Irradiation-induced amplification of electric fields at oxide interfaces as revealed by correlative DPC-STEM and DFT

Abstract

Heterointerfaces are ubiquitous in modern devices, found in technologies ranging from microelectronics to structural components for energy applications. Many of these emerging technologies are found in applications such as satellites, batteries, and next generation nuclear reactors, that are subject to harsh environments. In some scenarios, multiple extreme conditions, such as irradiation and corrosion, act on the material simultaneously. Extending the lifetime of these technologies is dependent on a detailed understanding of how their component materials platforms and interfaces respond in extreme environments, where irradiation and corrosion may couple in unique ways, distinct from corrosion under ambient conditions. Oxides, which form readily over metal underlayers, can act as protective coatings; enhancing the robustness of oxide overlayers to protect underlying metal alloys is a potential avenue towards corrosion mitigation. Here we study the impact of irradiation-induced non-equilibrium defects on charge segregation and electric fields at and near multi-phase oxide heterointerfaces. We perform a detailed study of irradiated Fe2O3-Cr2O3 thin film heterostructures using first-principles DFT electronic structure modeling paired with 4D-STEM DPC and EELS techniques to measure nanoscale changes in electric fields. Our results show clear evidence that irradiation drives substantial modulation of interfacial electric fields that can be tailored by controlling the atomistic chemical structure of the oxide interface. We show that irradiation can selectively induce built-in electric fields, thereby altering their direction; this suggests a pathway to engineering protective oxide heterostructure overlayers that can electrically control the spatial distribution of defects, with significant implications for the design of corrosion-resistant materials for extreme environments.

Figures (12)

• Figure 1: The atomistic chemical and structural details of each interface type and associated layer-resolved density of states. For the (a) abrupt interface, each cation layer only contains one type of cation, Fe (gold) or Cr (blue), while for the (d) mixed interface the interfacial cation layer contains both Fe and Cr. Oxygen ions are shown in red. The layer-resolved density of states and relative band edge positions of the (b,c) abrupt interface and (e,f) mixed interface.
• Figure 2: The setup and selected data for the 4D-STEM and DPC-PED measurements. HAADF image of (a) Cr$_{2}$O$_{3}$ / Fe$_{2}$O$_{3}$ mixed interface and (b) Fe$_{2}$O$_{3}$ / Cr$_{2}$O$_{3}$ abrupt interface after irradiation. (c) Schematic of the DPC-PED setup for oxide interfaces with a nearly parallel beam illumination. k$_{x}$ and k$_{z}$ are the x-axis and z-axis in reciprocal space when collecting data and calculating the CoM shifts. (d) Mean diffraction pattern of 4D-STEM dataset from irradiated Fe$_{2}$O$_{3}$ / Cr$_{2}$O$_{3}$ with an abrupt interface. Fe$_{2}$O$_{3}$ and Cr$_{2}$O$_{3}$ are on zone [$\bar{1}$010]. The (10$\bar{1}$0) direction in reciprocal space is the in-plane direction, while the (0002) direction is the out-of-plane direction. Dotted and dashed circles indicate the CoM of the center disk on the X$_{2}$O$_{3}$ and Y$_{2}$O$_{3}$ side of the interface, respectively. The black circle indicates the mask. (e) Absolute thickness mapping of the cross-sectional lamellae measured through EELS. (f) Zoom-in of absolute thickness mapping measured through EELS from dotted rectangle in (e). (g) Calculated E-field. (h) Zoom-in of E-field from dotted rectangle in (g). The scale bars in (a-c) and (e-h) indicate 50 nm.
• Figure 3: The out-of-plane electric fields and integrated potentials at the oxide interfaces. (a), (c), (e), and (g) are the electric distribution maps (MV/cm) for each of the four heterostructure samples with white arrows indicating the position of the interface. In irradiated samples, the side of the interface that was irradiated is indicated in orange text. In the mixed-pristine case, the area used to integrate the electric field to find the potential is enclosed in the black lines. (b), (d), (f), and (h) are plotted profiles of the out-of-plane mean electric field (left y-axis, gray line) and mean integrated potential (MIP) (right y-axis, blue line) plotted with respect to position along the (0001) axis (nm). The dotted line (burgundy) is a smoothed mean profile of E-field (3-points fast Fourier transform filter). The dashed line (green) denotes the change in MIP at the interface. Scale bars indicate 20 nm.
• Figure 4: The charge density along 2D sheets obtained from integrating the electric field across the oxide interfaces using Gauss's law.(a) Plots of the statistical density of the charge density in close proximity (10 nm) to the interface for the four test cases. (b) Plots of the statistical density of charge density integrated over a broad (50 nm) region on either side of the interface. The irradiated oxide is indicated in red boxes and arrows. Charge densities are separately shown for Fe$_{2}$O$_{3}$ (gold) and Cr$_{2}$O$_{3}$ (blue) sides of the interface, as indicated in the schematics (right). In all cases, the charge densities are ordered with the bottom oxide on the left and the top oxide on the right.
• Figure 5: The relative band offsets (neglecting in-gap defect states) of the abrupt and mixed interface heterostructures with oxygen vacancies 1.4 nm from the interface. For (a)(b) the abrupt interface, the Fe$_{2}$O$_{3}$ layer contains the oxygen vacancy. The Fe$_{2}$O$_{3}$ band edges shift down relative to the band edges of Cr$_{2}$O$_{3}$, with increasing magnitude as the oxygen vacancy charge state increases. For (c)(d) the mixed interface, the Cr$_{2}$O$_{3}$ layer contains the oxygen vacancy. The Cr$_{2}$O$_{3}$ band edges shift down relative to the band edges of Fe$_{2}$O$_{3}$, also with increasing magnitude as the oxygen vacancy charge state increases.