Why some interfaces cannot be sharp

Abstract

A central goal of modern materials physics and nanoscience is the control of materials and their interfaces to atomic dimensions. For interfaces between polar and nonpolar layers, this goal is thwarted by a polar catastrophe that forces an interfacial reconstruction. In traditional semiconductors, this reconstruction is achieved by an atomic disordering and stoichiometry change at the interface, but a new option is available in multivalent oxides: if the electrons can move, the atoms do not have to. Using atomic-scale electron energy loss spectroscopy, we have examined the microscopic distribution of charge and ions across the (001) LaAlO3/SrTiO3 interface. We find that there is a fundamental asymmetry between the ionically compensated AlO2/SrO/TiO2 interface, and the electronically compensated AlO2/LaO/TiO2 interface, both in interfacial sharpness and charge density. This suggests a general strategy to design sharp interfaces, remove interfacial screening charges, control the band offset and, hence, markedly improve the performance of oxide devices.

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Figure 1: The polar catastrophe illustrated for atomically abrupt (001) interfaces between LaAlO3 and SrTiO3.
Figure 2: ADF-STEM images of the interface structures.
Figure 3: SrTiO3/LaAlO3/SrTiO3 multilayer.
Figure 4: Repeated growth of LaAlO3/SrTiO3 multilayers.
Figure 5: O-K edge EELS profile across a AlO2/SrO/TiO2 p-type interface.
Figure 6: Chemical profiles of LaAlO3 on (001) SrTiO3 for both interface terminations.
Figure 7: Tuning the band offset.

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Acknowledgements

We thank A. Ohtomo and M. Kawasaki for helpful discussions. This work was supported by the Mitsubishi Foundation, a Grant-in-Aid for Scientific Research on Priority Areas, and the US Office of Naval Research through the ONR EMMA MURI monitored by Colin Wood. N.N. acknowledges partial support from QPEC, Graduate School of Engineering, University of Tokyo. The Cornell Electron Microscope facilities have been supported by the NSF through the MRSEC and IMR programs.

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Correspondence to David A. Muller.

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Nakagawa, N., Hwang, H. & Muller, D. Why some interfaces cannot be sharp. Nature Mater 5, 204–209 (2006). https://doi.org/10.1038/nmat1569

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