A Complementary Metal Oxide Semiconductor Process-Compatible Ferroelectric Tunnel Junction

Silicon-Compatible Ferroelectric Tunnel Junctions with a SiO2/Hf0.5Zr0.5O2 Composite Barrier as Low-Voltage and Ultra-High-Speed Memristors

The big data era requires ultrafast, low-power, and silicon-compatible materials and devices for information storage and processing. Here, ferroelectric tunnel junctions (FTJs) based on SiO2/Hf0.5Zr0.5O2 composite barrier and both conducting electrodes are designed and fabricated on Si substrates. The FTJ achieves the fastest write speed of 500 ps under 5 V (2 orders of magnitude faster than reported silicon-compatible FTJs) or 10 ns speed at a low voltage of 1.5 V (the lowest voltage among FTJs at similar speeds), low write current density of 1.3 × 104 A cm-2, 8 discrete states, good retention > 105 s at 85 °C, and endurance > 107. In addition, it provides a large read current (88 A cm-2) at 0.1 V, 2 orders of magnitude larger than reported FTJs. Interestingly, in FTJ-based synapses, gradually tunable conductance states (128 states) with high linearity (<1) are obtained by 10 ns pulses of <1.2 V, and a high accuracy of 91.8% in recognizing fashion product images is achieved by online neural network simulations. These results highlight that silicon-compatible HfO2-based FTJs are promising for high-performance nonvolatile memories and electrical synapses.

Physics, Structures, and Applications of Fluorite-Structured Ferroelectric Tunnel Junctions

The interest in ferroelectric tunnel junctions (FTJ) has been revitalized by the discovery of ferroelectricity in fluorite-structured oxides such as HfO2 and ZrO2 . In terms of thickness scaling, CMOS compatibility, and 3D integration, these fluorite-structured FTJs provide a number of benefits over conventional perovskite-based FTJs. Here, recent developments involving all FTJ devices with fluorite structures are examined. The transport mechanism of fluorite-structured FTJs is explored and contrasted with perovskite-based FTJs and other 2-terminal resistive switching devices starting with the operation principle and essential parameters of the tunneling electroresistance effect. The applications of FTJs, such as neuromorphic devices, logic-in-memory, and physically unclonable function, are then discussed, along with several structural approaches to fluorite-structure FTJs. Finally, the materials and device integration difficulties related to fluorite-structure FTJ devices are reviewed. The purpose of this review is to outline the theories, physics, fabrication processes, applications, and current difficulties associated with fluorite-structure FTJs while also describing potential future possibilities for optimization.

Electrically induced cancellation and inversion of piezoelectricity in ferroelectric Hf0.5Zr0.5O2

HfO2-based thin films hold huge promise for integrated devices as they show full compatibility with semiconductor technologies and robust ferroelectric properties at nanometer scale. While their polarization switching behavior has been widely investigated, their electromechanical response received much less attention so far. Here, we demonstrate that piezoelectricity in Hf0.5Zr0.5O2 ferroelectric capacitors is not an invariable property but, in fact, can be intrinsically changed by electrical field cycling. Hf0.5Zr0.5O2 capacitors subjected to ac cycling undergo a continuous transition from a positive effective piezoelectric coefficient d33 in the pristine state to a fully inverted negative d33 state, while, in parallel, the polarization monotonically increases. Not only can the sign of d33 be uniformly inverted in the whole capacitor volume, but also, with proper ac training, the net effective piezoresponse can be nullified while the polarization is kept fully switchable. Moreover, the local piezoresponse force microscopy signal also gradually goes through the zero value upon ac cycling. Density functional theory calculations suggest that the observed behavior is a result of a structural transformation from a weakly-developed polar orthorhombic phase towards a well-developed polar orthorhombic phase. The calculations also suggest the possible occurrence of a non-piezoelectric ferroelectric Hf0.5Zr0.5O2. Our experimental findings create an unprecedented potential for tuning the electromechanical functionality of ferroelectric HfO2-based devices.

Graphene-based RRAM devices for neural computing

Resistive random access memory is very well known for its potential application in in-memory and neural computing. However, they often have different types of device-to-device and cycle-to-cycle variability. This makes it harder to build highly accurate crossbar arrays. Traditional RRAM designs make use of various filament-based oxide materials for creating a channel that is sandwiched between two electrodes to form a two-terminal structure. They are often subjected to mechanical and electrical stress over repeated read-and-write cycles. The behavior of these devices often varies in practice across wafer arrays over these stresses when fabricated. The use of emerging 2D materials is explored to improve electrical endurance, long retention time, high switching speed, and fewer power losses. This study provides an in-depth exploration of neuro-memristive computing and its potential applications, focusing specifically on the utilization of graphene and 2D materials in RRAM for neural computing. The study presents a comprehensive analysis of the structural and design aspects of graphene-based RRAM, along with a thorough examination of commercially available RRAM models and their fabrication techniques. Furthermore, the study investigates the diverse range of applications that can benefit from graphene-based RRAM devices.

Revival of Ferroelectric Memories Based on Emerging Fluorite-Structured Ferroelectrics

Over the last few decades, the research on ferroelectric memories has been limited due to their dimensional scalability and incompatibility with complementary metal-oxide-semiconductor (CMOS) technology. The discovery of ferroelectricity in fluorite-structured oxides revived interest in the research on ferroelectric memories, by inducing nanoscale nonvolatility in state-of-the-art gate insulators by minute doping and thermal treatment. The potential of this approach has been demonstrated by the fabrication of sub-30 nm electronic devices. Nonetheless, to realize practical applications, various technical limitations, such as insufficient reliability including endurance, retention, and imprint, as well as large device-to-device-variation, require urgent solutions. Furthermore, such limitations should be considered based on targeting devices as well as applications. Various types of ferroelectric memories including ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction should be considered for classical nonvolatile memories as well as emerging neuromorphic computing and processing-in-memory. Therefore, from the viewpoint of materials science, this review covers the recent research focusing on ferroelectric memories from the history of conventional approaches to future prospects.

