In ferroelectric materials, spontaneous symmetry breaking leads to a switchable electric polarization, which offers significant promise for nonvolatile memories. In particular, ferroelectric tunnel junctions (FTJs) have emerged as a new resistive switching memory which exploits polarization-dependent tunnel current across a thin ferroelectric barrier. This work integrates FTJs with complementary metal-oxide-semiconductor-compatible Zr-doped HfO2 (Zr:HfO2) ferroelectric barriers of just 1 nm thickness, grown by atomic layer deposition on silicon. These 1 nm Zr:HfO2 tunnel junctions exhibit large polarization-driven electroresistance (>20 000%), the largest value reported for HfO2-based FTJs. In addition, due to just a 1 nm ferroelectric barrier, these junctions provide large tunneling current (>1 A cm−2) at low read voltage, orders of magnitude larger than reported thicker HfO2-based FTJs. Therefore, this proof-of-principle demonstration provides an approach to simultaneously overcome three major drawbacks of prototypical FTJs: a Si-compatible ultrathin ferroelectric, large electroresistance, and large read current for high-speed operation.

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 microsystems^2-5. 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 readiness^2,3,6, 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 effect^7-12, 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 hierarchy^1,16. 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 microsystems^17-19.

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO₂) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO₂, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

Ultrathin ferroelectric materials could potentially enable low-power perovskite ferroelectric tetragonality logic and nonvolatile memories^1,2. 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.

With the scaling of lateral dimensions in advanced transistors, an increased gate capacitance is desirable both to retain the control of the gate electrode over the channel and to reduce the operating voltage¹. This led to a fundamental change in the gate stack in 2008, the incorporation of high-dielectric-constant HfO₂ (ref. ²), which remains the material of choice to date. Here we report HfO₂-ZrO₂ superlattice heterostructures as a gate stack, stabilized with mixed ferroelectric-antiferroelectric order, directly integrated onto Si transistors, and scaled down to approximately 20 ångströms, the same gate oxide thickness required for high-performance transistors. The overall equivalent oxide thickness in metal-oxide-semiconductor capacitors is equivalent to an effective SiO₂ thickness of approximately 6.5 ångströms. Such a low effective oxide thickness and the resulting large capacitance cannot be achieved in conventional HfO₂-based high-dielectric-constant gate stacks without scavenging the interfacial SiO₂, which has adverse effects on the electron transport and gate leakage current³. Accordingly, our gate stacks, which do not require such scavenging, provide substantially lower leakage current and no mobility degradation. This work demonstrates that ultrathin ferroic HfO₂-ZrO₂ multilayers, stabilized with competing ferroelectric-antiferroelectric order in the two-nanometre-thickness regime, provide a path towards advanced gate oxide stacks in electronic devices beyond conventional HfO₂-based high-dielectric-constant materials.