Tunneling Electroresistance Research Articles

Microscopic physical origin of polarization induced large tunneling electroresistance in tetragonal-phase BiFeO3

Ferroelectric tunnel junctions have attracted intensive research interest due to the fundamental physics and potential applications in high-density data storage and neuromorphic computation. However, the intrinsic physical origin, especially at atomic scale, of polarization-controllable tunneling current is still controversial due to the degradation of ferroelectric polarization and the extrinsic conduction induced by defects or oxygen vacancies. Here, a large tunneling electroresistance effect of over 10,000% in a thick (∼15 nm) tetragonal-phase BiFeO3 thin film is observed, where a nanoscale point-contact geometry is delicately designed to reduce the extrinsic defect effects. By combining transmission electron microscopy and first-principles calculations, the atomic and electronic structures of BiFeO3 tunneling layer are investigated. The corresponding results indicate the different charge transfer occurs at the top and bottom interface, which induces distinct tunneling barrier asymmetry when the polarization direction is opposite.

Giant tunneling electroresistance in epitaxial ferroelectric ultrathin films directly integrated on Si

Retaining information with the least loss of energy is of interest for ubiquitous mobile devices. Various forms of nonvolatile memory have been pursued based on the resistance change in the memory architecture. The nonvolatility of spontaneous polarization in a ferroelectric material also has been considered a promising ingredient for information storage applications, including traditional nonvolatile memory applications and emerging neuromorphic synaptic devices. Here, we demonstrate a colossal resistance change in the ferroelectric tunnel junction with an On/Off ratio of 106 in a ferroelectric HfO2 thin film integrated directly on a silicon wafer, which is inevitable for practical application. To achieve this large On/Off ratio, we integrated the epitaxial fluorite-structure HfO2 thin film on the silicon substrate. The polarization direction in the metal-ferroelectric-semiconductor junction altered the depletion width, leaving behind the change in the tunnel barrier. Industry-relevant HfO2 with high CMOS compatibility could lead to fast adoption of a ferroelectric tunnel barrier with newly observed ferroelectricity in fluorite-structured HfO2 films.

Van der Waals Antiferroelectric Magnetic Tunnel Junction: A First-Principles Study of a CrSe2/CuInP2S6/CrSe2 Junction.

Magnetic tunnel junctions (MTJs), ferroelectric/antiferroelectric tunnel junctions (FTJs/AFTJs), and multiferroic tunnel junctions (MFTJs) have recently attracted significant interest for technological applications of nanoscale memory devices. Until now, most of them are based on perovskite oxide heterostructures with a relatively high resistance-area (RA) product and low resistance difference unfavorable for practical applications. The recent discovery of the two-dimensional (2D) van der Waals (vdW) ferroelectric (FE) and magnetic materials has opened a new route to realize tunnel junctions with high performance and atomic-scale dimensions. Here, using first-principles calculations, we propose a new type of 2D tunnel junction: an antiferroelectric magnetic tunnel junction (AFMTJ), which inherits the features of both MTJ and AFTJ. This AFMTJ is composed of monolayer CuInP2S6 (CIPS) sandwiched between 2D magnetic electrodes of CrSe2. The AFTJ with nonmagnetic electrodes of TiSe2 on both sides of CIPS and the asymmetric AFTJ with both CrSe2 and TiSe2 electrodes are also investigated. Based on quantum-mechanical modeling of the electronic transport, sizeable tunneling electroresistance effects and multiple nonvolatile resistance states are demonstrated. More importantly, a remarkably low RA product (less than 0.1 Ω·μm2) makes the proposed vdW AFMTJs superior to the conventional MFTJs in terms of their promising nonvolatile memory applications. Our calculations provide new guidance for the experiment and application of nanoscale memory devices.