Ferroelectric synaptic devices based on CMOS-compatible HfAlOx for neuromorphic and reservoir computing applications

The hafnium oxide-based ferroelectric tunnel junction (FTJ) has been actively researched because of desirable advantages such as low power and CMOS compatibility to operate as a memristor. In the case of HfAlO_x (HAO), the remanent polarization (P_r) value is high and the atomic radius of Al is smaller than that of Hf; therefore, ferroelectricity can be better induced without mechanical force. In this paper, we propose an FTJ using HAO as a ferroelectric layer through electrical analysis and experiments; further, we experimentally demonstrate its capability as a synaptic device. Moreover, we evaluate the maximum 2P_r and TER value of the device according to the difference in conditions of thickness and cell area. The optimized device conditions are analyzed, and a large value of 2P_r (>∼43 μC cm^-2) is obtained. Furthermore, we show that paired-pulse facilitation, paired-pulse depression, and spike-timing-dependent plasticity can be utilized in HAO-based FTJs. In addition, this study demonstrates the use of an FTJ device as a physical reservoir to implement reservoir computing. Through a series of processes, the synaptic properties of FTJs are verified for the feasibility of their implementation as an artificial synaptic device.

Cycling Waveform Dependent Wake-Up and ON/OFF Ratio in Al2O3/Hf0.5Zr0.5O2 Ferroelectric Tunnel Junction Devices

The wake-up behavior and ON/OFF current ratio of TiN-Al₂O₃-Hf_0.5Zr_0.5O₂-W ferroelectric tunnel junction (FTJ) devices were investigated for different wake-up voltage waveforms. We studied triangular and square waves, as well as square pulse trains of equal or unequal voltage amplitudes for positive and negative polarities. We find that the wake-up behavior in these FTJ stacks is highly influenced by the field cycling waveform. A square waveform is observed to provide wake-up with the lowest number of cycles, concomitantly resulting in higher remnant polarization and a higher ON/OFF ratio in the devices, compared to a triangular waveform. We further show that wake-up is dependent on the number of cycles rather than the total time of the applied electric field during cycling. We also demonstrate that different voltage magnitudes are necessary for positive and negative polarities during field cycling for efficient wake-up. Utilizing an optimized waveform with unequal magnitudes for the two polarities during field cycling, we achieve a reduction in wake-up cycles and a large enhancement of the ON/OFF ratio from ∼5 to ∼35 in our ferroelectric tunnel junctions.

Ferroelectric Electroresistance after a Breakdown in Epitaxial Hf0.5Zr0.5O2 Tunnel Junctions

The recent discovery of ferroelectricity in doped HfO₂ has opened perspectives on the development of memristors based on ferroelectric switching, including ferroelectric tunnel junctions. In these devices, conductive channels are formed in a similar manner to junctions based on nonferroelectric oxides. The formation of the conductive channels does not preclude the presence of ferroelectric switching, but little is known about the device ferroelectric properties after conduction path formation or their impact on the electric modulation of the resistance state. Here, we show that ferroelectricity and related sizable electroresistance are observed in pristine 4.6 nm epitaxial Hf_0.5Zr_0.5O₂ (HZO) tunnel junctions grown on Si. After a soft breakdown induced by the application of suitable voltage, the resistance decreases by about five orders of magnitude, but signatures of ferroelectricity and electroresistance are still observed. Impedance spectroscopy allows us to conclude that the effective ferroelectric device area after the breakdown is reduced, most likely by the formation of conducting paths at the edge.

A perspective on the physical scaling down of hafnia-based ferroelectrics

HfO₂-based ferroelectric thin films have attracted significant interest for semiconductor device applications due to their compatibility with complementary metal oxide semiconductor (CMOS) technology. One of the benefits of HfO₂-based ferroelectric thin films is their ability to be scaled to thicknesses as low as 10 nm while retaining their ferroelectric properties; a feat that has been difficult to accomplish with conventional perovskite-based ferroelectrics using CMOS-compatible processes. However, reducing the thickness limit of HfO₂-based ferroelectric thin films below the sub 5 nm thickness regime while preserving their ferroelectric property remains a formidable challenge. This is because both the structural factors of HfO₂, including polymorphism and orientation, and the electrical factors of HfO₂-based devices, such as the depolarization field, are known to be highly dependent on the HfO₂thickness. Accordingly, when the thickness of HfO₂drops below 5 nm, these factors will become even more crucial. In this regard, the size effect of HfO₂-based ferroelectric thin films is thoroughly discussed in the present review. The impact of thickness on the ferroelectric property of HfO₂-based thin films and the electrical performance of HfO₂-based ferroelectric semiconductor devices, such as ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction, is extensively discussed from the perspective of fundamental theory and experimental results. Finally, recent developments and reports on achieving ferroelectric HfO₂at sub-5 nm thickness regime and their applications are discussed.

