Abstract
Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1,2,3,4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5,6,7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.
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Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
The team at MIT and the University of Wisconsin-Madison acknowledge support primarily by the Defense Advanced Research Projects Agency (DARPA) (award number 027049-00001, W. Carters and J. Gimlett). The work at University of Wisconsin-Madison is also supported by the Army Research Office through grant W911NF-17-1-0462. C.A.R. and J.B. acknowledge support from the SMART Center sponsored by NIST and SRC. J.A.R. and S. Subramanian acknowledge support from NSF CAREER award 1453924. J.H.L. acknowledges support from a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (number 2018R1D1A1B07050484). The work at Cornell University is supported by the National Science Foundation (Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM)) under Cooperative Agreement Number DMR-1539918. J.K. thanks the Masdar Institute/Khalifa University, the LG Electronics R&D Center, Amore Pacific, the LAM Research Foundation, Analogue Devices, and Rocky Mountain Vacuum Tech for general support. We are grateful to J. Li for assistance with the TEM measurements. We especially thank R. Bliem and B. Yildiz of MIT for early help in preparation of STO films.
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Authors and Affiliations
Contributions
J.K. and C.B.E. conceived the idea and directed the team. H.S.K. designed and coordinated the experiments and characterization. H.S.K., H. Lee., S. Lindemann, W.K. and K.Q. performed epitaxial growth (pulsed-laser deposition and sputtering), characterization and heterogeneous integration development under the guidance of C.-B.E. and J.K. Epitaxial growth via MBE was performed by J.H.L. and S.X. under the guidance of D.G.S. Material characterization was done by H.S.K., P.C., L.R., S. Seo, C.C., S.-H.B. and K.L. Magnetoelectric coupling data analyses were performed by H.S.K, J.I. under the guidance of M.S.R. Device fabrication was carried out by H.S.K. and J.S. Magnetostatic and magnetoelastic data were analysed by H.S.K, S. Lee and J.B. under the guidance of C.A.R. The epitaxial graphene was grown by S. Subramanian under the guidance of J.A.R. Density functional theory calculations were performed by H. Li. All TEM imaging and analyses were performed by S.K. The manuscript was written by H.S.K., J.K. and C.B.E. All authors contributed to the analysis and discussion of the results leading to the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Density functional theory simulation of substrate surface potential penetrating through graphene layers on a STO substrate.
a, Illustration of the simulated structure. b, The potential fluctuation through graphene as a function of monolayer (1ML) and bilayer (2ML) graphene thickness d. The inset shows the potential fluctuation map on the surface of graphene-coated STO substrates for monolayer graphene (left) and bilayer graphene (right). c, The potential fluctuation map (colour scale) with three monolayers of graphene on top of the STO surface. d, Cross-sectional potential profile along the red line shown in c.
Extended Data Fig. 2 Pulsed-laser deposition of STO on graphene-coated STO substrate.
a, RHEED pattern during growth of STO on graphene-coated STO substrate, showing crystalline growth through the entire growth process. The yellow arrows indicate RHEED patterns caused by the transferred graphene. b, RHEED oscillation during the growth of STO on the graphene-coated STO substrate.
Extended Data Fig. 3 Cross-sectional TEM analysis of exfoliated STO membrane.
a, Low-magnification TEM image of STO membrane supported by the Ni stressor layer. b, Zoom-in of polycrystalline domains caused by residues left on graphene after transfer. c, High-resolution TEM image of the STO membrane (centre), with selected area electron diffraction (left) and high-resolution TEM (right) images, confirming the overall single-crystallinity of the membrane. The single-crystalline STO membrane has a cubic structure in the \(Pm\bar{3}m\) space group with lattice distances d100 and d001 of 0.4 nm.
Extended Data Fig. 4 STEM analysis of the SrTiO3 buffer layer grown in vacuum.
a, and b, show the bright-field (BF) and HAADF-STEM images of the exfoliated STO membrane (the same TEM sample as shown in Extended Data Fig. 3) at low magnification, respectively. c and d show higher-resolution bright-field and HAADF-STEM images of the sample, respectively. e, Electron energy loss spectroscopy spectra and line profile from the exfoliated surface to the bulk region (A–B in d), verifying that the composition of the buffer layer grown in vacuum is identical to the region grown under oxygen overpressure. f, High-resolution HAADF, showing individual atoms of the STO membrane at the exfoliation surface (the region grown in vacuum). The annular bright-field (ABF) and contrast inverted annular bright-field images (g and h, respectively) clearly show the absence of oxygen vacancies and no discernible differences are observed between the regions grown in vacuum and in oxygen.
