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Giant energy storage and power density negative capacitance superlattices

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Abstract

Dielectric electrostatic capacitors1, because of their ultrafast charge–discharge, are desirable for high-power energy storage applications. Along with ultrafast operation, on-chip integration can enable miniaturized energy storage devices for emerging autonomous microelectronics and microsystems2,3,4,5. Moreover, state-of-the-art miniaturized electrochemical energy storage systems—microsupercapacitors and microbatteries—currently face safety, packaging, materials and microfabrication challenges preventing on-chip technological readiness2,3,6, leaving an opportunity for electrostatic microcapacitors. Here we report record-high electrostatic energy storage density (ESD) and power density, to our knowledge, in HfO2–ZrO2-based thin film microcapacitors integrated into silicon, through a three-pronged approach. First, to increase intrinsic energy storage, atomic-layer-deposited antiferroelectric HfO2–ZrO2 films are engineered near a field-driven ferroelectric phase transition to exhibit amplified charge storage by the negative capacitance effect7,8,9,10,11,12, which enhances volumetric ESD beyond the best-known back-end-of-the-line-compatible dielectrics (115 J cm−3) (ref. 13). Second, to increase total energy storage, antiferroelectric superlattice engineering14 scales the energy storage performance beyond the conventional thickness limitations of HfO2–ZrO2-based (anti)ferroelectricity15 (100-nm regime). Third, to increase the storage per footprint, the superlattices are conformally integrated into three-dimensional capacitors, which boosts the areal ESD nine times and the areal power density 170 times that of the best-known electrostatic capacitors: 80 mJ cm−2 and 300 kW cm−2, respectively. This simultaneous demonstration of ultrahigh energy density and power density overcomes the traditional capacity–speed trade-off across the electrostatic–electrochemical energy storage hierarchy1,16. Furthermore, the integration of ultrahigh-density and ultrafast-charging thin films within a back-end-of-the-line-compatible process enables monolithic integration of on-chip microcapacitors5, which can unlock substantial energy storage and power delivery performance for electronic microsystems17,18,19.

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Fig. 1: Ultrahigh energy storage by ferroic phase engineering and NC in HZO.
Fig. 2: Scaling total energy storage by HZO–Al2O3 NC superlattices.
Fig. 3: Three-dimensional integrated antiferroelectric NC microcapacitors on Si.

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The data supporting the findings of this study are available in the paper and the Supplementary Information.

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Acknowledgements

This research was primarily supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05-CH11231 (Codesign of Ultra-Low-Voltage Beyond CMOS Microelectronics) for the development of materials for low-power microelectronics. This study was also supported by the Defense Threat Reduction Agency (DTRA) as part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA) under contract no. HDTRA1-20-2-0002 and the Berkeley Center for Negative Capacitance Transistors (BCNCT). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. This material is based on work supported by the Secretary of Defense for Research and Engineering under Air Force contract no. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Secretary of Defense for Research and Engineering. S.S.C. and N.S. would like to thank R. Ramesh for characterization facilities and M. Hoffman for the help with LabView setup.

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  1. Suraj S. Cheema

    Present address: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA

  2. These authors contributed equally: Suraj S. Cheema, Nirmaan Shanker, Shang-Lin Hsu

Authors and Affiliations

  1. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, USA

    Suraj S. Cheema, Nirmaan Shanker, Shang-Lin Hsu, Joseph Schaadt, Nathan M. Ellis, Robert C. N. Pilawa-Podgurski & Sayeef Salahuddin

  2. Department of Mechanical Engineering, University of California, Berkeley, CA, USA

    Joseph Schaadt

  3. Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA, USA

    Matthew Cook, Ravi Rastogi & Mohamed Mohamed

  4. National Center for Electron Microscopy Facility, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Jim Ciston

  5. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Sayeef Salahuddin

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Contributions

S.S.C. conceived the idea and experiments. S.S.C. and N.S. performed material synthesis and ferroic phase optimization. N.S. performed dielectric and electrical measurements. J.S., N.M.E. and N.S. developed the pulsed high-voltage setup, guided by R.C.N.P.-P. S.S.C. performed X-ray characterization. S.-L.H. performed the transmission electron microscopy, guided by J.C. M.M., R.R. and M.C. designed the 3D capacitor structures. N.S., R.R. and M.C. performed the capacitor fabrication. The 2D and 3D capacitor structures were fabricated at the UC Berkeley Marvell Nanofabrication Laboratory and the MIT Lincoln Laboratory Microelectronics Laboratory, respectively. S.S.C. and N.S analysed all results. S.S.C. and N.S. wrote the Article. S.S. supervised the research. All authors contributed to discussions and paper preparations.