High-performance ferroelectric nonvolatile memory based on Gd-and Ni-codoped BiFeO3 films

BiFeO₃ (BFO), Bi_0.92Gd_0.08FeO₃ (BGFO) and Bi_0.92Gd_0.08Fe_0.95Ni_0.05O₃ (BGFNO) films are epitaxially grown on 0.7 wt% Nb-SrTiO₃ (NSTO) substrates. The strong ferroelectric property in BGFNO film is confirmed by piezoresponse force microscopy (PFM) and polarization versus voltage (P-V) measurement. It is also found that the Au/BGFNO/NSTO devices possess a ferroelectric resistance switching (RS) effect. Gd- and Ni-codoped BiFeO₃ is found to strongly enhance the resistance on/off ratio. A resistance on/off ratio as large as 3 × 10⁶ is achieved with an applied pulse voltage of -8 V and +4 V. In addition, the devices exhibit excellent retention and anti-fatigue characteristics. The memristor behavior of Au/BGFNO/NSTO is attributed to the switching of polarization states, which modulate the width and height of the barrier at the BGFNO/NSTO interface. The excellent resistive switching properties in Au/BGFNO/NSTO devices indicate the promising application in nonvolatile memory.

Highly enhanced ferroelectricity in HfO2-based ferroelectric thin film by light ion bombardment

Continuous advancement in nonvolatile and morphotropic beyond-Moore electronic devices requires integration of ferroelectric and semiconductor materials. The emergence of hafnium oxide (HfO₂)-based ferroelectrics that are compatible with atomic-layer deposition has opened interesting and promising avenues of research. However, the origins of ferroelectricity and pathways to controlling it in HfO₂ are still mysterious. We demonstrate that local helium (He) implantation can activate ferroelectricity in these materials. The possible competing mechanisms, including He ion-induced molar volume changes, vacancy redistribution, vacancy generation, and activation of vacancy mobility, are analyzed. These findings both reveal the origins of ferroelectricity in this system and open pathways for nanoengineered binary ferroelectrics.

Si-Doped HfO2-Based Ferroelectric Tunnel Junctions with a Composite Energy Barrier for Non-Volatile Memory Applications

Ferroelectric tunnel junctions (FTJs) have attracted attention as devices for advanced memory applications owing to their high operating speed, low operating energy, and excellent scalability. In particular, hafnia ferroelectric materials are very promising because of their high remanent polarization (below 10 nm) and high compatibility with complementary metal-oxide-semiconductor (CMOS) processes. In this study, a Si-doped HfO₂-based FTJ device with a metal-ferroelectric-insulator-semiconductor (MFIS) structure was proposed to maximize the tunneling electro-resistance (TER) effect. The potential barrier modulation effect under applied varying voltage was analyzed, and the possibility of its application as a non-volatile memory device was presented through stability assessments such as endurance and retention tests.

Effect of Process Temperature on Density and Electrical Characteristics of Hf0.5Zr0.5O2 Thin Films Prepared by Plasma-Enhanced Atomic Layer Deposition

HfxZr1−xO2 (HZO) thin films have excellent potential for application in various devices, including ferroelectric transistors and semiconductor memories. However, such applications are hindered by the low remanent polarization (Pr) and fatigue endurance of these films. To overcome these limitations, in this study, HZO thin films were fabricated via plasma-enhanced atomic layer deposition (PEALD), and the effects of the deposition and post-annealing temperatures on the density, crystallinity, and electrical properties of the thin films were analyzed. The thin films obtained via PEALD were characterized using cross-sectional transmission electron microscopy images and energy-dispersive spectroscopy analysis. An HZO thin film deposited at 180 °C exhibited the highest o-phase proportion as well as the highest density. By contrast, mixed secondary phases were observed in a thin film deposited at 280 °C. Furthermore, a post-annealing temperature of 600 °C yielded the highest thin film density, and the highest 2Pr value and fatigue endurance were obtained for the film deposited at 180 °C and post-annealed at 600 °C. In addition, we developed three different methods to further enhance the density of the films. Consequently, an enhanced maximum density and exceptional fatigue endurance of 2.5 × 107 cycles were obtained.

High-Speed Switching and Giant Electroresistance in an Epitaxial Hf0.5Zr0.5O2-Based Ferroelectric Tunnel Junction Memristor

HfO₂-based ferroelectric materials are good candidates for constructing next-generation nonvolatile memories and high-performance electronic synapses and have attracted extensive attention from both academia and industry. Here, a Hf_0.5Zr_0.5O₂-based ferroelectric tunnel junction (FTJ) memristor is successfully fabricated by epitaxially growing a Hf_0.5Zr_0.5O₂ film on a 0.7 wt % Nb-doped SrTiO₃ (001) substrate with a buffer layer of La_2/3Sr_1/3MnO₃ (∼1 u.c.). The FTJ shows a high switching speed of 20 ns, a giant electroresistance ratio of ∼834, and multiple states (eight states or three bits) with good retention >10⁴ s. As a solid synaptic device, tunable synapse functions have also been obtained, including long-term potentiation, long-term depression, and spike-timing-dependent plasticity. These results highlight the promising applications of Hf_0.5Zr_0.5O₂-based FTJ in ultrafast-speed and high-density nonvolatile memories and artificial synapses.