Extended Data Fig. 5 Cross-sectional TEM analysis of exfoliated CFO membrane.
a, Cross-sectional TEM of exfoliated CFO on the Ti/Ni stressor layer. The red dotted line indicates a polycrystalline domain caused by residues left during graphene transfer. b, Zoomed-in TEM of the polycrystalline area marked by red dotted lines. c, Higher-resolution cross-sectional TEM of the CFO film (centre), with selected area electron diffraction (left) and high-resolution TEM (right) images, confirming the overall single-crystallinity of the membrane. The single-crystalline CFO membrane has a cubic structure in the \(Fd\bar{3}m\) space group with a lattice distance d200 and d002 of 0.41 nm.
Extended Data Fig. 6 Exfoliation and characterization of BaTiO3 membrane grown via MBE.
a, Photograph of exfoliated BTO membrane (50 nm) grown via remote epitaxy. b, EBSD of the exfoliated BTO membrane showing single-crystalline (100) orientation. c, The inverse-pole map of the EBSD data shown in b. d, Electron backscattering patterns (also known as Kikuchi patterns) of the BTO membrane seen over the entire area of the sample.
Extended Data Fig. 7 Reusability of a graphene-coated MAO substrate.
a, b, Microscope images of a MAO substrate after exfoliating a CFO film grown on monolayer (a) and bilayer (b) graphene, where severe damage on the surface of the MAO substrate after exfoliation of CFO grown on monolayer graphene was observed, caused by crack propagation into the substrate. No evidence of damage was observed on substrates coated with bilayer graphene because the second graphene transfer covers the macroscopic defective areas of the first graphene layer. The scale bar indicates 1 mm. c, AFM of the pristine MAO substrate surface (left) and after one cycle of CFO exfoliation (right) with a root-mean-square roughness of approximately 5.5 Å before and after exfoliation. Scale bar indicates 1 µm. d, e, Raman spectra showing the characteristic D peak, G peak and 2D peak of graphene (d) and Raman intensity mapping (e) of the 2D peak (2,685 cm−1) of graphene on the MAO substrate after one cycle of CFO exfoliation, showing evidence that graphene is preserved on the MAO substrate after exfoliation, probably because the non-specific adhesion between graphene and CFO is weaker than that between graphene and MAO. Where this is not the case, we could etch off any graphene remaining on the substrate and re-deposit graphene before epitaxy. The scale bar indicates 10 µm. f, Magnetic hysteresis M of the three exfoliated CFO membranes produced on a single graphene-coated MAO substrate measured by vibrating sample magnetometry at room temperature. a.u., arbitrary units.
Extended Data Fig. 8 STEM imaging and strain analysis of the PMN-PT/SRO/STO interfaces.
a, b, Cross-sectional HAADF-STEM images of the PMN-PT/SRO/STO interfaces with and without a Ni stressor layer. Clear straining at the PMN-PT/SRO interface can be seen with a Ni stressor layer, whereas the SRO/STO interface remains unstrained. c, Atomic-resolution STEM image of one of the periodic edge dislocations observed at the PMN-PT/SRO interface. d, Geometric phase analysis of the PMN-PT/SRO and SRO/STO interfaces in the x direction (2nd column), y direction (3rd column) and rotational geometry (last column) with and without the Ni stressor layer. The white arrows indicate edge dislocations. The colour scale indicates the strain fraction with reference to the SRO substrate.
Extended Data Fig. 9 Description of the in situ TEM CFO/PMN-PT heterostructure device.
a, A cross-sectional SEM of the in situ CFO/PMN-PT magnetoelectric device. A thick Pt layer (labelled TEM probe contact) was deposited on top of the 7-nm-thick Pt layer to enable the TEM probe tip to establish electrical contact. The TEM probe contact was intentionally made much thicker and further away from the actively observed region (distance greater than 5 µm) to prevent effects caused by bending of the sample. b, High-resolution TEM image of the CFO/Pt/PMN-PT interface, showing a thin amorphous oxide layer that has formed between the CFO and Pt, enabling efficient strain coupling.
Extended Data Fig. 10 CFO magnetic hysteresis as a function of voltage applied across PMN-PT.
CFO magnetism with a varying voltage bias across a PMN-PT measured via vibrating sample magnetometry. a, In the clamped structure, the PMN-PT film is grown on a SRO/STO substrate, and the CFO membrane is transferred on top of a thin Pt layer deposited on top of PMN-PT. b, In the freestanding structure, the PMN-PT membrane is transferred onto a PDMS substrate after exfoliation. The rest of the stack is identical.
Supplementary information
Supplementary Information
This file contains Supplementary Section 1, the graphene transfer process and Supplementary Section 2, Supplementary Figures 1-5.
Video 1
In situ TEM of (100) CFO as a function of voltage applied across (100) PMN-PT for a freestanding CFO/PMN-PT magnetoelectric coupled membrane device. The voltage is swept from 0 to 10 V.
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Kum, H.S., Lee, H., Kim, S. et al. Heterogeneous integration of single-crystalline complex-oxide membranes. Nature 578, 75–81 (2020). https://doi.org/10.1038/s41586-020-1939-z
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DOI: https://doi.org/10.1038/s41586-020-1939-z
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