Abstract
Ultrathin ferroelectric materials could potentially enable low-power logic and nonvolatile memories1,2. As ferroelectric materials are made thinner, however, the ferroelectricity is usually suppressed. Size effects in ferroelectrics have been thoroughly investigated in perovskite oxides—the archetypal ferroelectric system3. Perovskites, however, have so far proved unsuitable for thickness scaling and integration with modern semiconductor processes4. Here we report ferroelectricity in ultrathin doped hafnium oxide (HfO2), a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to a thickness of one nanometre. Our results indicate not only the absence of a ferroelectric critical thickness but also enhanced polar distortions as film thickness is reduced, unlike in perovskite ferroelectrics. This approach to enhancing ferroelectricity in ultrathin layers could provide a route towards polarization-driven memories and ferroelectric-based advanced transistors. This work shifts the search for the fundamental limits of ferroelectricity to simpler transition-metal oxide systems—that is, from perovskite-derived complex oxides to fluorite-structure binary oxides—in which ‘reverse’ size effects counterintuitively stabilize polar symmetry in the ultrathin regime.
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The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
Change history
06 May 2020
A Correction to this paper has been published: https://doi.org/10.1038/s41586-020-2297-6
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Acknowledgements
This research was supported in part by the Berkeley Center for Negative Capacitance Transistors (BCNCT), ASCENT (Applications and Systems-Driven Center for Energy-Efficient Integrated NanoTechnologies), one of the six centres in the JUMP initative (Joint University Microelectronics Program), an SRC (Semiconductor Research Corporation) programme sponsored by DARPA, the DARPA T-MUSIC (Technologies for Mixed-mode Ultra Scaled Integrated Circuits) programme and the UC MRPI (University of California Multicampus Research Programs and Initiatives) project. This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract number DE-AC02-05CH11231. Use of the Stanford Synchrotron Radiation Light source, SLAC National Accelerator Laboratory, is supported by the US DOE, Office of Science, Office of Basic Energy Sciences under contract number DE-AC02-76SF00515. Electron microscopy was performed at the Molecular Foundry, LBNL, supported by the Office of Science, Office of Basic Energy Sciences, US DOE (DE-AC02-05CH11231). J.C. and R.d.R. acknowledge additional support from the Presidential Early Career Award for Scientists and Engineers (PECASE) through the US DOE. J.X and X.Z acknowledge support from the National Science Foundation (NSF) under grant 1753380 and the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research award OSR-2016-CRG5-2996.
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Contributions
Film synthesis was performed by S.S.C., G.K. and D.K. Device fabrication was performed by D.K. Electron microscopy was performed by R.d.R. and S.-L.H. under the supervision of J.C. and R.R., respectively, and analysis was performed by L.-C.W. under the supervision of S.S. Scanning probe microscopy was performed by S.S.C. and N.S. IDS measurements were performed and developed by R.W. and R.P. SCM was performed by H.Z. X-ray structural characterization was performed by S.S.C., N.S. and M.R.M. under the supervision of A.M. and E.K. X-ray spectroscopy and microscopy was performed by S.S.C. under the supervision of R.V.C., P.S. and E.A. Second harmonic generation was performed by J.X. under the supervision of X.Z. Electrical measurements were performed by S.S.C., N.S. and A.D. S.S.C. and S.S. co-wrote the manuscript. S.S. supervised the research. All authors contributed to discussions and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Size effects in fluorite- and perovskite-structure ferroelectrics.
