Taming the Untamable: Designing Symmetry-Forbidden Oxide Interfaces for Next-Generation Electronics

Atomically sharp interfaces between 3D complex oxides with different lattice symmetries and spacings have been fabricated. This shows a novel interfacial reconstruction dictated by moiré patterns as observed through scanning transmission electron microscopy.

Interfaces play a critical role in generating emergent electronic behaviors that are driving revolutionary advances in modern technology through applications in light-emitting diodes, photovoltaic solar cells, and field-effect transistors. Atomic-scale engineering of interfaces has led to important discoveries such as quantum Hall effects, giant magnetoresistance, and interfacial superconductivity.

Epitaxial growth has long been the cornerstone for fabricating high-quality interfaces between three-dimensional (3D) materials with ionically bonded lattices. However, it imposes severe structural constraints that limit the combinations of interfacing materials to those with compatible symmetries and lattice parameters. These constraints hinder the creation of interfaces between materials with different lattice symmetries, limiting the exploration of new material combinations and their potential properties.

Mechanical stacking, a non-epitaxial growth method, has successfully created excellent interfaces for two-dimensional materials by exploiting the weak van der Waals bonds between the layers. The success of such a method suggests new possibilities for overcoming the epitaxial constraints of 3D materials. Much research has been devoted to stacking membranes of 3D materials to create interfaces. While some attempts have produced interfaces clean enough for electronic applications, the state-of-the-art falls short of producing atomically clean and sharp interfaces.

In their recent paper published in Advanced Materials, Wang et al. devised a three-step process to fabricate interfaces between 3D materials, using the cubic-to-hexagonal SrTiO₃–sapphire interface as a case study. Scanning transmission electron microscopy (STEM) studies demonstrated that the interfaces were atomically clean and structurally well-defined, devoid of impurities or voids. Electron energy loss spectroscopy (EELS) mapping revealed a compositionally sharp interface between the SrTiO₃ and the sapphire with no evidence of cation intermixing on either side of the interface.

The study revealed an unprecedented moiré-type interface reconstruction dictated by the moiré patterns formed by the overlapping crystal lattices of the SrTiO₃ and sapphire. Complementary electron energy loss fine structure analysis, quantitative STEM, and 4D-STEM studies revealed an electronic structure modulation at the interface coupled with interfacial oxygen defects and polar distortion in the first SrTiO₃ layer adjacent to the interface.

This research opens up new possibilities for engineering symmetry-broken interfaces of 3D materials with unprecedented precision. The flexibility to stack materials and twist them at will opens up a new realm of potential moiré patterns and related phenomena.

The exciting horizon and challenge now lies ahead: Can we achieve novel and useful functionalities by designing symmetry-broken interfaces? Can this developed method be used to fabricate interfaces beyond oxides, such as oxide-carbide interfaces or oxide-nitride interfaces? These questions pave the way for exciting future research and potential technological breakthroughs.