Sub-Ångstrom imaging and 3D characterization of interface dislocations in epitaxial superconducting nitride films on sapphire using multislice electron ptychography

Some of the main sources of losses in superconducting nitride-based quantum devices arise from nanoscale defects, amorphous dielectrics, contaminants, impurities, and lattice mismatches [1]. The interface between materials with dissimilar lattice constants are natural hosts for dislocations and defects, with macroscopic columnar structures and leakage pathways often forming from these dislocation cores [2]. With the development of aberration-corrected scanning transmission electron microscopy (STEM), we can routinely resolve projected atomic structure of materials with sub-Ångstrom precision. Even higher spatial resolution while reliably resolving light elements such as oxygen and nitrogen, not just in projection, but in 3 dimensions, is now possible using multislice electron ptychography (MEP) [3]. We utilize MEP to reveal the atomic structure at the interface of an epitaxially-grown TiN on sapphire substrate. From our reconstruction, we observe the 3D atomic-scale behavior of misfit dislocations through the thickness of the sample, as well as the microscopic origins of screw and threading dislocation formation. Despite the abundance of dislocations and columnar growths, TiN resonators have demonstrated high quality factor (Q_i ~ 10⁶) compared to niobium-based systems that suffer from similar defects, and it would be of high interest to further investigate how their microscopic differences affect the device properties.

[1] Muller et al., Rep. Prog. Phys. 82, 124501 (2019)

[2] Sutton and Balluffi, Interfaces in Crystalline Materials (1995)

[3] Chen et al., Science 372, 826-831 (2021)