Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Enhanced ferroelectricity in ultrathin films grown directly on silicon

A Publisher Correction to this article was published on 06 May 2020

This article has been updated

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.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Size effects in fluorite-structure ferroelectrics.
Fig. 2: Electric polarization switching in ultrathin HZO.
Fig. 3: Emergence of ‘reverse’ size effects in ultrathin HZO.

Similar content being viewed by others

Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Change history

References

  1. Junquera, J. & Ghosez, P. Critical thickness for ferroelectricity in perovskite ultrathin films. Nature 422, 506–509 (2003).

    ADS  CAS  PubMed  Google Scholar 

  2. Mikolajick, T., Slesazeck, S., Park, M. & Schroeder, U. Ferroelectric hafnium oxide for ferroelectric random-access memories and ferroelectric field-effect transistors. MRS Bull. 43, 340–346 (2018).

    ADS  CAS  Google Scholar 

  3. Dawber, M., Rabe, K. M. & Scott, J. F. Physics of thin-film ferroelectric oxides. Rev. Mod. Phys. 77, 1083–1130 (2005).

    ADS  CAS  Google Scholar 

  4. Schlom, D. G., Guha, S. & Datta, S. Gate oxides beyond SiO2. MRS Bull. 33, 1017–1025 (2008).

    CAS  Google Scholar 

  5. Lines, M. E. & Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials (Oxford Univ. Press, 1977).

  6. Ahn, C., Rabe, K. & Triscone, J. Ferroelectricity at the nanoscale: local polarization in oxide thin films and heterostructures. Science 303, 488–491 (2004).

    ADS  CAS  PubMed  Google Scholar 

  7. Fong, D. D. et al. Ferroelectricity in ultrathin perovskite films. Science 304, 1650–1653 (2004).

    ADS  CAS  PubMed  Google Scholar 

  8. Tenne, D. A. et al. Probing nanoscale ferroelectricity by ultraviolet Raman spectroscopy. Science 313, 1614–1616 (2006).

    ADS  CAS  PubMed  Google Scholar 

  9. Warusawithana, M. P. et al. A ferroelectric oxide made directly on silicon. Science 324, 367–370 (2009).

    ADS  CAS  PubMed  Google Scholar 

  10. Dubourdieu, C. et al. Switching of ferroelectric polarization in epitaxial BaTiO3 films on silicon without a conducting bottom electrode. Nat. Nanotechnol. 8, 748–754 (2013).

    ADS  CAS  PubMed  Google Scholar 

  11. Schlom, D. G. & Haeni, J. H. A thermodynamic approach to selecting alternative gate dielectrics. MRS Bull. 27, 198–204 (2002).

    CAS  Google Scholar 

  12. Böscke, T. S., Müller, J., Bräuhaus, D., Schröder, U. & Böttger, U. Ferroelectricity in hafnium oxide thin films. Appl. Phys. Lett. 99, 102903 (2011).

    ADS  Google Scholar 

  13. Park, M. H. et al. Ferroelectricity and antiferroelectricity of doped thin HfO2-based films. Adv. Mater. 27, 1811–1831 (2015).

    CAS  PubMed  Google Scholar 

  14. Robertson, J. High dielectric constant gate oxides for metal oxide Si transistors. Rep. Prog. Phys. 69, 327 (2006).

    ADS  CAS  Google Scholar 

  15. Muller, J. et al. Ferroelectric hafnium oxide: a CMOS-compatible and highly scalable approach to future ferroelectric memories. In 2013 IEEE Int. Electron Devices Meet. (IEDM) 10.8.1–10.8.4 (IEEE, 2013).

  16. Park, M., Lee, Y., Mikolajick, T., Schroeder, U. & Hwang, C. Review and perspective on ferroelectric HfO2-based thin films for memory applications. MRS Commun. 8, 795–808 (2018).

    CAS  Google Scholar 

  17. Wong, J. C. & Salahuddin, S. Negative capacitance transistors. Proc. IEEE 107, 49–62 (2019).

    CAS  Google Scholar 

  18. Kwon, D. et al. Improved subthreshold swing and short channel effect in FDSOI n-channel negative capacitance field effect transistors. IEEE Electron Device Lett. 39, 300–303 (2018).

