Emergent phenomena at complex oxide interfaces continue to attract attention as the basis for a variety of next-generation devices, including photovoltaics and spintronics. Tremendous progress has been made toward understanding the role of interfacial defects, cation intermixing, and film stoichiometry in single heterojunction systems; however, the techniques commonly used to study these interfaces, such as X-ray photoelectron and absorption spectroscopies, are either sensitive only to near-surface regions or do not offer depth resolution to probe individual interfaces. Here we explore the induced polarization in superlattices of LaCrO3 (LCO) and SrTiO3 (STO) using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy loss spectroscopy (STEM-EELS). We show that a correlative approach, utilizing an array of local and non-local probes, is necessary to fully understand the defect-mediated origin of the induced polarization in this system.
We have conducted detailed structural characterization of several LCO-STO superlattices, as shown in Figure 1. We employ high-angle annular dark field imaging (STEM-HAADF) to directly measure the induced ferroelectric polarization in the STO layers. We first acquire a relatively high-speed time series of multiple fast frames (0.4 µs px-1), which are then aligned using both rigid and non-rigid registration to remove both sample drift and scan distortion . Using this procedure we directly measure the induced polarization with picometer precision, as we have demonstrated elsewhere . Our results reveal that the built-in asymmetric potential across the LCO / STO interfaces is sufficient to induce a sizable polarization, on the order of 40-70 µC cm-2, in good agreement with ab initio calculations .
We next perform detailed characterization of chemical intermixing and local electronic fine structure changes to explore how defects affect the induced polarization. Figure 2 shows the result of monochromated EELS measurements of the Ti L23 edge fine structure, overlaid onto the integrated Ti L23 edge signal. An improved energy resolution of better than 0.120 eV allows us to observe significant Ti intermixing through the superlattice, as well as subtle fine structure changes in the vicinity of the LCO layers not apparent in earlier data. Mapping the Ti L3 t2g – eg crystal field splitting across the film, we find evidence consistent with a slight reduction in Ti valence from 4+ to 3+ in the vicinity of the LCO layers, possibly the consequence of La3+ substitution for Sr2+ or oxygen vacancies. Measurements of the Ti L3 t2g / eg ratio also point toward such a trend: moving from the STO toward the LCO layers the ratio begins to decrease within the intermixed region, indicating a redistribution of electrons from t2g to eg states, suggesting a reduction in valence. In light of these results, our experimental STEM-HAADF measurements and accompanying ab initio calculations indicate that the induced polarization is robust against even sizable chemical intermixing and defect formation.
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 Spurgeon, S. R. et al, “Polarization screening-induced magnetic phase gradients at complex oxide interfaces.” Nat. Commun. 6, 1–11 (2015).
 Comes, R.B. et al, “Interface-induced Polarization in SrTiO3-LaCrO3 Superlattices.” Adv. Mater. Int. (2016). DOI: 10.1002/admi.201500779
To cite this abstract:Steven Spurgeon, Despoina Kepaptsoglou, Lewys Jones, Ryan Comes, Quentin Ramasse, Phuong-Vu Ong, Peter Sushko, Scott Chambers; Monochromated STEM-EELS Analysis of Interface-Induced Polarization in LaCrO3-SrTiO3 Superlattices. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/monochromated-stem-eels-analysis-of-interface-induced-polarization-in-lacro3-srtio3-superlattices/. Accessed: March 21, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/monochromated-stem-eels-analysis-of-interface-induced-polarization-in-lacro3-srtio3-superlattices/