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Correspondence to Suraj S. Cheema or Sayeef Salahuddin.

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Competing interests

S.S.C, N.S., S.S. and S.-L.H. have filed a US provisional patent (No. 63/625,727) through the University of California, Berkeley (Disclosure BK-2024-082) titled ‘Giant Energy and Power Density Microcapacitors via Ferroic Order Superlattices’.

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Extended data figures and tables

Extended Data Fig. 1 Energy storage engineering strategy.

The energy storage density in HZO thin films was optimized through a three-pronged approach: (i) field-driven NC optimization through ferroic phase engineering in ∼10 nm films (left), (ii) scaling up the field-driven NC behavior to ∼100 nm through amorphous-templated superlattices (lower right), and (iii) integration of NC superlattices into 3D Si capacitors to increase the energy storage density per footprint area (upper right).

Extended Data Fig. 2 Dielectric energy storage measurement and methodology.

(a) Experimental setup for pulsed I-V measurements. Voltage is applied with a pulse generator unit (Methods), while the current is measured through the oscilloscope’s 50 Ω input impedance. (b, c, d) The time-dependent applied voltage pulses (b), measured current response (c), and integrated charge (d) for a 9 nm HZO (80% Zr) film. (e) The maximum charge Qmax, residual charge Qres, and their difference Qrev, derived from the charge versus time curve for each voltage pulse (Methods). Qrev is the charge that is reversibly stored and released from the capacitor. (f) Experimental setup for impedance analyzer measurements. (g, h) Measured frequency-dependent magnitude (|Z|, g) and phase (θ, h) of the complex impedance at different biases. (i, j) Fitted real (i) and imaginary (j) parts of the complex impedance assuming a three-component circuit model (k). The series resistance is extracted from the real part at high frequency (Methods) and should be bias-independent as shown in (i). The series resistance was found to be 155 Ω. (k) The three-component circuit model used to correct for the series resistance, where Rs is the series resistance and Rp is the parallel resistance which models the leakage through the capacitor, C. (l) The measured hysteretic charge-voltage curve from pulsed I-V measurements without any series resistance correction. (m) The Rs-corrected hysteretic Q-V curves corresponding to each voltage pulse applied. Note that 50 Ω was added to Rs extracted in (i) due to the additional 50 Ω from the oscilloscope’s input impedance. (n) The extracted energy storage density (ESD) is taken to be the shaded green area (Methods). The shaded blue area represents the hysteretic energy loss during the charging-discharging cycle. (o) ESD as a function of electric field for 9 nm HZO (80% Zr).

Extended Data Fig. 3 Superlattice structure and morphology characterization.

(a, b, c) AFM topography for the 9 nm HZO film (80% Zr, a), HZOx10 superlattice (b), and HZOx10 continuous (c) films. (d) Extracted rms roughness values for each film, demonstrating the persistence of smooth films for the HZO-Al2O3 superlattice. (e, f, g) Schematic of 9 nm HZO (e), HZOx10 superlattice (f), and HZOx10 continuous (g) films. (h) X-ray reflectivity (XRR) of HZOx2-10 superlattice films. Clear superlattice reflections are observed, which indicate that the thin (0.5 nm) Al2O3 layers serve as a sufficient barrier to separate the HZO layers, consistent with TEM (Extended Data Fig. 4) and recent HfO2-based superlattice engineering studies62,108,109,110,111,112,113. In fact, ALD of binary oxide superlattices have demonstrated the absence of chemical intermixing down to atomic layer periodicity114.

Extended Data Fig. 4 Ferroic phase identification in HZOx10 superlattice.

