The critical size limit of electric polarization remains a fundamental question in nanoscale ferroelectric research 1 . As such, the viability of ultrathin ferroelectricity greatly impacts emerging low-power logic and nonvolatile memories 2 . Size effects in ferroelectrics have been thoroughly investigated for perovskite oxides -the archetypal ferroelectric system 3 . Perovskites, however, have so far proved unsuitable for thickness-scaling and integration with modern semiconductor processes 4 . Here, we report ultrathin ferroelectricity in doped-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 1 nm. Our results indicate not only the absence of a ferroelectric critical thickness, but also enhanced polar distortions as film thickness is reduced, contradictory to perovskite ferroelectrics. This work shifts the focus on the fundamental limits of ferroelectricity to simpler transition metal oxide systems -from perovskite-derived complex oxides to fluoritestructure binary oxides -in which 'reverse' size effects counter-intuitively stabilize polar symmetry in the ultrathin regime.Ferroelectric materials exhibit stable states of collectively ordered electrical dipoles whose polarization can be reversed under an applied electric field 5 . Consequently, ultrathin ferroelectrics are of great technological interest for high-density electronics, particularly field-effect transistors and nonvolatile memories 2 . However, ferroelectricity is typically suppressed at the few nanometer scale in the ubiquitous perovskite oxides 6 . First-principles calculations predict six unit cells as the critical thickness in perovskite ferroelectrics 1 due to incomplete screening of depolarization fields 3 . Atomic-scale ferroelectricity in perovskites often fail to demonstrate polarization switching 7,8 , a crucial ingredient for application. Furthermore, attempts to synthesize ferroelectric perovskite films on silicon 9,10 are plagued by chemical incompatibility 4,11 and high temperatures required for epitaxial growth. Since the discovery of ferroelectricity in HfO2-based thin films in 2011 12 , fluorite-structure binary oxides (fluorites) have attracted considerable interest 13 as they enable lowtemperature synthesis and conformal growth in three-dimensional (3D) structures on silicon 14,15 , thereby overcoming many of the issues that restrict its perovskite counterparts in terms of complementary metal-oxide-semiconductor (CMOS) compatibility and thickness scaling 16 .

We demonstrate ferroelectric (FE) memory transistors on a crystalline silicon channel with endurance exceeding 10 10 cycles. The ferroelectric transistors (FeFETs) incorporate a high-κ interfacial layer (IL) of thermally grown silicon nitride (SiN x ) and a thin 4.5 nm layer of Zr-doped FE-HfO 2 (HZO) on a ∼30 nm silicon on insulator (SOI) channel. The device shows a ∼1V memory window (MW) in a DC sweep of just ± 2.5V, and can be programmed and erased with voltage pulses of V G = ± 3V at a pulse width of 250 ns. The device also shows very good retention behavior. These results indicate that appropriate engineering of the IL layer could substantially improve FeFET device performance and reliability.

incompatibilities with silicon and modern semiconductor processes. [5] Since the discovery of ferroelectricity in HfO 2 -based thin films in 2011, [6] fluorite-structure binary oxides have attracted considerable interest as they are compatible with complementary metal-oxide-semiconductor (CMOS) processes. [7] Accordingly, HfO 2based ferroelectric memory has received significant attention in recent years, [1,8,9] primarily focused on charge-based ferroelectric random access memory (FeRAM) and ferroelectric field effect transistors (FeFETs). [2,10] Meanwhile, resistiveswitching materials-which exhibit electrically-induced resistance changes in metal-dielectric-metal junctions or heterostructures with multi-dielectric barriershave emerged as promising candidates for novel beyond-CMOS data-centric computing paradigms. [11][12][13] In this context, ferroelectric tunnel junctions (FTJs) present a promising energy-efficient resistive switching memory [12,13] as FTJs exploit the ferroic polarization functionality of the insulating barrier. [14] Voltage-controlled polarization-dependent tunneling through the ferroelectric layer (tunnel electroresistance, TER) can yield much larger ON/OFF conductance ratios [15,16] than, for example, current-controlled magnetic tunnel junctions, [12] another two-terminal tunneling resistive switching device.A critical requirement for FTJs is to achieve a sufficiently high tunneling current (J ON ) at the ON state to ensure that a scaled device can be read rapidly, while still exhibiting a large TER ((J ON -J OFF )/J OFF × 100%). [13] Considering the large band gap of HfO 2 (≈6 eV), the thickness of HfO 2 in the FTJ will need to be reduced to the ultrathin limit for adequate tunnel current. Tunnel junctions implementing CMOS-compatible HfO 2 -based ferroelectric barriers have been recently demonstrated, [17][18][19] but even three nanometer Zr-doped HfO 2 (Zr:HfO 2 ) barriers were found to be too thick to obtain nano-ampere level current in micron-sized capacitors. [20] Therefore, high ON current is a critical consideration; however, the increased ON state current from an ultrathin barrier will coincide with an increased OFF state current. For array-level implementations, where sneak leakage paths can lead to increased power consumption, selector devices may be required in conjunction with the FTJ memory elements to reduce such sneak currents. [13] Here, we demonstrate FTJs utilizing one nanometer Zr:HfO 2 as the ferroelectric barrier, grown by atomic layer deposition (ALD) directly on silicon, thereby scaling down the tunnel barrier ABSTRACT: In ferroelectric materials, spontaneous symmetry breaking leads to a switch-able 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 polarizationdependent tunnel current across a thin ferroelectric barrier. This work integrates FTJs with com-plementary metal-oxide-semiconductor-compatible Zr-doped HfO 2 (Zr...

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 (ZrO 2 ) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO 2 , 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.

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