Magnetoelectric Coupling at the Ni/Hf0.5Zr0.5O2 Interface

Composite multiferroics containing ferroelectric and ferromagnetic components often have much larger magnetoelectric coupling compared to their single-phase counterparts. Doped or alloyed HfO₂-based ferroelectrics may serve as a promising component in composite multiferroic structures potentially feasible for technological applications. Recently, a strong charge-mediated magnetoelectric coupling at the Ni/HfO₂ interface has been predicted using density functional theory calculations. Here, we report on the experimental evidence of such magnetoelectric coupling at the Ni/Hf_0.5Zr_0.5O₂(HZO) interface. Using a combination of operando XAS/XMCD and HAXPES/MCDAD techniques, we probe element-selectively the local magnetic properties at the Ni/HZO interface in functional Au/Co/Ni/HZO/W capacitors and demonstrate clear evidence of the ferroelectric polarization effect on the magnetic response of a nanometer-thick Ni marker layer. The observed magnetoelectric effect and the electronic band lineup of the Ni/HZO interface are interpreted based on the results of our theoretical modeling. It elucidates the critical role of an ultrathin NiO interlayer, which controls the sign of the magnetoelectric effect as well as provides a realistic band offset at the Ni/HZO interface, in agreement with the experiment. Our results hold promise for the use of ferroelectric HfO₂-based composite multiferroics for the design of multifunctional devices compatible with modern semiconductor technology.

Effect of ferroelectric and interface films on the tunneling electroresistance of the Al2O3/Hf0.5Zr0.5O2based 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 Hf_0.5Zr_0.5O₂/Al₂O₃based FTJ device. Integrating a thick ferroelectric layer (i.e. 12 nm Hf_0.5Zr_0.5O₂) 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 Hf_0.5Zr_0.5O₂-Al₂O₃films and Ge-Al₂O₃interfaces are examined via time-of-flight secondary ion mass spectrometry depth profiling mode. A bilayer oxide heterostructure (Hf_0.5Zr_0.5O₂/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 Hf_0.5Zr_0.5O₂film deposition mode is changed from exposure (H₂O) ALD to sequential plasma (sequential O₂-H₂) ALD. Moreover, the interfacial engineering approach based on thein situALD 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-Al₂O₃interface and producing Al₂O₃with 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 Hf_0.5Zr_0.5O₂and Al₂O₃films in controlling the ON/OFF current ratio of the FTJ.

A novel physical unclonable function (PUF) using 16 × 16 pure-HfOxferroelectric tunnel junction array for security applications

As the computing paradigm has shifted toward edge computing, improving the security of edge devices is attracting significant attention. However, because edge devices have limited resources in terms of power and area, it is difficult to apply a conventional cryptography system to protect them. On the other hand, as a simple security application, a physical unclonable function (PUF) can be implemented without power and area problems because it provides a security key by utilizing process variations without additional external circuits. Ferroelectric tunnel junctions (FTJs) are 2-terminal devices that store information by changing the resistance of a ferroelectric material, where the resistance is determined by the polarization states of the ferroelectric domains. Because polycrystalline ferroelectric materials have a multi-domain nature, domain variation can also be used as a randomness source to induce cell-to-cell variations along with process variations. In this paper, we demonstrate PUF operations of a low-power, small area 16 × 16 hafnium oxide (pure-HfO_x)-based FTJ array using certain metrics. It is clear that the proposed array consisting of scaled FTJs has adequate randomness for security applications such that the array-level PUF operations are robust against model-based machine learning attacks.

Defects in ferroelectric HfO2

The discovery of ferroelectricity in polycrystalline thin films of doped HfO2 has reignited the expectations of developing competitive ferroelectric non-volatile memory devices. To date, it is widely accepted that the performance of HfO2-based ferroelectric devices during their life cycle is critically dependent on the presence of point defects as well as structural phase polymorphism, which mainly originates from defects either. The purpose of this review article is to overview the impact of defects in ferroelectric HfO2 on its functional properties and the resulting performance of memory devices. Starting from the brief summary of defects in classical perovskite ferroelectrics, we then introduce the known types of point defects in dielectric HfO2 thin films. Further, we discuss main analytical techniques used to characterize the concentration and distribution of defects in doped ferroelectric HfO2 thin films as well as at their interfaces with electrodes. The main part of the review is devoted to the recent experimental studies reporting the impact of defects in ferroelectric HfO2 structures on the performance of different memory devices. We end up with the summary and perspectives of HfO2-based ferroelectric competitive non-volatile memory devices.

Incorporating a redox active entity to attain electrical bistability in a polymer semiconductor

Owing to the advantages of 3-D printable stack, scalability and low cost solution state production, polymer-based resistive memory devices have been identified as the promising alternative for conventional oxide technology. Resistive memory devices based on the redox switch mechanism is particularly found to yield high precision with respect to the operational voltages. Reversible non-volatile resistive state switching was realized with high device yield (>80%), with a redox-active chemical entity conjugated to the polymeric semiconductor, and the control experiments with the model compound confirmed the imperative role of the redox-active anthraquinone center in the polymeric backbone. Highly uniform nanodomains and the trap free layers excluded the possibilities of other known switching mechanisms. Optical studies and the molecular modelling data assert the presence of strong charge transfer characteristics upon optical excitation due to the insertion of the anthraquinone unit, which was detrimental in exhibiting bistable conductive states in electrical bias as well.