    ADS  CAS  Google Scholar 

  19. Salahuddin, S., Ni, K. & Datta, S. The era of hyper-scaling in electronics. Nat. Electron. 1, 442–450 (2018).

    Google Scholar 

  20. Merz, W. J. The effect of hydrostatic pressure on the Curie point of barium titanate single crystals. Phys. Rev. 78, 52 (1950).

    ADS  CAS  Google Scholar 

  21. Ohtaka, O. et al. Phase relations and volume changes of hafnia under high pressure and high temperature. J. Am. Ceram. Soc. 84, 1369–1373 (2001).

    CAS  Google Scholar 

  22. Materlik, R., Künneth, C. & Kersch, A. The origin of ferroelectricity in Hf1−xZrxO2: a computational investigation and a surface energy model. J. Appl. Phys. 117, 134109 (2015).

    ADS  Google Scholar 

  23. Xiao, J. et al. Intrinsic two-dimensional ferroelectricity with dipole locking. Phys. Rev. Lett. 120, 227601 (2018).

    ADS  CAS  PubMed  Google Scholar 

  24. Vasudevan, R. K., Balke, N., Maksymovych, P., Jesse, S. & Kalinin, S. V. Ferroelectric or non-ferroelectric: why so many materials exhibit “ferroelectricity” on the nanoscale. Appl. Phys. Rev. 4, 021302 (2017).

    ADS  Google Scholar 

  25. Balke, N. et al. Differentiating ferroelectric and nonferroelectric electromechanical effects with scanning probe microscopy. ACS Nano 9, 6484–6492 (2015).

    CAS  PubMed  Google Scholar 

  26. Collins, L., Liu, Y., Ovchinnikova, O. S. & Proksch, R. Quantitative electromechanical atomic force microscopy. ACS Nano 13, 8055–8066 (2019).

    CAS  PubMed  Google Scholar 

  27. Disa, A. S. et al. Orbital engineering in symmetry-breaking polar heterostructures. Phys. Rev. Lett. 114, 026801 (2015).

    ADS  CAS  PubMed  Google Scholar 

  28. Wei, Y. et al. A rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films. Nat. Mater. 17, 1095–1100 (2018).

    ADS  CAS  PubMed  Google Scholar 

  29. Lyu, J., Fina, I., Solanas, R., Fontcuberta, J. & Sánchez, F. Growth window of ferroelectric epitaxial Hf0.5Zr0.5O2 thin films. ACS Appl. Electron. Mater. 1, 220–228 (2019).

    CAS  Google Scholar 

  30. Schlom, D. G. et al. Elastic strain engineering of ferroic oxides. MRS Bull. 39, 118–130 (2014).

    CAS  Google Scholar 

  31. Park, M. H. et al. Evolution of phases and ferroelectric properties of thin Hf0.5Zr0.5O2 films according to the thickness and annealing temperature. Appl. Phys. Lett. 102, 242905 (2013).

    ADS  Google Scholar 

  32. Tian, X. et al. Evolution of ferroelectric HfO2 in ultrathin region down to 3 nm. Appl. Phys. Lett. 112, 102902 (2018).

    ADS  Google Scholar 

  33. Richter, C. et al. Si doped hafnium oxide—a “fragile” ferroelectric system. Adv. Electron. Mater. 3, 1700131 (2017).

    Google Scholar 

  34. Stengel, M. & Spaldin, N. A. Origin of the dielectric dead layer in nanoscale capacitors. Nature 443, 679–682 (2006).

    ADS  CAS  PubMed  Google Scholar 

  35. Kim, S. J., Mohan, J., Summerfelt, S. R. & Kim, J. Ferroelectric Hf0.5Zr0.5O2 thin films: a review of recent advances. JOM 71, 246–255 (2019).