a, In perovskite ferroelectrics, the polar ‘tetragonal’ distortion (c/a) can be represented as the centre cation displacement with respect to its surrounding oxygen octahedron. b, In fluorite-structure ferroelectrics, the polar ‘rhombic’ distortion (2c/(a + b)) can be represented as the centre anion displacement with respect to its surrounding cation tetrahedron; in the nonpolar T-phase, the oxygen atom (blue) lies in the polyhedral centre of the tetrahedron. The evolution of the bulk-stable M-phase to the high-symmetry T-phase and polar O-phase in the fluorite-structure structure illustrates the role of size effects (surface energies favour higher symmetry) and confinement strain (distortions favour lower symmetry) on stabilizing inversion asymmetry. Surface energies are critical when considering the role of size effects on ferroelectricity; higher-symmetry phases are energetically favourable at reduced dimensions owing to lower unit cell volumes. In fluorite structures (perovskites), the noncentrosymmetric O-phase (T-phase) has higher (lower) symmetry than the bulk-stable centrosymmetric M-phase (C-phase). Consequently, surface energies help to counteract depolarization fields in fluorite-structure ferroelectrics—already diminished in fluorite structures relative to perovskites owing to its lower dielectric constant4—in the ultrathin regime. Therefore, both intrinsic (surface energies) and extrinsic (confinement strain) mechanisms can favour ultrathin inversion symmetry breaking in fluorite structures. Meanwhile, both surface and depolarization energies tend to destabilize inversion asymmetry in perovskite ferroelectrics, while epitaxial strain can stabilize symmetry-lowering polar distortions36.
Extended Data Fig. 2 Thickness verification of ultrathin HZO films from XRR.
a, Laboratory diffractometer XRR of HZO thickness series, demonstrating clear fringes for thickness extraction present down to 20-cycle HZO. b, Synchrotron XRR of ultrathin HZO films, enabling thickness fitting analysis for sub-20-cycle films. c, HZO thickness as a function of ALD cycles, as determined by fitting analysis from XRR. The growth rate is about 11 cycles nm−1, verified across 10–100 ALD cycle films. Squares (circles) represent thicknesses extracted from fitting to synchrotron (laboratory diffractometer) XRR measurements.
Extended Data Fig. 3 Thickness verification of ultrathin HZO films using TEM.
a, HZO thickness as a function of ALD cycles, as determined by Si atomic lattice calibration from TEM imaging. The growth rate is ∼11 cycles nm−1, verified across 10–50 ALD cycle films, consistent with XRR (Extended Data Fig. 2). The red error bars reflect 2σ variation. b, Cross-sectional ADF STEM image of 20 cycles HZO. c–e, Cross-section TEM images of ten-cycle HZO (c), 15-cycle HZO (d) and 40-cycle HZO (e). f–h, Wide field-of-view TEM images of ten-cycle HZO (f), 15-cycle HZO (g) and 40-cycle HZO (h) to provide a perspective of the heterostructure uniformity. The Si substrate is oriented along the [110] zone axis for all TEM images.
Extended Data Fig. 4 Emergence of highly-textured films in the ultrathin regime.
a, Synchrotron GI-XRD scans (λ = 0.775 Å) of HZO thickness series endmembers: 10-cycle and 100-cycle. The 100-cycle HZO film is indexed according to the polar orthorhombic phase Pca21. Many of the polycrystalline reflections, most notably the (111), are no longer present at an appreciable intensity in the ultrathin limit owing to the geometric constraints of one-dimensional spectra (unable to probe all reflections present in highly oriented films) (Methods). Instead tilted-geometry diffraction (pole figures) are used to access the oriented reflections. b, Pole figure of ten-cycle HZO, taken at a Qx slice corresponding to the film (111) lattice spacing. The radial direction represents χ, while the azimuthal direction represents φ (0°–360° range). The presence of four intense peaks corresponding to the four (111)-projections indicate the highly textured nature of the ultrathin HZO film. The four Si (111)-projections would be expected at φ = 45° off from the Qx,y principal axes at a smaller value of Qz. c, Schematic of the (311) (left) and (111) (right) close-packed planes in the fluorite-structure structure. All the cation sites lie on such planes, which minimize surface energy effects because only metal-oxygen dangling bonds are present out-of-plane. We note that all schematics reflect stacking of the respective planes to a total thickness of 1 nm, although ultrathin HZO films may not exhibit such stacking throughout the film. For ten-cycle films, {311} indexing is consistent with the relevant intensity (about 30°) observed in the out-of-plane one-dimensional GI-XRD pattern (a), and the (111) reflections are present from the two-dimensional pole figure pattern (b).
Extended Data Fig. 5 Inversion symmetry breaking in ultrathin HZO via SHG.
a, Schematic of the SHG experimental setup, using a 960-nm pump and SHG intensity detected around 480 nm under tilt incidence, which is sensitive to out-of-plane polarization (Methods). NIR, near-infrared; 1D, one-dimensional; PBS, polarized beam splitter; Obj, objective; LP, BP and SP represent long-pass, band-pass and short-pass filters; DMSP, dichroic short-pass mirror; M1, M2 and M3 refer to mirrors; OPO, optical parametric oscillator. b, Schematic of the ten-cycle HZO islands probed by SHG (Methods); micrometre-sized islands enabled identification of specific HZO regions either poled with an electric field (applied by a PFM tip) or left as is. For these experiments, heavily doped (1019 cm−3) p-type Si substrates (p++ Si) are used to serve as the bottom electrode. c, SHG spectrum on a ten-cycle HZO film, comparing poled versus unpoled SHG intensity. Spontaneous polarization is demonstrated by the presence of SHG—allowed only for inversion asymmetric systems—in unpoled ten-cycle HZO. This is consistent with PFM phase contrast in unpoled HZO regions (Fig. 2c), indicating elimination of the ‘wake-up’ effects for ferroelectricity in ultrathin HZO. The enhanced SHG contrast in poled films—possibly due to the electric field converting a small fraction of the film to the polar phase or aligning polar domains—indicates that the mechanism behind the SHG contrast is field-tunable. This field-enhanced SHG is consistent with ferroelectric origins and would probably eliminate SHG contrast from surface effects.
Extended Data Fig. 6 Role of ultrathin confinement for polar phase stabilization.
a, b, Schematic structure (left) probed by PFM (tip location indicated by arrows), topography (centre), and PFM phase contrast images (right) on ten-cycle HZO in a region that was uncapped (a) versus confined (b) by W (represented by ‘M’ for metal in the schematic) during phase annealing. Robust 180° phase contrast is only present for the confined HZO. c, Phase (left) and amplitude (right) switching spectroscopy loops (Vdc = 0, ‘OFF’ state) as a function of bias voltage on ten-cycle HZO films, demonstrating the critical role of confinement during phase annealing in stabilizing ferroelectricity in ultrathin HZO. 180° phase contrast and butterfly-shaped amplitude are present only for confined HZO. Therefore, both switching-spectroscopy PFM and PFM imaging illustrate the critical role of confinement during phase annealing for stabilizing the ferroelectric phase. For the PFM images, ±7 V was applied in a ‘box-in-box’ poling pattern directly on the HZO surface, and switching-spectroscopy PFM loops were measured on capacitor structures (Methods). d, Schematic structure (left) probed by PFM (tip location indicated by arrows) and PFM phase and amplitude hysteresis loops (right) as a function of bias voltage on 100-cycle HZO in a region that was confined by W during phase annealing. Thicker 100-cycle HZO also demonstrates ferroelectric behaviour.
Extended Data Fig. 7 Eliminating artefacts from scanning probe microscopy.