Excellent Reliability and High-Speed Antiferroelectric HfZrO2 Tunnel Junction by a High-Pressure Annealing Process and Built-In Bias Engineering

Hafnia-based ferroelectric tunnel junctions (FTJs) have great potential for use in logic in nonvolatile memory because of their complementary metal-oxide-semiconductor process compatibility, low power consumption, high scalability, and nondestructive readout. However, typically, ferroelectrics have a depolarization field, resulting in poor endurance owing to the early dielectric breakdown. Herein, an outstandingly reliable and high-speed antiferroelectric HfZrO tunnel junction (AFTJ) is probed to understand whether it is a promising candidate for next-generation nonvolatile memory applications. High-reliability AFTJ can be explained by less charge injection due to the low depolarized field. The formation of two stable nonvolatile states, even with antiferroelectric materials, is possible if asymmetric work function electrodes and fixed oxide charges are employed, generating a built-in bias and shifting the polarization-voltage curve. In addition, via high-pressure annealing, a critical voltage that determines the transition from the t-phase to the o-phase is effectively reduced (22%). The AFTJ shows a higher endurance property (>10⁹ cycles) and faster switching speed (<30 ns) than FTJ. Hence, it is proposed that with the help of internal bias modulation and high-pressure annealing, AFTJs can be employed in next-generation memory devices.

Data retention and low voltage operation of Al2O3/Hf0.5Zr0.5O2 based ferroelectric tunnel junctions

Ferroelectric random-access memories based on conventional perovskite materials are non-volatile but suffer from lack of CMOS compatibility, scalability limitation, and a destructive reading scheme. On the other hand, ferroelectric tunnel junctions based on CMOS compatible hafnium oxide are a promising candidate for future non-volatile memory technology due to their simple structure, scalability, low power consumption, high operation speed, and non-destructive read-out operation. Herein, we report an efficient strategy based on the interface-engineering approach to improve upon the tunneling electroresistance effect and data retention by depositing bilayer oxide heterostructure (Al₂O₃/Hf_0.5Zr_0.5O₂) using atomic layer deposition (ALD) on Ge substrate which is treated in-situ ALD chamber with H₂-plasma before film deposition. Integrating a thin ferroelectric layer i.e. Hf_0.5Zr_0.5O₂ (8.4 nm) with a thin interface layer i.e. Al₂O₃ (1 nm) allowed us to reduce the operation (read and write) voltage to 1.4 V, and 4.3 V, respectively, while maintaining a good tunneling electroresistance or ON/OFF ratio above 10. Furthermore, an extrapolation to 1000 years at room temperature gives a residual ON/OFF ratio of 4.

Optimizing the Piezoelectric Strain in ZrO2- and HfO2-Based Incipient Ferroelectrics for Thin-Film Applications: An Ab Initio Dopant Screening Study

HfO₂ and ZrO₂ have increasingly drawn the interest of researchers as lead-free and silicon technology-compatible materials for ferroelectric, pyroelectric, and piezoelectric applications in thin films such as ferroelectric field-effect transistors, ferroelectric random access memories, nanoscale sensors, and energy harvesters. Owing to the environmental regulations against lead-containing electronic components, HfO₂ and ZrO₂ offer, along with AlN, (K,Na)NbO₃- and (Bi_0.5Na_0.5)TiO₃-based materials, an alternative to Pb(Zr_xTi_1-x)O₃-based materials, which are the overwhelmingly used ceramics in industry. HfO₂ and ZrO₂ thin films may show field-induced phase transformation from the paraelectric tetragonal to the ferroelectric orthorhombic phase, leading to a change in crystal volume and thus strain. These field-induced strains have already been measured experimentally in pure and doped systems; however, no systematic optimization of the piezoelectric activity was performed, either experimentally or theoretically. In this screening study, we calculate the ultimate size of this effect for 58 dopants depending on the oxygen supply and the defect incorporation type: substitutional or interstitial. The largest piezoelectric strain values are achieved with Yb, Li, and Na in ZrO₂ and exceed 40 pm V^-1 or 0.8% maximal strain, which exceeds the best experimental findings by a factor of 2. Furthermore, we discovered that Mo, W, and Hg make the polar-orthorhombic phase in the ZrO₂ bulk stable under certain circumstances, which would count in favor of these systems for the ceramic crystallization process. Our work guides the development of the performance of a promising material system by rational design of the essential mechanisms so as to apply it to unforeseen applications.

The effects of thin film homogeneity on the performance of ferroelectric tunnel junctions

The compelling physical properties of the recently discovered ferroelectric phase in thin film Hf _x Zr_1-x O₂ have opened a window for applications such as non-volatile resistive switching memory devices with high retention known as ferroelectric tunnel junctions. In this article, we investigate the stability of these two-terminal, polarization induced resistance-switching devices with respect to the statistical reproducibility of constitutive electrical parameters based on surface thickness inhomogeneities. We provide a straightforward, quantitative model to estimate tunneling currents dependent on thickness variations, and the resulting tunneling electroresistance (TER) ratios and breakdown probability. An analytical expression for the probability distribution of tunneling currents for normally distributed thicknesses is given. Using material parameters of a TiN/HZO/Pt heterostructure, practical design requirements are deduced and an estimation with respect to the surface roughness is given for practical ferroelectric layer thicknesses and voltages below 4 nm and 1 V, respectively. In this regime, the simple model of a ballistic, direct tunneling mechanism can be used to adequately model the thickness and voltage dependence of the resistivity.

Back-End, CMOS-Compatible Ferroelectric Field-Effect Transistor for Synaptic Weights

Neuromorphic computing architectures enable the dense colocation of memory and processing elements within a single circuit. This colocation removes the communication bottleneck of transferring data between separate memory and computing units as in standard von Neuman architectures for data-critical applications including machine learning. The essential building blocks of neuromorphic systems are nonvolatile synaptic elements such as memristors. Key memristor properties include a suitable nonvolatile resistance range, continuous linear resistance modulation, and symmetric switching. In this work, we demonstrate voltage-controlled, symmetric and analog potentiation and depression of a ferroelectric Hf_0.57Zr_0.43O₂ (HZO) field-effect transistor (FeFET) with good linearity. Our FeFET operates with low writing energy (fJ) and fast programming time (40 ns). Retention measurements have been performed over 4 bit depth with low noise (1%) in the tungsten oxide (WO_x) readout channel. By adjusting the channel thickness from 15 to 8 nm, the on/off ratio of the FeFET can be engineered from 1 to 200% with an on-resistance ideally >100 kΩ, depending on the channel geometry. The device concept is using earth-abundant materials and is compatible with a back end of line (BEOL) integration into complementary metal-oxide-semiconductor (CMOS) processes. It has therefore a great potential for the fabrication of high-density, large-scale integrated arrays of artificial analog synapses.

