Quantitative mapping of strain and displacement fields in complex oxides by STEM studies

Abstract

International audienceComplex oxides, which have a wide range of characteristics, have a lot of potential in the microelectronics industry. They are made up of two or more types of atoms, each of which has its own sublattice constraint. However, the epitaxial integration of piezoelectric and ferroelectric crystals results in a variety of material properties being altered. The ability to measure polarization locally is therefore essential for furthering understanding. Scanning transmission electron microscopy (STEM) is a versatile tool which provides nanoscale information on structural and chemical properties. Here, we present different developments on the analysis of STEM images in the aim to study piezoelectric and ferroelectric materials.Strain and atom displacements are particularly concerned because they alter spontaneous polarization and may result in the creation of inverse polarization domains. The deformed sub-lattices can be strained and/or shifted differently, although sharing a common Bravais lattice. Thus, atomic displacements must be measured for each sub-lattice individually and in relation to others. Such an objective can be achieved by a treatment of high-resolution STEM (HR-STEM) images which allows direct visualization of atomic columns in a crystal. In order to quantify their positions in the image, a real-space method called "peak findings" has been used thus far. One of the method's major flaws is its small field of view (15 nm x 15 nm). Here, we present AbSTEM, a method for HR-STEM image treatment in reciprocal space that allows: i) extraction of sub-lattice images; ii) absolute correction of an ensemble of distortions present in HR-STEM image; iii) mapping of absolute values of interplanar distances and angles, strain tensor components, and displacements measured for each sub-lattice individually and in relation to others, all with high precision and accuracy for large (60 nm x 60 nm) field of view. The limits of the technique will be discussed