(a) Cross-sectional HR-TEM image for the HZOx10 superlattice demonstrating separated Al2O3 and HZO layers. (b, c, d) HR-TEM (left) and zoom-in HR-TEM (right) images for the top (b), middle (c), and bottom HZO (d) layers in the superlattice. (e-j) Top (e, h), middle (f, i), and bottom (g, j) HZO layers are indexed to the t-phase by oxygen-sensitive negative spherical aberration, demonstrating that the superlattice maintains the t-phase to the ~100 nm regime. For inverted contrast images (e, f, g), the light (dark) atoms represent O (Hf, Zr). For the top HZO layer (e, h), the cation atomic arrangements match to the t-phase [021] zone axis. Along this zone axis, the oxygen atoms overlap with the cations, so only the cations can be matched. Presence of the o-phase and m-phase is ruled out as they do not have a hexagonal-shaped cation arrangement along a zone axis. For the middle (f, i) and bottom HZO layers (g, j), the cation and anion atomic arrangements match to the t-phase [001] and [101] zone axes, respectively. Overall, the structural identification of the t-phase in the superlattice film is consistent with electrical measurements indicating antiferroelectric behavior (Fig. 2, Extended Data Fig. 6). (k-m) Wide field-of-view TEM (k), selected area diffraction pattern (l), and corresponding radial profile integration of diffraction pattern (m) for the HZOx10 superlattice, which indexes to the t-phase, consistent with the oxygen imaging analysis. Furthermore, the HZOx10 superlattice shows similar d101,T lattice spacing as prior work on ∼9 nm ZrO255, suggesting that the superlattice approach maintains a similar strain state as the conventional 9 nm antiferroelectric building block. Therefore, the amorphous Al2O3 template allows the t-phase to persist across the entire thickness, underscoring the important role of amorphous templating108,112,113,115 in HZO antiferroelectric-ferroelectric phase stability116,117.

Extended Data Fig. 5 Ferroic phase identification of continuous HZOx10 film from cation analysis.

(a) Cross-sectional HR-TEM image for the continuous HZOx10 film integrated in an TiN-HZO-TiN MIM capacitor. (b-g) HR-TEM imaging of two HZO regions (b, e), inverted contrast images zoomed-in on single grains (c, f), and corresponding HR-TEM simulations (d, g). The cation arrangement corresponds to the fluorite-structure o-phase (Pca21) [100] and [001] zone axes for region 1 (b-d) and 2 (e-g), respectively (Supplementary Fig. 9). (h-j) Wide field-of-view TEM (h), selected area diffraction pattern (i), and corresponding radial profile integration of diffraction pattern (j) for the HZOx10 continuous film, which primarily indexes to the o-phase, consistent with the cation analysis. The HZOx10 continuous film also shows the presence of some m-phase, which is consistent with the expected phase evolution with increasing thickness15. Additionally, the continuous ∼100 nm HZO film demonstrates similar d111,O lattice spacing as bulk o-phase ZrO2118, suggesting the presence of minimal thin film induced strain contributions which would otherwise be expected from small size effects in thinner films26. Overall, the structural identification of the o-phase in the continuous thick HZO film is consistent with electrical measurements indicating ferroelectric behavior (Fig. 2, Extended Data Fig. 6).

Extended Data Fig. 6 Thickness-dependent evolution of ferroic behavior for continuous and superlattice HZO films.

(a-i) Pulsed Q-E behavior for both continuous (top) and superlattice (bottom) HZO thickness series to the 100 nm thickness regime. Inset: corresponding C-V loops. For the continuous HZO thickness series, an antiferroelectric (HZOx2, a) to mixed antiferroelectric-ferroelectric (HZOx3-4, b,c) to ferroelectric (HZOx5-10, d-i) is observed from the pulsed Q-E curve as regime II (and III) eventually disappear for the HZOx5-10 samples, corresponding to the loss of an antiferroelectric-to-ferroelectric field-dependent phase transition. This is additionally confirmed by the emergence of ferroelectric-like hysteresis in the C-V characteristics. Meanwhile, for the superlattice HZO thickness series, the antiferroelectric behavior is maintained throughout, as regimes I-III are present for each thickness and antiferroelectric-like C-V characteristics are observed. This confirms the ultrathin Al2O3 interlayers reset the HZO grain growth and maintains the desired antiferroelectric behavior. For all figures, the enhanced slope in Regime II is highlighted in gray.

Extended Data Fig. 7 Thickness-dependent evolution of energy storage for continuous and superlattice HZO films.

(a-i) Thickness-dependent areal ESD versus electric field for both continuous (top) and superlattice (bottom) thickness series, extracted from hysteretic charge-field measurements (insets). For the continuous HZO thickness series, ESD saturates with increasing thickness (~250 µJ/cm2) as the continuous HZO films become fully ferroelectric with increasing thickness (Extended Data Fig. 6). Meanwhile for the superlattice HZO thickness series, the areal ESD scales approximately linearly with thickness due to the persistence of the desired antiferroelectric behavior (Extended Data Fig. 6).

Extended Data Fig. 8 Reliability characterization.

(a) Leakage current versus electric field for continuous (blue) and superlattice (purple) HZO thickness series films. For all thicknesses, the superlattice approach shows 2-3 orders of magnitude of lower leakage current, derived from the presence of the high-bandgap Al2O3 interlayer and increased number of interfaces. (b, c) Energy storage and efficiency for 2D planar (b) and 3D trench (c) capacitors integrating the HZOx10 superlattice after cycling at two electric fields (1 µs pulses): one near the onset of Regime II (3.0 MV/cm and 2.5 MV/cm for planar and trench, respectively) and another at 0.5 MV/cm higher field. Near the onset of Regime II, the endurance of both planar and trench capacitors show endurance larger than 108 cycles. At 0.5 MV/cm higher electric field, the endurance is beyond 106 cycles. (d, e) Weibull distribution of the breakdown field, comparing the HZOx10 continuous and HZOx10 superlattice films in 2D planar capacitors (d) and the HZOx10 superlattice films integrated into 2D planar and 3D trench capacitors (e). The 3D trench capacitors show a 24% lower breakdown field compared to the 2D planar capacitors, likely derived from surface inhomogeneities, which can lead to an uneven distribution of electric fields.

Extended Data Fig. 9 Power Density Extraction.

(a, e) Schematic of 2D (a) and 3D trench (e) capacitor structures. (b, f) Measured discharged current versus time during 200 ns fall time of voltage pulse for both 2D planar (f) and 3D trench (f) capacitors. The sub-microsecond discharge time is consistent with other reported electrostatic capacitors119. (c, g) Power density as a function of time for both 2D (c) and 3D (g) capacitors. The power was calculated by multiplying the measured voltage and current during the discharging stage of the voltage pulse (Methods). (d, h) Energy storage density as a function of time for both 2D (d) and 3D (h) capacitors. The characteristic discharging time was taken to be when 90% of the stored energy had discharged (Methods).

Extended Data Fig. 10 Permittivity-breakdown trade-off.

Permittivity-breakdown strength (κ-EBD) relationship for various dielectric materials, which tend to follow an EBD ∼ κ−0.5 empirical trend120,121,122. In this work, we engineer the field-induced nonpolar-to-polar phase transition and its associated NC effect to enhances permittivity during the charging-discharging process (the maximum permittivity extracted from pulsed I-V measurements during Regime II is reported). This approach can overcome the conventional κ-EBD trend which limits energy storage in dielectric capacitors (Supplementary Text), ultimately leading to the largest volumetric ESD value reported for a BEOL-compatible dielectric (Supplementary Table 1). Additional promising systems to overcome this trend towards breakthrough ESD values include other (anti)ferroelectric superlattices demonstrating field-driven ferroic phase transitions74,75,76,123, relaxor ferroelectrics124,125,126,127,128, morphotropic phase boundary systems129,130, amorphous-engineered oxides131, 2D-layered perovskites132, super-high-κ nanolaminates133, and interface engineering in polymer nanocomposites134.

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Cheema, S.S., Shanker, N., Hsu, SL. et al. Giant energy storage and power density negative capacitance superlattices. Nature (2024). https://doi.org/10.1038/s41586-024-07365-5

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