Charge Transfer-Mediated Blue Luminescence in Plasmonic Ag-Cu2O Quantum Nanoheterostructures

Metal-semiconductor hybrid nanoheterostructures have the possibility to exhibit new synergic properties other than the combination of properties from discrete components due to the interaction of metal and semiconductor components at the interfaces. Here, we have synthesized Ag-Cu₂O eyeball-shaped quantum nanoheterostructures with diameter ranging between 8 and 12 nm using a single-step low-cost solvothermal process. It is observed that the presence of a minimum 3% of Ag is required for the formation of Ag-Cu₂O quantum nanoheterostructures. The formation of nanoheterostructures has introduced new synergic properties like intense blue luminescence and surface-enhanced Raman scattering due to the interactions between Ag and Cu₂O atoms at the interfaces. The significant presence of charge transfer through the interfaces is identified from the peak shift of Raman modes. The increase in the electron density at the metal surface due to the charge transfer and the recombination of these electrons with sp- or d-band holes of Ag could be the effective mechanism of the observed blue luminescence. The blue luminescence of Ag-Cu₂O quantum nanoheterostructures together with its low band gap value (≈2.3 eV) is believed to have important applications in optoelectronic devices.

Memristor with a ferroelectric HfO2 layer: in which case it is a ferroelectric tunnel junction

New interest in the implementation of ferroelectric tunnel junctions has emerged following the discovery of ferroelectric properties in HfO2 films, which are fully compatible with silicon microelectronics technology. The coercive electric field to switch polarization direction in ferroelectric HfO2 is relatively high compared to classical perovskite materials, and thus it can cause the migration of non-ferroelectric charges in HfO2, namely charged oxygen vacancies. The charge redistribution would cause the change of the tunnel barrier shape and following change of the electroresistance effect. In the case of ambiguous ferroelectric properties of HfO2 ultrathin films, this oxygen-driven resistive switching effect can mimic the tunnel electroresistance effect. Here, we demonstrate two separate resistive switching regimes, depending on the applied voltage, in the same memristor device employing a ferroelectric Hf0.5Zr0.5O2 (4.5 nm) layer. The first regime originates from the polarization reversal, whereas the second one is attributed to the accumulation/depletion of the oxygen vacancies at the electrode interface. The modulation of the tunnel barrier causes the enhancement of R OFF/R ON ratio in ∼20 times compared to the tunnel electroresistance effect. The developed device was used to formulate the criteria for unambiguous discrimination between the ferroelectric-and non-ferroelectric resistive switching effects in HfO2-based ferroelectric tunnel junctions.

Ferroelectric Tunneling Junctions Based on Aluminum Oxide/ Zirconium-Doped Hafnium Oxide for Neuromorphic Computing

Ferroelectric tunneling junctions (FTJs) with tunable tunneling electroresistance (TER) are promising for many emerging applications, including non-volatile memories and neurosynaptic computing. One of the key challenges in FTJs is the balance between the polarization value and the tunneling current. In order to achieve a sizable on-current, the thickness of the ferroelectric layer needs to be scaled down below 5 nm. However, the polarization in these ultra-thin ferroelectric layers is very small, which leads to a low tunneling electroresistance (TER) ratio. In this paper, we propose and demonstrate a new type of FTJ based on metal/Al₂O₃/Zr-doped HfO₂/Si structure. The interfacial Al₂O₃ layer and silicon substrate enable sizable TERs even when the thickness of Zr-doped HfO₂ (HZO) is above 10 nm. We found that F-N tunneling dominates at read voltages and that the polarization switching in HZO can alter the effective tunneling barrier height and tune the tunneling resistance. The FTJ synapses based on Al₂O₃/HZO stacks show symmetric potentiation/depression characteristics and widely tunable conductance. We also show that spike-timing-dependent plasticity (STDP) can be harnessed from HZO based FTJs. These novel FTJs will have high potential in non-volatile memories and neural network applications.

Ferroelectric Tunneling Junctions Based on Aluminum Oxide/ Zirconium-Doped Hafnium Oxide for Neuromorphic Computing

Ferroelectric tunneling junctions (FTJs) with tunable tunneling electroresistance (TER) are promising for many emerging applications, including non-volatile memories and neurosynaptic computing. One of the key challenges in FTJs is the balance between the polarization value and the tunneling current. In order to achieve a sizable on-current, the thickness of the ferroelectric layer needs to be scaled down below 5 nm. However, the polarization in these ultra-thin ferroelectric layers is very small, which leads to a low tunneling electroresistance (TER) ratio. In this paper, we propose and demonstrate a new type of FTJ based on metal/Al₂O₃/Zr-doped HfO₂/Si structure. The interfacial Al₂O₃ layer and silicon substrate enable sizable TERs even when the thickness of Zr-doped HfO₂ (HZO) is above 10 nm. We found that F-N tunneling dominates at read voltages and that the polarization switching in HZO can alter the effective tunneling barrier height and tune the tunneling resistance. The FTJ synapses based on Al₂O₃/HZO stacks show symmetric potentiation/depression characteristics and widely tunable conductance. We also show that spike-timing-dependent plasticity (STDP) can be harnessed from HZO based FTJs. These novel FTJs will have high potential in non-volatile memories and neural network applications.

