Ultrahigh dielectric permittivity in Hf0.5Zr0.5O2 thin-film capacitors

Abstract

The ever-shrinking electrostatic capacitor, which is capable of storing substantial quantities of electrical charge, has found widespread applications in high-storage-density dynamic random access memory and energy-efficient complementary metal-oxide-semiconductor devices. Despite the high energy storage densities (133-152 J/cm³) and efficiencies (75-90%) that have been realized using relaxor ferroelectric thick films, low-permittivity interfacial layers in the ultrathin films have caused the overall permittivity to be one to two orders of magnitude lower than expected. However, innovative use of complementary metal-oxide-semiconductor-compatible HfO₂-based materials with high permittivities (~52) could enable integration of these capacitors into few-nanometre-scale devices. This study reports an ultrahigh dielectric permittivity of 921, stored charge density of 349 μC/cm², and energy density of 584 J/cm³ with nearly 100% efficiency within near-edge plasma-treated Hf_0.5Zr_0.5O₂ thin-film capacitors when the Hf-based material's ferroelectricity disappears suddenly after polarization fatigue. The ultrahigh dielectric permittivity originates from a distorted orthorhombic phase with ordered oxygen vacancies that enables high-density integration of extremely scaled logic and memory devices for low-voltage applications.

Fig. 1. Dielectric permittivities of size-scaled capacitors.

a Area dependences of the capacitance at 1 MHz for Samples 1 and 2. The solid lines represent the best fit for the data when considering the edging-area contribution under the top electrodes in the geometries illustrated in the inset based on the assumptions of r₀ = 150 nm and 0 nm, respectively. The large deviation of data from the red-line fit is due to the variation of ion implantation across different areas of the film. b, c Frequency dependences of ε′ and tanδ for virgin HZO capacitors of various sizes (Sample 1). The solid lines represent data fittings according to Eq. (4). d D-E hysteresis loops for the size-scaled virgin capacitors when characterized at 1 MHz. e Cycling number dependences of the remanent polarization values of the size-scaled capacitors when using square pulses of ±4 V/500 ns at a repeat frequency of 1 MHz. f D-E hysteresis loops at 1 MHz for the size-scaled capacitors after fatigue, where the parenthetic values of ε′ are estimated from the slopes of the loops.

Fig. 2. Ultrahigh dielectric permittivities.

a Frequency dependences of ε′ and tanδ when E_osc = 50 kV/cm for the size-scaled capacitors after the occurrence of polarization fatigue. Larger capacitor sizes generally result in smaller values of ε′. The solid lines represent data fittings according to Eqs. (5) and (6). b Frequency dependences of the dielectric permittivity and loss tangent at various temperatures for the 4.39-μm-long fatigued capacitor. The solid lines represent data fittings according to Eqs. (5) and (6). c D-E hysteresis loops at 252 kHz for the 4.39-μm-long fatigued capacitor at different temperatures. d Cycling number dependences of the stored charge density within the 4.39-μm-long virgin/fatigued capacitors under application of unipolar square pulses with voltage/width characteristics of 1.2 V/50 ns with a repetition frequency of 10 MHz at room temperature. The charge density increases by 10 times after fatigue. e Comparison of the ultrahigh dielectric permittivity realized in this work with the corresponding values for the pure ZrO₂ and HfO₂^,, Al-doped HfO₂ (HAO)^,, Si-doped HfO₂ (HSO), Y-doped HfO₂ (HYO), Zr-doped HfO₂ (HZO)^,, and HfO₂-ZrO₂ superlattices^,, reported previously in the literature. f Synchrotron Laue micro-diffractions of the O $\left(111\right)$ reflections extracted from the virgin and fatigued areas nearby a 4.35 μm-long capacitor at a wavelength of 0.6209 Å (Supplementary Fig. S13a–c).

Fig. 3. Phase structures (before and after fatigue).

a Low-magnification cross-sectional STEM-HAADF image of virgin HZO thin film (left panel). The framed yellow dashed rectangle is the oxygen vacancy penetration zone (~150 nm). The right-hand panel shows the simulated atomic structure for the [010]-Pca2₁ phase. b, c STEM-iDPC images of the O phases beyond and within the penetration zone framed in a, respectively. d, e Line scans over the rows of the oxygen columns framed in b, c, respectively. The dashed lines represent the intensities of the reference fully O-occupied columns, and the peak intensities that are lower than the reference line in e indicate the appearance of random oxygen vacancies at the O₅ and O₈ sites. f STEM-iDPC image of the [010]-oriented Pca2₁’ phase after fatigue. The inset in the top-left corner shows the simulation of this phase. g, h Two line scans of the atomic columns within the dashed rectangles framed in (f). Periodic changes in the oxygen intensities that deviate from the reference full-O dashed lines at the O₂, O₄ and O₅ sites consistently implies the appearance of ordered oxygen vacancies when measured along two different $\left(\left[10\overline{1}\right]\mathrm{and}\left[\overline{1}0\overline{1}\right]\right)$ directions.

Fig. 4. Oxygen vacancy ordering process and simulations.

a, b Cycling number dependences of the D-E hysteresis loop and the imprint field for the 5.42 μm-long HZO capacitor when using square fatigue pulses of ±4 V/500 ns at a repetition frequency of 1 MHz. The solid line in b represents the fit of an oscillating imprint field according to Eq. (1) that accompanies the V_O^·· ordering illustrated in the inset figures. c Schematic of the preferred V_O^·· accumulation near the top/bottom electrodes during fatigue that is interdependent on the E_i direction (thin arrows). The built-up internal field caused by the V_O^·· accumulation contradicts E_i and changes the dipole orientation (thick arrows). d Minimum energy paths for the O-anion movement within the (001) crystal plane in a 96-atom supercell for Cases i–iv. The dotted balls represent oxygen vacancies, the red balls represent the displaced oxygen anions, and the arrows indicate the movement directions. e Minimum energy paths for bipolar polarization switching with different numbers of three-coordinated oxygen vacancies within a 96-atom supercell. The arrows indicate the polarization (P) orientations in the initial and final states.