The Impact of Sample Tilt on Scanning Transmission Electron Microscopy in Strontium Titanate

Annular Bright Field Scanning Transmission Electron Microscopy (ABF-STEM) allows microscopers to image the location of atoms in films as thin as a single atomic layer. In high signal to noise images sub-picometer localization precision is achievable. Recent work has used ABF-STEM to measure oxygen displacements in complex oxides heterostructures, with intent of showing ferromagnetic and multiferroic properties. However, previous work on the accuracy of ABFSTGEM imaging has shown that when a sample is tilted by 6 mrad relative to the electron beam, it creates artificial displacements of 11.9 pm between oxygen and cation columns. Artifacts of this magnitude make picometer-scale measurements of oxygen displacements impossible. However, use of Convergent Beam Electron Diffraction (CBED) can aid sample alignment in the STEM and mistilts can typically be reduced to approximately 1 mrad or better. Thus, there remains an open question as to what kinds of tilt-induced artifacts exist at sample tilts expected in experimental ABF-STEM. In order to quantify the effects of a sample tilt of 1 mrad, I performed multislice image simulations on cubic SrTiO3 over a range of thicknesses from one atomic layer to just over 30 nm. I found that even with only 1 mrad of tilt, artificial displacements so large as 11:8pm between titanium/oxygen and oxygen columns and 4.2 pm between strontium and titanium/oxygen columns are present in ABF-STEM images. I further found that these displacements are not present in HAADF-STEM images, as the displacement between strontium and titanium/oxygen columns was below 0.2 pm at 1 mrad of sample tilt and less than 1.5 pm at sample tilts up to 10 mrad. Because tilts of this magnitude are difficult to control for experimentally, the apparent location of atomic positions in ABF-STEM images may not accurately reflect true atomic structure and measurements of picometer-scale oxygen distortions in complex oxides may not be possible unless sample tilt is carefully controlled