Defect Engineering in Complex Oxide Thin Films Using Nanoscale Patterning

Thin film complex oxides provide one of the most versatile platforms to engineer functionalities in next-generation materials, where their complex crystal structure, tri-valent element composition, different structural symmetries, oxygen octahedral rotations and tolerance to strain result in remarkable material properties like ferroelectricity, metal-insulator transitions, and superconductivity to state a few. This also allows them to host defects in their structure, such as 0D point, 1D line, 2D planar defects like stacking faults and 3D bulk defects, introducing unique material and electronic properties.^1,2 Defect engineering in thin films hence provides a platform to modify properties down at the atomic level, where extended defects such as line defects and stacking faults are especially ideal, as they allow property-tuning along a specific dimension in the thin film. Here, we demonstrate the use of a nanoscale patterning technique employed to modify the substrate surfaces using an ion beam, creating atomic-scale roughness along patterned substrate channels that act as defect nucleation sites during film growth by molecular beam epitaxy.
Modified SrTiO₃ substrates with nanometer-scale channels were create by patterning the surface of a SrTiO₃ wafer using a Ga ion beam in a dual-beam FIB-SEM.³ The substrates were then heated at 900 °C to recrystallize the SrTiO₃ around the patterned regions. BaSnO₃ and SrSnO₃ thin films were grown on the modified substrates using hybrid MBE.⁴ Film growth in the modified regions were studied using a combination of SEM imaging of the film surfaces, scanning transmission electron microscopy (STEM) imaging and energy dispersive X-ray spectroscopy (EDX). Cross-section and plan-view samples for defect study of the films in the modified growth areas were prepared using a FIB.⁵
The thin film structure for BaSnO₃ and SrSnO₃ in the patterned SrTiO₃ substrate regions created under different Ga ion doses were studied for defect nucleation. In each case, film structure in the regions away from the patterned areas showed atomically sharp film-substrate interfaces, while growth over the recrystallized channels showed a much rougher interface, depending on the shape of the channel. These roughened interfaces at the substrate channel slopes coupled with additional in- and out-of-plane strain enabled nucleation of different types of extended defects. In the BaSnO₃ films growing over the recrystallized channels, atomic resolution STEM analysis of the film revealed a 5-fold increase in density of threading dislocations when compared to the bulk reference. This allows a pathway to increase the density of these metallic-like dislocations in the insulating host film, providing unidirectional conduction pathways as well as ability to tailor properties of these films at the atomic level. A similar study on the SrSnO₃ films revealed a high density of planar Ruddlesden Popper faults in the film over the channels, with the concentration of the defects approaching the theoretical limit, three orders of magnitude higher than the bulk, allowing modification and control of the electronic properties in these films. With this method, localized high density of extended defects can hence be engineered and used for selective property modulation in a wide range of other complex oxides for future studies.⁶
References:
1. H. Yun et al., Nano Lett., 2021, 21, 4357-4364.
2. H. Yun et al., Sci. Adv., 2021, 7, eabd4449.
3. S. Ghosh et al., (Under Review), 2024, arXiv:2408.02995
4. A. Prakash et al., J. Vac. Sci. Technol. A 2015, 33, (6), 060608.
5. S. Ghosh et al., (Under Review), 2024, arXiv:2401.02538.
6. This work was supported primarily by the National Science Foundation (NSF) through award No. DMR-2309431, and partially by University of Minnesota MRSEC under award No. DMR-2011401.