Local Electric Property Modification of Ferroelectric Tunnel Junctions Induced by Variation of Polarization Charge Screening Conditions under Measurement with Scanning Probe Techniques

Scanning tunneling spectroscopy in ultrahigh vacuum conditions and conductive atomic-force microscopy in ambient conditions were used to study local electroresistive properties of ferroelectric tunnel junctions SrTiO3/La0.7Sr0.3MnO3/BaTiO3. Interestingly, experimental current-voltage characteristics appear to strongly depend on the measurement technique applied. It was found that screening conditions of the polarization charges at the interface with a top electrode differ for two scanning probe techniques. As a result, asymmetry of the tunnel barrier height for the opposite ferroelectric polarization orientations may be influenced by the method applied to study the local tunnel electroresistance. Our observations are well described by the theory of electroresistance in ferroelectric tunnel junctions. Based on this, we reveal the main factors that influence the polarization-driven local resistive properties of the device under study. Additionally, we propose an approach to enhance asymmetry of ferroelectric tunnel junctions during measurement. While keeping the high locality of scanning probe techniques, it helps to increase the difference in the value of tunnel electroresistance for the opposite polarization orientations.

Selector-less Ferroelectric Tunnel Junctions by Stress Engineering and an Imprinting Effect for High-Density Cross-Point Synapse Arrays

In the quest for highly scalable and three-dimensional (3D) stackable memory components, ferroelectric tunnel junction (FTJ) crossbar architectures are promising technologies for nonvolatile logic and neuromorphic computing. Most FTJs, however, require additional nonlinear devices to suppress sneak-path current, limiting large-scale arrays in practical applications. Moreover, the giant tunneling electroresistance (TER) remains challenging due to their inherent weak polarization. Here, we present that the employment of a diffusion barrier layer as well as a bottom metal electrode having a significantly low thermal expansion coefficient has been identified as an important way to enhance the strain, stabilize the ferroelectricity, and manage the leakage current in ultrathin hafnia film, achieving a high TER of 100, negligible resistance changes even up to 108 cycles, and a high switching speed of a few tens of nanoseconds. Also, we demonstrate that the usage of an imprinting effect in a ferroelectric capacitor induced by an ionized oxygen vacancy near the electrode results in highly asymmetric current-voltage characteristics with a rectifying ratio of 1000. Notably, the proposed FTJ exhibits a high density array size (>4k) with a securing read margin of 10%. These findings provide a guideline for the design of high-performance and selector-free FTJ devices for large-scale crossbar arrays in neuromorphic applications.

Room‐Temperature Multiferroicity and Magnetization Dynamics in Fe/BTO/LSMO Tunnel Junction

AbstractThis article describes the static, dynamic, and temperature dependent magneto‐transport properties of the Fe/BTO/LSMO mutliferroic tunnel junction (MFTJ). The multilayer structure is grown on a high quality crystalline STO substrate by means of pulsed laser deposition, which enables epitaxial layer‐by‐layer growth. The MFTJ is patterned into micrometer‐size devices by means of the ion‐etching‐free lithography process ensuring that the properties of the oxide layers are preserved after the device fabrication. The measured static properties indicate that the MFTJ has multiferroic properties at room temperature with the tunneling electroresistance (TER) and tunneling magnetoresistance (TMR) reaching 270% and 0.4%, respectively. The temperature measurements indicate the exponentially decreasing dependence of TMR with increasing temperature, whereas TER is temperature independent. The electric ferromagnetic resonance measurements based on the spin‐diode effect shows the existence of two resonance peaks, from both ferromagentic electrodes, one identified as LSMO‐derived and characterized by low magnetization damping of α = 0.002 and the other from the Fe.

Oxygen-vacancy enhanced tunnel electroresistance in LaNiO3/BaTiO3/LaNiO3 ferroelectric tunnel junctions

Oxygen vacancies (OVs) usually exist in perovskite oxides in ferroelectric tunnel junctions (FTJs), which significantly influence electron transport properties of FTJ. However, the role of OVs is currently not well understood since the OVs concentration is difficult to detect in experiments or to simulate using traditional first-principles methods. Here, using the density functional theory combined with nonequilibrium Green's function and coherent potential approximation (NECPA-DFT), we investigate electron transport properties of the LaNiO3/BaTiO3/LaNiO3 FTJ, which has a low concentration OVs in the left LaNiO3/BaTiO3 interface. The tunnel barrier height monotonously decreases with the increase in the OVs concentration for the rightward polarization in BaTiO3, leading to an increased electron tunneling coefficient. In contrast, for a leftward polarization, the barrier height only slightly decreases with the increasing OVs concentration, leading to a nearly invariant electron tunneling coefficient. The tunnel electroresistance ratio, therefore, increases monotonously with the OVs concentration and reaches to 5898% for a OVs concentration of 9%. Our results show that OVs play a critical role in determining electron transport properties of an FTJ as well as provide an alternative avenue to realize a natural asymmetric FTJ to improve its performance.