a, Topography and PFM phase contrast images for ten-cycle HZO which did not (left) and did (right) undergo annealing after ALD deposition. The terraced topography in the non-annealed film indicates that the weak phase contrast is falsely caused by field-induced topographic changes. This is consistent with charge injection or ion migration, which plague amorphous HfO2 films25. Phase-annealed films do not display such field-induced topographic distortions yet demonstrate much clearer phase contrast, indicating the origin of PFM phase contrast in crystalline HZO films is different than that of amorphous HZO films. In the images shown, ±7 V were applied in a ‘box-in-box’ poling sequence. b, Time-dependent PFM phase contrast images on a ten-cycle HZO film across a 24-h period. In the images shown, ±7 V was applied in the indicated checkerboard poling pattern. c, Collapse of the PFM loop from Vac-series. Schematic capacitor structure probed by PFM (top) and piezoresponse as a function of Vac in the ‘OFF’ (Vdc = 0) state (bottom), demonstrating the collapse of the PFM loop as Vac approaches the coercive voltage. This provides further confirmation of the ferroelectric origin of the PFM signal as opposed to tip bias-induced mechanisms46. The non-ideal shape of the piezoresponse loops, particularly at higher voltages, is probably caused by non-ferroelectric contributions from the additional dielectric SiO2 layer through which most of the voltage is dropped (Methods). d, IDS switching-spectroscopy measurements on ten-cycle (1 nm) HZO, demonstrating hysteresis for the PFM tip on-surface (top) versus no hysteresis for the tip off-surface (bottom). The on-surface loops indicate 180° phase hysteresis and butterfly-shaped d33, indicative of ferroelectric behaviour. IDS PFM measurements (Methods) remove the long-range electrostatics and cantilever resonance artefacts that plague typical voltage-modulated PFM switching spectroscopy26. This ferroelectric origin of the hysteresis is further supported by non-hysteretic off-surface loops26, which probe electrostatic contributions.
Extended Data Fig. 8 High-frequency capacitance characterization of ultrathin HZO.
a, Schematic heterostructure of ultrathin HZO on metallic TiN probed the microwave capacitance measurements to eliminate contributions from the semiconducting Si substrate. b, PFM phase contrast (left) and topography (right) imaging for 10 cycles HZO on TiN-buffered Si. Ultrathin ferroelectricity persists on top of metallic underlayers as well as dielectric SiO2, although the topography is rougher than the films on SiO2 due to the inhomogeneity introduced by the sputtered TiN. c, SCM dC–dV spectroscopy loops taken on multiple bare regions of an ultrathin ten-cycle HZO film, demonstrating reproducible SCM response. The square 180° phase hysteresis and dC/dV loops, which integrates into the classic butterfly-shaped capacitance–voltage plot (Fig. 2c), provides conclusive evidence of ferroelectric polarization switching beyond PFM loops (Fig. 2e, Extended Data Fig. 6). The microwave-frequency nature of the SCM enables leakage-mitigated differential capacitance measurements of ultrathin films (Methods). d, PFM switching-spectroscopy loops taken on the same region of the ten-cycle HZO as the SCM measurements, confirming the ferroelectric-like phase and amplitude hysteresis. We note that the SCM and PFM switching spectroscopy was done using the Asylum Cypher scanning probe microscope at Asylum Research (Methods).
Extended Data Fig. 9 Ultrathin-enhanced distortions and polar signatures from spectroscopy.
a, Crystal field splitting diagram for the fluorite-structure structural polymorphs; symmetry-induced e-splitting provides a spectroscopic signature for the polar O-phase (Methods). b, Delineating symmetry-split energy regimes in oxygen K-edge XAS. Just as convergent beam electron diffraction provides signatures to demonstrate inversion symmetry breaking49, XAS provides spectroscopic signatures to distinguish between the nonpolar tetragonal and polar orthorhombic polymorphs (difficult to resolve from GI-XRD). Left, simulated XAS spectrum for tetragonal ZrO2 (P42/nmc) and right, polar orthorhombic ZrO2 (Pca21), both courtesy of the Materials Project56,57. The background colour shading denotes the symmetry-split regimes explained in the crystal field splitting diagram. c, Experimental XAS data on ultrathin HZO displays similar spectroscopic XAS features as the simulated polar O-phase (Pca21)—namely, relative e/t2 spectral weight and splittings corresponding to tetrahedral (∆T) and rhombic (∆R) distortions. Left, XAS of the HZO thickness series at the O K-edge, zooming in on the e- and t2-regimes. Right, O K-edge spectral weight trends as a function of HZO thickness. The relative spectral weights from the t2/e and e-split regimes indicate enhanced tetrahedral (∆T) and rhombic distortions (∆R) in ultrathin films, respectively, consistent with C2v symmetry of the polar O-phase. d, Schematic representation of the cation nearest-neighbour coordination dropping from NN = 8 (T-phase) to NN = 7 (polar O-phase) as the crystal symmetry is lowered. The disorder in oxygen polyhedral coordination (note the different oxygen atoms denoted by the blue and cyan atoms in the polar O-phase) manifests as spectral weight in the pre-edge regime62. e, The experimental pre-edge spectral weight as a function of thickness, indicating ultrathin-enhanced polyhedral disorder. f, Top: PEEM-XLD images of ten-cycle (1 nm) HZO at the O K-edge. Pre-edge images (left) exhibit no XLD contrast, while on-edge images (right)—at the energy corresponding to the polar-distortion split e-regime—demonstrate XLD contrast. This suggests that XLD is indeed sensitive to polar features in ultrathin highly textured HZO. Bottom, line profile of the XLD intensity, demonstrating substantial variations in on-edge XLD data compared to noise for pre-edge XLD. g, Crystal field splitting energies in HZO-related transition metal oxide systems. The material system, primary crystal electric field (∆1), secondary crystal electric field (∆2), and structure for various systems related to HZO and perovskite ferroelectrics are shown, where ∆O, ∆t, ∆T and ∆R corresponds to octahedral, tetragonal, tetrahedral, and rhombic crystal electric field (CEF), respectively. The reference crystal electric field values are taken from the Materials Project database57 (reference codes denoted by ‘mp’), and the experimental values are extracted via XAS energy-split features (b). The large tetrahedral (∆T) and rhombic (∆R) crystal field splitting energies present in ten-cycle HZO films are much larger than expected values for the polar fluorite-structure ZrO2 (b), which highlights the enhanced distortion present in ultrathin films subject to confinement strain, and is consistent with anomalously large structural distortions extracted from diffraction (Fig. 3g).
Extended Data Fig. 10 Ultrathin HZO ferroelectric tunnel junction.
a, c, Tunnel current–voltage characterization of Si(p++)/SiO2(1 nm)/HZO(∼1 nm)/W capacitor devices—demonstrated for ten-cycle HZO with Hf:Zr composition 4:1 (a) and 1:1 (c)—as a function of the write pulse (to set the ferroelectric polarization state). Tunnelling electroresistance behaviour is demonstrated for ±2 V write and 100 mV read. Insets, linear-scale current–voltage characteristics of the two polarization-driven current states. b, d, Tunnelling electroresistance hysteresis map as a function of write voltage (demonstrated for ten-cycle HZO with Hf:Zr composition 4:1 (b) and 1:1 (d)) measured at 200 mV read voltage. The abrupt hysteretic behaviour and saturating tunnelling electroresistance is characteristic of polarization-driven switching67, as opposed to filamentary-based switching caused by electrochemical migration and/or oxygen vacancy motion (Methods). e, Current–voltage hysteresis sweeps ruling out non-polarization-driven resistive switching mechanisms (Methods). The device demonstrates current–voltage hysteresis at low voltage and voltage polarity-independent current–voltage hysteresis sense: both negative-positive-negative voltage polarity (left) and positive-negative-positive voltage polarity (right) demonstrate counter-clockwise hysteresis. Such behaviour rules out resistive switching mediated by dielectric breakdown and filamentary mechanisms66 and is consistent with polarization-driven switching.
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Cheema, S.S., Kwon, D., Shanker, N. et al. Enhanced ferroelectricity in ultrathin films grown directly on silicon. Nature 580, 478–482 (2020). https://doi.org/10.1038/s41586-020-2208-x
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DOI: https://doi.org/10.1038/s41586-020-2208-x
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