Polarization-dependent electric potential distribution across nanoscale ferroelectric Hf0.5Zr0.5O2 in functional memory capacitors

The emergence of ferroelectricity in nanometer-thick films of doped hafnium oxide (HfO₂) makes this material a promising candidate for use in Si-compatible non-volatile memory devices. The switchable polarization of ferroelectric HfO₂ controls functional properties of these devices through the electric potential distribution across the capacitor. The experimental characterization of the local electric potential at the nanoscale has not so far been realized in practice. Here, we develop a new methodology which allows us, for the first time, to experimentally quantify the polarization-dependent potential profile across few-nanometer-thick ferroelectric Hf_0.5Zr_0.5O₂ thin films. Using a standing-wave excitation mode in synchrotron based hard X-ray photoemission spectroscopy, we depth-selectively probe TiN/Hf_0.5Zr_0.5O₂/W prototype memory capacitors and determine the local electrostatic potential by analyzing the core-level line shifts. We find that the electric potential profile across the Hf_0.5Zr_0.5O₂ layer is non-linear and changes with in situ polarization switching. Combined with our scanning transmission electron microscopy data and theoretical modeling, we interpret the observed non-linear potential behavior in terms of defects in Hf_0.5Zr_0.5O₂, at both interfaces, and their charge state modulated by the ferroelectric polarization. Our results provide an important insight into the intrinsic electronic properties of HfO₂ based ferroelectric capacitors and are essential for engineering memory devices.

Pulse-Driven Nonvolatile Perovskite Memory with Photovoltaic Read-Out Characteristics

This paper presents a unique GdFe_0.8Ni_0.2O₃ perovskite thin film for use in pulse-controlled nonvolatile memory devices (combined with a SrTiO₃ (STO) substrate) without the need for an electrical-stressing read-out process. The use of pulse voltage imposes permanent downward/upward polarization states on GFNO, which enables greater energy density and higher energy efficiency than the unpoled state for memory. The two polarization states produce carrier migrations in opposing directions across the GFNO/STO interface, which alter the depletion region of the device, as reflected in photovoltaic short-circuit current density (J_sc) values. Modulating the duration (varying the number of sequential pulses but fixing the pulse width and delay time) and direction of continuous pulse voltage is an effective method for controlling J_sc, thereby allowing the fabrication of nondestructive, light-tunable, nonvolatile memory devices. In experiments, J_sc in the downward polarized state was approximately 6 times greater than that in the upward polarized state. It is promising that more memory states can be enabled by the proposed heterostructure by selecting appropriate pulse trains. Real-time interfacial changes (relative to the nonvolatile characteristics of the device) were obtained by applying synchrotron X-ray techniques simultaneously with pulse characterization. This made it possible to separately probe the electronic and chemical states of the GFNO (a p-type-like semiconductor) and STO (an n-type-like semiconductor) while varying the pulse direction, thereby making it possible to identify the mechanisms underlying the observed phenomena.

Ferroelectric Second-Order Memristor

While the conductance of a first-order memristor is defined entirely by the external stimuli, in the second-order memristor it is governed by the both the external stimuli and its instant internal state. As a result, the dynamics of such devices allows to naturally emulate the temporal behavior of biological synapses, which encodes the spike timing information in synaptic weights. Here, we demonstrate a new type of second-order memristor functionality in the ferroelectric HfO₂-based tunnel junction on silicon. The continuous change of conductance in the p⁺-Si/Hf_0.5Zr_0.5O₂/TiN tunnel junction is achieved via the gradual switching of polarization in ferroelectric domains of polycrystalline Hf_0.5Zr_0.5O₂ layer, whereas the combined dynamics of the built-in electric field and charge trapping/detrapping at the defect states at the bottom Si interface defines the temporal behavior of the memristor device, similar to synapses in biological systems. The implemented ferroelectric second-order memristor exhibits various synaptic functionalities, such as paired-pulse potentiation/depression and spike-rate-dependent plasticity, and can serve as a building block for the development of neuromorphic computing architectures.

Flexible Memristors Based on Single-Crystalline Ferroelectric Tunnel Junctions

Ferroelectric tunnel junction (FTJ) based memristors exhibiting continuous electric field controllable resistance states have been considered promising candidates for future high-density memories and advanced neuromorphic computational architectures. However, the use of rigid single crystal substrate and high temperature growth of the epitaxial FTJ thin films constitutes the main obstacles to using this kind of heterostructure in flexible computing devices. Here, we report the integration of centimeter-scale single crystalline FTJs on flexible plastic substrates, by water-etching based epitaxial oxide membrane lift-off and the following transfer. The resulting highly flexible FTJ membranes retain the single-crystalline structure along with stable and switchable ferroelectric polarization as the grown-on single crystal substrate state. We show that the obtained flexible memristors, i.e., FTJs on plastic substrates, present high speed and low voltage mediated memristive behaviors with resistance changes over 500% and are stable against shape change. This work is an essential step toward the realization of epitaxial ultrathin ferroelectric oxide film-based electronics on large-area, flexible, and affordable substrates.