Effect of ferroelectric and interface films on the tunneling electroresistance of the Al2O3/Hf0.5Zr0.5O2 based ferroelectric tunnel junctions

Ferroelectric random-access memory (FRAM) based on conventional ferroelectric materials is a non-volatile memory with fast read/write operations, high endurance, and 10 years of data retention time. However, it suffers from destructive read-out operation and lack of CMOS compatibility. HfO2-based ferroelectric tunnel junctions (FTJ) may compensate for the shortcomings of FRAM by its CMOS compatibility, fast operation speed, and non-destructive readout operation. In this study, we investigate the effect of ferroelectric and interface film thickness on the tunneling electroresistance or ON/OFF current ratio of the Hf0.5Zr0.5O2/Al2O3 based FTJ device. Integrating a thick ferroelectric layer (i.e. 12 nm Hf0.5Zr0.5O2) with a thin interface layer (i.e. 1 nm Al2O3) resulted in an ON/OFF current ratio of 78. Furthermore, to elucidate the relationship between ON/OFF current ratio and interfacial properties, the Hf0.5Zr0.5O2-Al2O3 films and Ge-Al2O3 interfaces are examined via time-of-flight secondary ion mass spectrometry depth profiling mode. A bilayer oxide heterostructure (Hf0.5Zr0.5O2/Al2O3) is deposited by atomic layer deposition (ALD) on the Ge substrate. The ON/OFF current ratio is enhanced by an order of magnitude when the Hf0.5Zr0.5O2 film deposition mode is changed from exposure (H2O) ALD to sequential plasma (sequential O2–H2) ALD. Moreover, the interfacial engineering approach based on the in situ ALD H2-plasma surface pre-treatment of Ge increases the ON/OFF current ratio from 9 to 38 by reducing the interfacial trap density state at the Ge-Al2O3 interface and producing Al2O3 with fewer oxygen vacancies as compared to the wet etch (HF + H2O rinse) treatment of the Ge substrate. This study provides evidence of strong coupling between Hf0.5Zr0.5O2 and Al2O3 films in controlling the ON/OFF current ratio of the FTJ.

Domain-wall induced giant tunneling electroresistance effect in two-dimensional Graphene/In2Se3 ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) are very promising as a new type of nonvolatile memory devices due to the tunneling electroresistance (TER) effect. In recent years, with the rise of two-dimensional (2D) materials, 2D ferroelectrics and their application in FTJs have attracted intensive attention, with the advantage of greatly reducing the FTJ based memory device sizes, as demanded by the ongoing device minituriazation in modern electronic circuits. However, all present schemes for realizing giant TER ratio with 2D FTJs are based on the polarization reversal of the whole ferroelectric layer upon an electrical field. In this work, we explore the quantum transport properties of the 2D FTJs with the partial reversal of polarization, namely, the formation of domain walls (DWs) by constructing two kinds of FTJs. One is in a uniform-polarization state and the other one is a state with domain walls. Structural relaxation confirms the stability of the domain-wall state. By quantum transport calculation, we obtain a TER ratio as high as 2.75 × 10 4 % . Further analysis of the electronic structure shows that there is charge accumulation or charge depletion at the two DWs. Such asymmetric interface polarization charges result in a built-in electrical field and thus affect the distribution of the effective potential along the transport direction. This leads to partial metal-insulator transition around the DWs and finally the giant TER ratio. Our results indicate that DWs may greatly affect the quantum transport and provide a new mechanism for realizing giant TER effect in 2D FTJs.

Resonant band engineering of ferroelectric tunnel junctions

We propose energy band engineering to enhance tunneling electroresistance (TER) in ferroelectric tunnel junctions (FTJs). We predict that an ultrathin dielectric layer with a smaller band gap, embedded into a ferroelectric barrier layer, acts as a switch controlling high and low conductance states of an FTJ depending on polarization orientation. Using first-principles modeling based on density functional theory, we investigate this phenomenon for a prototypical SrRuO3/BaTiO3/SrRuO3 FTJ with a BaSnO3 monolayer embedded in the BaTiO3 barrier. We show that in such a composite-barrier FTJ, ferroelectric polarization of BaTiO3 shifts the conduction band minimum of the BaSnO3 monolayer above or below the Fermi energy depending on polarization orientation. The resulting switching between direct and resonant tunneling leads to a TER effect with a giant ON/OFF conductance ratio. The proposed resonant band engineering of FTJs can serve as a viable tool to enhance their performance useful for device application.

Giant ferroelectric modulation of barrier height and width in multiferroic tunnel junctions

The high tunneling electroresistance (TER) effect, generally caused by ferroelectric (FE)-modulated barrier height or width, is essential for the applications of multiferroic tunnel junctions in data storage. It is traditionally obtained by distinct electrical screening lengths of electrodes. Interface engineering can enhance the TER effect further. In this work, taking $\mathrm{Co}\text{\ensuremath{-}}{({\mathrm{TiO}}_{2}\text{\ensuremath{-}}\mathrm{BaO})}_{N}\text{\ensuremath{-}}\mathrm{Co}$ tunnel junctions as examples, we demonstrate a distinct principle than the screening lengths for designing extraordinary TER effect. We reveal that when the interfacial FE displacement is much larger than that of the FE bulk, it will bend the barrier band near the interface violently, and the interfacial polarization direction pointing to or away from the interface determines whether the energy band rises or falls. The large interfacial Ba-O displacement and its corresponding polarization direction in $\mathrm{Co}\text{\ensuremath{-}}{\mathrm{BaTiO}}_{3}\text{\ensuremath{-}}\mathrm{Co}$ tunnel junctions can be significantly modulated by the direction of FE polarization, resulting in a metallic-insulating transition of the entire thin ${\mathrm{BaTiO}}_{3}$ barrier. For thick ${\mathrm{BaTiO}}_{3}$ barrier ($N=25$, $\ensuremath{\sim}10$ nm), the effective tunnel barrier width shifts between about 2 nm and 6.5 nm as the polarization of ${\mathrm{BaTiO}}_{3}$ switches direction, which can dramatically modulate the tunneling efficiency. This effect shed light on a novel route for enhancing TER through the interface engineering.

Electroresistance in metal/ferroelectric/semiconductor tunnel junctions based on a Hf0.5Zr0.5O2 barrier

Ferroelectric Hf0.5Zr0.5O2 films, 5.8 nm in thickness, were deposited on Nb:SrTiO3 semiconductor substrates to form a Pt/Hf0.5Zr0.5O2/Nb:SrTiO3 metal/ferroelectric/semiconductor ferroelectric tunnel junction (FTJ). A high tunneling electroresistance ratio of 800 was achieved at room-temperature. It is observed that in the low resistance state, the transport characteristic obeys direct tunneling, while in the high resistance state, it is dominated by thermal emission. It implies that the Schottky barrier on the surface of the semiconductive electrode is modulated by the polarization in the ferroelectric Hf0.5Zr0.5O2 barrier, generating the high electroresistance ratio. The FTJ also exhibits excellent retention for more than 10 000 s and good switching endurance for more than 1500 cycles. The results suggest the potential of this HfO2-based FTJ for next generation nonvolatile memories.

Large Tunnel Electroresistance with Ultrathin Hf0.5Zr0.5O2 Ferroelectric Tunnel Barriers

AbstractHafnia‐based ferroelectric tunnel junctions (FTJs) hold great promise for nonvolatile memory and emerging data storage applications. In this article, a large tunnel electroresistance effect with ultrathin Hf0.5Zr0.5O2 (HZO) barrier based FTJs is reported. Robust ferroelectricity is achieved with ≈1 nm films by stabilizing the rhombohedral polar phase of HZO (R‐HZO) through a large compressive strain, induced by growing the film epitaxially on a SrTiO3 (001) substrate. The OFF/ON ratio of the junction resistance at zero bias is about 135 with ≈1 nm thick barrier, which increases to ≈105 with increasing the barrier thickness to ≈2.5 nm. The resistance‐area product (RA) of tunnel junctions is reduced by nearly three orders of magnitude by using an ≈1 nm R‐HZO barrier as compared with typically reported RA values for doped‐HfO2 barrier based FTJs, which significantly improves signal‐to‐noise ratio during the read operation. These results set the stage for further exploration of Hafnia‐based FTJs for non‐volatile memory applications.

Low-Temperature Tunneling Electroresistance in Ferromagnetic Metal/Ferroelectric/Semiconductor Tunnel Junctions.

Ferroelectric tunnel junctions (FTJs) as artificial synaptic devices are promising candidates for the building block of nonvolatile data storage devices. However, a small ON/OFF ratio of FTJs limits their application in low-temperature operations. In this work, the influence of quantum interference effects on tunneling electroresistance in the La0.7Sr0.3MnO3/BaTiO3/Nb:SrTiO3 (ferromagnetic metal/ferroelectric/semiconductor) FTJ at low temperatures is investigated. The Current-voltage curves are observed in the tunnel junction from 300 to 10 K with a six-unit-cell thick BaTiO3 film by the ferroelectric polarization effect. First, the ON/OFF current ratio increases from 300 to 30 K due to the increase of polarization in the ferroelectric barrier, and then, it gradually decreases when the temperature drops below 30 K. An anomalous ON/OFF current ratio of ∼105 is obtained at 30 K. The low-temperature tunneling properties in the FTJ are associated with a low-temperature resistivity minimum in the ferromagnetic metal layer by the electron-electron interaction, which increases the La0.7Sr0.3MnO3/BaTiO3 interface resistance, leading to a higher resistance state and lower IOFF for the OFF state. As a result, the ON/OFF current ratio is abruptly enhanced at 30 K. Our results emphasize the crucial role of transport properties of La0.7Sr0.3MnO3 in FTJs and pave the way for the design and application of FTJs at low temperatures.

Amorphous ZrO2 Tunnel Junction Memristor With a Tunneling Electroresistance Ratio Above 400

An amorphous ZrO2 based tunneling junction memristor (TJM) with a tunneling electroresistance (TER) ratio above 400 is demonstrated. It is attributed to the modulation of tunneling barrier width induced by the accumulation of oxygen vacancies ( $V_{O}^{+}$ ) and negative charges near the ZrO2/semiconductor interface. The ferroelectric-like behaviors attributed to the voltage-modulated switching of the dipoles consisting of $V_{O}^{+}$ and negative charges in ZrO2 are characterized by a polarization-voltage test. The ZrO2 TJM achieves a TER ratio above 400 under 2.5/–1.5 V at 100 ns write/erase pulse condition, over 104 cycles program/erase endurance, and >104 s data retention.

Giant tunnel electroresistance in ferroelectric tunnel junctions with metal contacts to two-dimensional ferroelectric materials

Two-dimensional (2D) ferroelectric materials (FEMs) and their application in ferroelectric tunnel junctions (FTJs) have attracted a great deal of attention during the past several years due to their great potential in nonvolatile memory devices. Particularly, the all-2D FTJs, which have only atomic-layer thickness, have been demonstrated to show very high tunnel electroresistance (TER) ratio. Nevertheless, to better integrate with the present semiconductor technology, it is necessary to consider metal contacts in the construction of FTJs with 2D FEMs. However, due to the unknown interaction between traditional metals and 2D FEMs, it is not clear whether ferroelectricity still persists when the 2D FEMS are in contact with metals and whether the corresponding FTJs exhibit high TER effect as demanded for memory devices. To probe this, we construct FTJs with top contact between Au(010) and ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, a 2D FEM with out-of-plane ferroelectric polarization. By density functional calculations combined with a nonequilibrium Green function technique, we find that not only the ferroelectricity still persists in the metal/FEM contact, but also a giant TER ratio as high as ${10}^{4}%$ is achieved. The giant TER arises from the change of the metal/FEM contact from a Schottky type to an Ohmic type accompanying with the ferroelectric polarization reversal. In the meantime, the tunnel barrier height between Au(010) and ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ is zero, which means good ability of electron injection from metal to semiconductor and low contact resistance. Our study suggests that, by properly selecting the metal materials, giant TER ratio and high performance can be achieved in FTJs constructed with 2D FEMs and metal contacts.

Electroresistance effect in MoS2-Hf0.5Zr0.5O2 heterojunctions

Pairing two-dimensional semiconductors with ferroelectric films may allow for the development of hybrid electronic devices that would not only exhibit a combination of the functional properties of both material groups but would also reveal unusual characteristics emerging from coupling between these properties. Here, we report the observation of a considerable (up to 103 at 0.8 V read bias) polarization-mediated tunneling electroresistance (TER) effect in Hf0.5Zr0.5O2 (HZO) ferroelectric tunnel junctions (FTJs) employing MoS2 as one of the electrodes. It was found that for this type of hybrid FTJs, a change in resistance upon polarization reversal could be described by Fowler–Nordheim tunneling. The underlying mechanism for the enhanced TER effect is a polarization-mediated accumulation or depletion of the majority carriers at the MoS2/HZO interface, which results in a change in the effective barrier shape seen by the tunneling electrons. Given the compatibility of HfO2-family ferroelectrics with CMOS technology and a possibility of large scale growth and transfer of MoS2 films, our results provide a pathway for fabrication of high-density nonvolatile memory and data storage systems based on hybrid FTJs.

Two-Dimensional Antiferroelectric Tunnel Junction.

Ferroelectric tunnel junctions (FTJs), which consist of two metal electrodes separated by a thin ferroelectric barrier, have recently aroused significant interest for technological applications as nanoscale resistive switching devices. So far, most existing FTJs have been based on perovskite-oxide barrier layers. The recent discovery of the two-dimensional (2D) van der Waals ferroelectric materials opens a new route to realize tunnel junctions with new functionalities and nm-scale dimensions. Because of the weak coupling between the atomic layers in these materials, the relative dipole alignment between them can be controlled by applied voltage. This allows transitions between ferroelectric and antiferroelectric orderings, resulting in significant changes of the electronic structure. Here, we propose to realize 2D antiferroelectric tunnel junctions (AFTJs), which exploit this new functionality, based on bilayer In_{2}X_{3} (X=S, Se, Te) barriers and different 2D electrodes. Using first-principles density functional theory calculations, we demonstrate that the In_{2}X_{3} bilayers exhibit stable ferroelectric and antiferroelectric states separated by sizable energy barriers, thus supporting a nonvolatile switching between these states. Using quantum-mechanical modeling of the electronic transport, we explore in-plane and out-of-plane tunneling across the In_{2}S_{3} van der Waals bilayers, and predict giant tunneling electroresistance effects and multiple nonvolatile resistance states driven by ferroelectric-antiferroelectric order transitions. Our proposal opens a new route to realize nanoscale memory devices with ultrahigh storage density using 2D AFTJs.

CMOS-Compatible Fabrication of Low-Power Ferroelectric Tunnel Junction for Neural Network Applications

A low-cost fabrication process of Hf <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{1}-{x}}$ </tex-math></inline-formula> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) nonvolatile memory (NVM) was proposed and its characteristics were investigated. We successfully fabricated a ferroelectric tunnel junction (FTJ) device with tunable conductance for neural network applications. The proposed FTJ device exhibits excellent performances, such as large conductance ratio of ~40 for a 500-ns pulse and thus satisfied low-power consumption of write pulse (1 fJ per bit) and fast write speed (<500 ns) requirements. Furthermore, we revealed that the metal–ferroelectric–insulator–semiconductor structure had a higher tunneling electroresistance ratio than the metal–ferroelectric–insulator–metal structure. Moreover, the polarization operation of our FTJ devices achieved low-power analog-like conductance transition, multilevel operation, and even endurance characteristics is promising. These results demonstrate that the FTJ has high potential to be an ideal emerging memory for neuromorphic computing. Therefore, HZO-based devices are promising materials in neural network applications for next-generation devices.

Effect of Insertion of Dielectric Layer on the Performance of Hafnia Ferroelectric Devices

Hafnia-based ferroelectric (FE) devices have attracted significant attention as nonvolatile memory devices due to their compatibility with complementary metal–oxide–semiconductor processes. Among them, the FE tunnel junction (FTJ) has been considered as the next-generation of nonvolatile memory devices owing to its neuromorphic characteristics and nondestructive read operation as well as the high-density integration. However, degradation of ferroelectricity in a thin hafnia film causes a reduced tunneling electroresistance (TER) ratio. In this work, we inserted a TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer into the metal–FE–metal capacitor to provide an asymmetric structure for FTJ operations and recorded the enhanced remnant polarization with enhanced ferroelectricity. In this study, a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2{P}_{r}$ </tex-math></inline-formula> value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$44.4~\mu \text{C}$ </tex-math></inline-formula> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a notable TER ratio of 11.6 were observed for 6-nm thick hafnia films with 1-nm thick TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films.