Revealing buried layers in oxide heterostructures with scanning transmission electron microscopy using integrated differential phase contrast (STEM-iDPC) imaging.

Abstract

Atomic-scale characterisation of thin buried layers in oxide heterostructures is often limited by the reduced sensitivity of conventional Z-contrast high-angle annular dark-field (HAADF) imaging to light elements and low-density regions. Here, a Sc₂O₃/Er₂O₃/Si heterostructure was investigated using aberration-corrected scanning transmission electron microscope (STEM) with simultaneous HAADF and integrated differential phase contrast (iDPC) imaging, with site-specific cross-sectional specimens prepared by scanning electron microscope-plasma focused ion beam (SEM-PFIB) using a Xe-ion beam under identical acquisition conditions to enable direct contrast comparison. Although HAADF clearly resolves the heavy rare-earth oxide layers and reveals a broad interfacial region with contrast variations indicative of structural complexity, simultaneous STEM-iDPC imaging resolves a distinct ∼3 nm buried interfacial layer with substantially greater clarity. Additional high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) analyses reveal oxygen enrichment and local structural ordering within this region, indicating that it is chemically and structurally distinct from both crystalline Si and bulk Er₂O₃. The combined observations are consistent with an oxygen-rich Er-Si-O transition layer. These results demonstrate the capability of STEM-iDPC to reveal buried interfacial structure and highlight the value of combining HAADF, iDPC, HRTEM and spectroscopy for comprehensive characterisation of oxide heterostructures.