Freestanding Oxide Ferroelectric Tunnel Junction Memories Transferred onto Silicon

Crystalline oxide ferroelectric tunnel junctions enable persistent encoding of information in electric polarization, featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories. However, the well-established fabrication of epitaxial devices on oxide substrates is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics. In this work, we report ferroelectric tunnel junctions based on 2.8 nm-thick BaTiO₃ films grown epitaxially on SrTiO₃ growth substrates, released, and relaminated onto silicon. The performance of the transferred devices is comparable to devices characterized on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories broadly integrable with different materials platforms.

Epitaxial Integration on Si(001) of Ferroelectric Hf0.5Zr0.5O2 Capacitors with High Retention and Endurance

Epitaxial ferroelectric Hf_0.5Zr_0.5O₂ films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf_0.5Zr_0.5O₂ phase, [111] out-of-plane oriented, is stabilized in the films. The films present high remnant polarization P_r close to 20 μC/cm², rivaling with equivalent epitaxial films on single crystalline oxide substrates. Retention time is longer than 10 years for a writing field of around 5 MV/cm, and the capacitors show endurance up to 10⁹ cycles for a writing voltage of around 4 MV/cm. It is found that the formation of the orthorhombic ferroelectric phase depends critically on the bottom electrode, being achieved on La_0.67Sr_0.33MnO₃ but not on LaNiO₃. The demonstration of excellent ferroelectric properties in epitaxial films of Hf_0.5Zr_0.5O₂ on Si(001) is relevant toward fabrication of devices that require homogeneity in the nanometer scale, as well as for better understanding of the intrinsic properties of this promising ferroelectric oxide.

Nonvolatile Memory Materials for Neuromorphic Intelligent Machines

Recent progress in deep learning extends the capability of artificial intelligence to various practical tasks, making the deep neural network (DNN) an extremely versatile hypothesis. While such DNN is virtually built on contemporary data centers of the von Neumann architecture, physical (in part) DNN of non-von Neumann architecture, also known as neuromorphic computing, can remarkably improve learning and inference efficiency. Particularly, resistance-based nonvolatile random access memory (NVRAM) highlights its handy and efficient application to the multiply-accumulate (MAC) operation in an analog manner. Here, an overview is given of the available types of resistance-based NVRAMs and their technological maturity from the material- and device-points of view. Examples within the strategy are subsequently addressed in comparison with their benchmarks (virtual DNN in deep learning). A spiking neural network (SNN) is another type of neural network that is more biologically plausible than the DNN. The successful incorporation of resistance-based NVRAM in SNN-based neuromorphic computing offers an efficient solution to the MAC operation and spike timing-based learning in nature. This strategy is exemplified from a material perspective. Intelligent machines are categorized according to their architecture and learning type. Also, the functionality and usefulness of NVRAM-based neuromorphic computing are addressed.

The effect of the bottom electrode on ferroelectric tunnel junctions based on CMOS-compatible HfO2

Ferroelectric tunnel junctions (FTJs) have attracted research interest as promising candidates for non-destructive readout non-volatile memories. Unlike conventional perovskite FTJs, hafnia FTJs offer many advantages in terms of scalability and CMOS compatibility. However, so far, hafnia FTJs have shown poor endurance and relatively low resistance ratios and these have remained issues for real device applications. In our study, we fabricated HfZrO(HZO)-based FTJs with various electrodes (TiN, Si, SiGe, Ge) and improved the memory performance of HZO-based FTJs by using the asymmetry of the charge screening lengths of the electrodes. For the HZO-based FTJ with a Ge substrate, the effective barrier afforded by this FTJ can be electrically modulated because of the space charge-limited region formed at the ferroelectric/semiconductor interface. The optimized HZO-based FTJ with a Ge bottom electrode presents excellent ferroelectricity with a high remnant polarization of 18 μC cm^-2, high tunneling electroresistance value of 30, good retention at 85 °C and high endurance of 10⁷. The results demonstrate the great potential of HfO₂-based FTJs in non-destructive readout non-volatile memories.

Effect of Polarization Reversal in Ferroelectric TiN/Hf0.5Zr0.5O2/TiN Devices on Electronic Conditions at Interfaces Studied in Operando by Hard X-ray Photoemission Spectroscopy

Because of their compatibility with modern Si-based technology, HfO₂-based ferroelectric films have recently attracted attention as strong candidates for applications in memory devices, in particular, ferroelectric field-effect transistors or ferroelectric tunnel junctions. A key property defining the functionality of these devices is the polarization dependent change of the electronic band alignment at the metal/ferroelectric interface. Here, we report on the effect of polarization reversal in functional ferroelectric TiN/Hf_0.5Zr_0.5O₂/TiN capacitors on the potential distribution across the stack and the electronic band line-up at the interfaces studied in operando by hard X-ray photoemission spectroscopy. By tracking changes in the position of Hf_0.5Zr_0.5O₂ core-level lines with respect to those of the TiN electrode in both short- and open-circuit configurations following in situ polarization reversal, we derive the conduction band offset to be 0.7 (1.0) eV at the top and 1.7 (1.0) eV at the bottom interfaces for polarization, pointing up (down), respectively. Energy dispersive X-ray spectroscopy profiling of the sample cross-section in combination with the laboratory X-ray photoelectron spectroscopy reveal the presence of a TiO_x/TiON layer at  both interfaces. The observed asymmetry in the band line-up changes in the TiN/Hf_0.5Zr_0.5O₂/TiN memory stack is explained by different origin of these oxidized layers and effective pinning of polarization at the top interface. The described methodology and first experimental results are useful for the optimization of HfO₂-based ferroelectric memory devices under development.