    ADS  Google Scholar 

  36. Schlom, D. G. et al. Strain tuning of ferroelectric thin films. Annu. Rev. Mater. Res. 37, 589–626 (2007).

    ADS  CAS  Google Scholar 

  37. Haeni, J. H. et al. Room-temperature ferroelectricity in strained SrTiO3. Nature 430, 758–761 (2004).

    ADS  CAS  PubMed  Google Scholar 

  38. Zubko, P., Catalan, G. & Tagantsev, A. K. Flexoelectric effect in solids. Annu. Rev. Mater. Res. 43, 387–421 (2013).

    ADS  CAS  Google Scholar 

  39. Jariwala, D., Marks, T. J. & Hersam, M. C. Mixed-dimensional van der Waals heterostructures. Nat. Mater. 16, 170–181 (2017).

    ADS  CAS  PubMed  Google Scholar 

  40. Kwon, D. et al. Negative capacitance FET with 1.8-nm-thick Zr-doped HfO2 oxide. IEEE Electron Device Lett. 40, 993–996 (2019).

    ADS  CAS  Google Scholar 

  41. Lee, M. H. et al. Physical thickness 1.x nm ferroelectric HfZrOx negative capacitance FETs. In 2016 IEEE Int. Electron Devices Meet. (IEDM) 12.1.1–12.1.4, https://ieeexplore.ieee.org/document/7838400/ (IEEE, 2016).

  42. Karbasian, G. et al. Stabilization of ferroelectric phase in tungsten capped Hf0.8Zr0.2O2. Appl. Phys. Lett. 111, 022907 (2017).

    ADS  Google Scholar 

  43. Rodriguez, B. J., Callahan, C., Kalinin, S. V. & Proksch, R. Dual-frequency resonance-tracking atomic force microscopy. Nanotechnology 18, 475504 (2007).

    ADS  Google Scholar 

  44. Jesse, S., Lee, H. N. & Kalinin, S. V. Quantitative mapping of switching behavior in piezoresponse force microscopy. Rev. Sci. Instrum. 77, 073702 (2006).

    ADS  Google Scholar 

  45. Hong, S. et al. Principle of ferroelectric domain imaging using atomic force microscope. J. Appl. Phys. 89, 1377–1386 (2001).

    ADS  CAS  Google Scholar 

  46. Strelcov, E. et al. Role of measurement voltage on hysteresis loop shape in piezoresponse force microscopy. Appl. Phys. Lett. 101, 192902 (2012).

    ADS  Google Scholar 

  47. Leu, C.-C. et al. Domain structure study of SrBi2Ta2O9 ferroelectric thin films by scanning capacitance microscopy. Appl. Phys. Lett. 82, 3493–3495 (2003).

    ADS  CAS  Google Scholar 

  48. Chernikova, A. et al. Ultrathin Hf0.5Zr0.5O2 ferroelectric films on Si. ACS Appl. Mater. Interf. 8, 7232–7237 (2016).

    CAS  Google Scholar 

  49. Sang, X., Grimley, E. D., Schenk, T., Schroeder, U. & LeBeau, J. M. On the structural origins of ferroelectricity in HfO2 thin films. Appl. Phys. Lett. 106, 162905 (2015).

    ADS  Google Scholar 

  50. Böscke, T. Crystalline Hafnia and Zirconia Based Dielectrics for Memory Applications PhD thesis, Hamburg University of Technology, https://cuvillier.de/en/shop/publications/763-crystalline-hafnia-and-zirconia-based-dielectrics-for-memory-applications (2010).

  51. Zhao, C., Roebben, G., Heyns, M. M. & Van der Biest, O. Crystallisation and tetragonal-monoclinic transformation in ZrO2 and HfO2 dielectric thin films. Key Eng. Mater. 206–213, 1285–1288 (2001).

    Google Scholar 

  52. Kriegner, D., Wintersberger, E. & Stangl, J. xrayutilities: a versatile tool for reciprocal space conversion of scattering data recorded with linear and area detectors. J. Appl. Cryst. 46, 1162–1170 (2013).

    CAS  Google Scholar 

  53. Lichtensteiger, C., Triscone, J., Junquera, J. & Ghosez, P. Ferroelectricity and tetragonality in ultrathin PbTiO3 films. Phys. Rev. Lett. 94, 047603 (2005).

    ADS  PubMed  Google Scholar 

  54. Park, M. H. et al. A comprehensive study on the structural evolution of HfO2 thin films doped with various dopants. J. Mater. Chem. C 5, 4677–4690 (2017).

