffusion through lattice defects in epitaxial oxide thin films on the water-soluble buffer layer Sr₃Al₂O₆ using atomic resolution electron microscopy,” by David J. Baek, Di Lu, Yasuyuki Hikita, Harold Y. Hwang, and Lena F. Kourkoutis, APL Materials (2017). The article can be accessed at https://doi.org/10.1063/1.4994538 .