    CAS  Google Scholar 

  55. Young, A. T. et al. Variable linear polarization from an x-ray undulator. J. Synch. Rad. 9, 270–274 (2002).

    CAS  Google Scholar 

  56. Jain, A. et al. The Materials Project: a materials genome approach to accelerating materials innovation. APL Mater. 1, 011002 (2013).

    ADS  Google Scholar 

  57. Mathew, K. et al. High-throughput computational X-ray absorption spectroscopy. Sci. Data 5, 180151 (2018).

    CAS  PubMed  PubMed Central  Google Scholar 

  58. Ong, S. P. et al. Python Materials Genomics (pymatgen): a robust, open-source Python library for materials analysis. Comput. Mater. Sci. 68, 314–319 (2013).

    CAS  Google Scholar 

  59. Cho, D.-Y., Jung, H.-S. & Hwang, C. S. Structural properties and electronic structure of HfO2-ZrO2 composite films. Phys. Rev. B 82, 094104 (2010).

    ADS  Google Scholar 

  60. Wu, M. et al. Strain and composition dependence of orbital polarization in nickel oxide superlattices. Phys. Rev. B 88, 125124 (2013).

    ADS  Google Scholar 

  61. de Groot, F. et al. Oxygen 1s X-ray-absorption edges of transition-metal oxides. Phys. Rev. B 40, 5715–5723 (1989).

    ADS  Google Scholar 

  62. de Groot, F. Multiplet effects in X-ray spectroscopy. Coord. Chem. Rev. 249, 31–63 (2005).

    Google Scholar 

  63. Cohen, R. E. Origin of ferroelectricity in perovskite oxides. Nature 358, 136–138 (1992).

    ADS  CAS  Google Scholar 

  64. Pesquera, D. et al. Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films. Nat. Commun. 3, 1189 (2012).

    ADS  CAS  PubMed  Google Scholar 

  65. Denev, S. A., Lummen, T. T. A., Barnes, E., Kumar, A. & Gopalan, V. Probing ferroelectrics using optical second harmonic generation. J. Am. Ceram. Soc. 94, 2699–2727 (2011).

    CAS  Google Scholar 

  66. Bersuker, G. & Gilmer, D. Metal oxide resistive random-access memory (RRAM) technology. In Advances in Non-Volatile Memory and Storage Technology 288–340 (Elsevier, 2014).

  67. Chanthbouala, A. et al. Solid-state memories based on ferroelectric tunnel junctions. Nat. Nanotechnol. 7, 101–104 (2012).

    ADS  CAS  Google Scholar 

  68. Gruverman, A. et al. Tunneling electroresistance effect in ferroelectric tunnel junctions at the nanoscale. Nano Lett. 9, 3539–3543 (2009).

    ADS  CAS  PubMed  Google Scholar 

  69. Müller, J. et al. Ferroelectricity in simple binary ZrO2 and HfO2. Nano Lett. 12, 4318–4323 (2012).

    ADS  PubMed  Google Scholar 

  70. Park, M. H. et al. Surface and grain boundary energy as the key enabler of ferroelectricity in nanoscale hafnia-zirconia: a comparison of model and experiment. Nanoscale 9, 9973–9986 (2017).

    CAS  PubMed  Google Scholar 

  71. Fujii, S. et al. First demonstration and performance improvement of ferroelectric HfO2-based resistive switch with low operation current and intrinsic diode property. In 2016 IEEE Symposium on VLSI Technology 1–2 (IEEE, 2016).

  72. Max, B., Hoffmann, M., Slesazeck, S. & Mikolajick, T. Ferroelectric tunnel junctions based on ferroelectric-dielectric Hf0.5Zr0.5O2/Al2O3 capacitor stacks. In 2018 48th European Solid-State Device Research Conference (ESSDERC) 142–145 (IEEE, 2018).

Download references

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.

Author information

Authors and Affiliations

Authors

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.

Corresponding authors

Correspondence to Suraj S. Cheema or Sayeef Salahuddin.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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. ce, Cross-section TEM images of ten-cycle HZO (c), 15-cycle HZO (d) and 40-cycle HZO (e). fh, 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.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-020-2208-x

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing