Nonvolatile Ferroelectric Random Access Memory Research Articles

Roadmap for ferroelectric domain wall memory

Commercial nonvolatile Ferroelectric Random Access Memory employs a destructive readout scheme based on charge sensing, which limits its cell scalability in sizes above 100 nm. Ferroelectric domain walls are two-dimensional topological interfaces with thicknesses approaching the unit cell level between two antiparallel domains and exhibit electrical conductivity, distinguishing them from insulating matrices that are uniformly ordered. Recently, novel research has been devoted to utilizing this extraordinary interface for the application in nonvolatile memory with nanometer-sized scalability and low energy consumption. Here, we pay more attention to the development of the domain wall memory technologies in the future with challenges and opportunities to design planar and vertical arrays of the memory cells in the CMOS platform.

Effect of ZnSnO3 on dielectric and ferroelectric properties of Sr2Bi4Ti5O18 ceramics

AbstractSr2Bi4Ti5O18 is one of the promising candidates for ferroelectric nonvolatile random access memory device applications. Albeit, this ceramic material is not suitable for certain ferroelectric device applications because of the merits and demerits associated with this material. Solid‐solution or composite ceramics in bulk configuration is the better choice to overcome the drawbacks associated with the individual compounds. Concurrently, solid‐solution of functional electro‐ceramics comprising Aurivillius family‐based lead‐free bismuth‐layered structured ceramics (1 − x) Sr2Bi4Ti5O18 (SBT) and x mol% of ilmenite structured ZnSnO3 (ZS) (where x = 0, 5, 10, and 20 mol%) were synthesized via conventional solid‐state reaction route to investigate the effect of ZS on the physical properties of SBT. X‐ray diffraction diffractograms revealed that a single phase was formed maximum up to 5 mol% of ZS in SBT and scanning electron microscopy studies suggested platelike microstructure of the samples. Intriguingly, positive‐up and negative‐down results indicated that the true switchable polarization for the 10 mol% ZS in SBT (10.87 μC/cm2), 33% higher than that of pure SBT ceramics in addition to the lower leakage current density and enhanced dielectric response in the SBT–ZS samples. The observed enhancement in electrical characteristics of SBT–ZS composite ceramics compared to pure SBT highlights the decisive role played by the ZS in lowering the thermodynamic barrier resulting in ease of polarization switching and mitigating the leakage current of SBT ceramics.

Fast Operations of Nonvolatile Ferroelectric Domain Wall Memory with Inhibited Space Charge Injection

The microampere-level domain wall currents in LiNbO3 single crystals have promising applications in nonvolatile ferroelectric domain wall random access memory and logic with high-density integration, ultrafast operation speeds, and almost unlimited switching cycles. For the memory commercialization, the improvements of the reliability and operation speed of the devices are challenging due to the high-field charge injection. The injected charge could compensate the domain-wall boundary charge that screens the domain switching field and reduces the domain wall current. In this work, two kinds of memory nanocells were fabricated on the surfaces of X-cut LiNbO3 single crystals to study the geometry-dependent charge injection. The striped memory cell due to the appearance of the size-driven reconstruction has a smaller coercive field than that of a clamped memory cell without relaxation of the lattice matching stress, which reduces low-frequency charge injection and increases the domain switching speed. At an operating voltage of 5 V, we observed a retention time of more than 1 week and an on/off current ratio of 2 × 104 for a striped-like cell, paving the route to integrate energy-efficient high-density domain wall memory in high reliability.

A facile synthesis of Al-doped BaTiO3 thin films by a hydrothermal-galvanic couple method on TiAlN film electrodes

Al-doped barium titanate (ABT) thin films were produced by a facile hydrothermal-galvanic couple (HT-GC) method using ternary TiAlN seeding layers. TiAlN thin films serving as the working electrode were deposited onto Si substrates by reactive unbalanced magnetron sputtering. HT-GC synthesis was conducted in the alkaline solutions consisting of 0.5 M Ba(CH3COOH)2 and 2 M NaOH. Although no external power sources were applied, substantial galvanic currents occurred during the synthesis. X-ray diffraction patterns show that cubic ABT films were formed over TiAlN. Field-emission scanning electronic microscopy revealed ABT films exhibiting hemispherical grains over the nitride surface with single-layered structures. X-ray photoelectron spectroscopy verified Al doping in the obtained ABT coatings. Reaction paths and formation mechanisms of the ABT coatings were also proposed. Moreover, the ABT thin films possessed higher residual polarization (Pr) and piezoelectricity coefficient (d33) compared with un-doped BaTiO3 (BTO), deduced from polarization–electric field loops results. The deduced Pr-value for BTO thin films was 0.8 μC/cm2 and could be enhanced to 1.6 μC/cm2 for ABT thin films, meanwhile the value of d33 for BTO films was 13 ± 1 pC/N and increased to 18 ± 1 pC/N for ABT films. Such a facile synthesis of ABT thin films may bring in more technological applications such as piezoelectric and triboelectric materials in the hybrid nanogenerator, and non-volatile ferroelectric random access memory.

Atomic-scale polar vortices in Na0.5Bi0.5TiO3 grains

Ferroelectric materials have great potential applications in non-volatile ferroelectric random access memories because of the singularity of one-dimensional topological defects. In this paper, we have found for the first time the atomic-level dipole rotations of vortex structures in crystalline materials. By the aberration-corrected high resolution scanning transmission electron microscopy (STEM), the atomic-scale multiple rotation domains and polar vortices in Na0.5Bi0.5TiO3 (NBT) grains have been clearly observed in lead-free perovskite NBT. Meanwhile, nanobeam electron diffraction experiments provided the strong evidences of the coexistence of tetragonal, orthorhombic and rhombohedra phases at room temperature. These polar vortices are constructed by the competition of these atomic-scale polar domains, which are originated from the multiphase coexistence of NBT and can be well confirmed by our theoretical calculations. These observations have put forward new implications for the creation of vortex topologies in crystalline materials without sophisticated thin-film technologies, and it can also help to understand the complicated polar mechanism in lead-free perovskite ferroelectric materials.

BEOL Integrated Ferroelectric HfO₂-Based Capacitors for FeRAM: Extrapolation of Reliability Performance to Use Conditions

Si doped HfO2 based ferroelectric capacitors integrated into Back-End-Of-Line (BEOL) 130 nm CMOS technology were investigated in regard to critical reliability parameters for their implementation in non-volatile one-transistor one-capacitor ferroelectric random-access memory applications. The assessed reliability parameters are electric field, capacitor area, and temperature and are evaluated on single and parallel structured capacitors to understand their respective impact on wake-up, fatigue, imprint, and retention.

Analytical properties of switching current transients in ferroelectrics

The increasing application of ferroelectrics in technological applications such as nonvolatile ferroelectric random access memories, capacitors, and energy harvesters highlights the importance of investigating how the characteristics of switching current transients change in response to variation of factors that affect it. This work presents a completely analytical approach based on the Landau-Devonshire (LD) model of phase transitions to quickly study the changes to the properties of switching current transients and back-switching currents in ferroelectric material. The non-linear nature of polarization decay is analyzed based on an exact solution of the Landau-Khalatnikov equation. Exact expressions of the temporal changes in polarization and back-switching current that incorporate the cubic polarization are derived. Comparison of differences between the exact non-linear model of polarization decay phenomena from the linearized model of polarization decay are discussed. This work shows that the characteristics of the critical switching time phenomenon in partial switching current experiments can be explained by the analytical LD model. Many features of complete and partial switching current transients produced by bipolar electric pulses are successfully modeled analytically by varying factors such as temperature, electric field amplitude, pulse-width and off-time. This work shows that pulse powered switching current behaviour in ferroelectric material can be modeled analytically instead of relying only on numerical approaches to the Landau-Khalatnikov equation.

Investigating Effect of Strain and Geometric Defects on Single Polarization Vortex in PbTiO3 Nanowires

The single polarization vortex structure in nanowire can be used to store binary data in Non-Volatile Ferroelectric Random Access Memories (NVFRAM or FRAM). However, at the nanoscale, mechanical strains or geometry defects (cracks) can affect the polarization vortex and they are one of the reasons to reduce the service life as well as the reliability of the device. In this study, the atomic simulation method using the interactive potential function based on the core-shell model is selected to investigate the effects of strain, cracks and domain wall deviations (DW) on the single polarization vortex in PbTiO3 (PTO) nanowires. The results obtained showed that the polarization vortex can appear or disappear depending on the position and size of the crack. Deviations in the DW position make the polarization vortex change the size and shape. Besides, the magnitude of the vortex investigated increases under tension strain and decreases under compression strain. Especially, in large compression strain (10%), the vortex can be disappeared.

Interfacial Regulation of Dielectric Properties in Ferroelectric Hf0.5Zr0.5O2 Thin Films

The discovery of ferroelectricity in hafnium zirconium oxide (HZO) thin films has attracted wide attention from academia to industry due to the application in ferroelectric non-volatile random access memories (FeRAM) with prominent performance in scalability and CMOS process compatibility. Dielectric behavior of ferroelectric HZO thin films is a key factor affecting the dynamic effect, piezoelectric and electrostrictive effect. Interface between HZO and capping electrodes plays an important role in regulating the dielectric properties. In this paper, the impedance frequency response and dielectric spectrum of ferroelectric HZO thin films were analyzed. Parameters of the interface were extracted to analyze the regulating effect on the dielectric properties based on an impedance model with constant phase element (CPE). Besides, dielectric spectrums at elevated temperatures were identified to verify this analysis.

High temperature ferroelectric domain wall memory

For automotive and space applications, the nonvolatile ferroelectric domain wall random access memory (DWRAM) is required to work at a wide temperature range of −40 °C to +150 °C. However, the wall current generally decays with the retention time at high temperatures with the uncertain mechanism. Here we observed the peaked domain wall (DW) currents to decay with time at 450 K in LiNbO3 single crystals, which was correlated with the charge injection into domain wall regions in the mobility of 6.4 × 10−9 cm2/V·s. The extrinsic charge injection suppresses the intrinsic diode-like DW current due to the local Li/O vacancy redistribution to compensate the domain boundary charge. But the injected charge can be removed at an opposite poling voltage and can be frozen upon cooling down to 300 K. The traces of frozen wall regions become conducting upon the wall erasure. Once the memory write and read times are far shorter than the charge injection time, the DWRAM shows good retention of written information with a large on/off current ratio of ~104 at 450 K, suitable to the applications in some extreme conditions.

Origin of the retention loss in ferroelectric Hf0.5Zr0.5O2-based memory devices

For the decade, ferroelectric hafnium oxide films are attracting the interest as a promising functional material for nonvolatile ferroelectric random access memory due to full scalability and complementary metal-oxide-semiconductor integratability. Despite the significant progress in key performance parameters, particularly, the readout charge and voltage as well as the endurance, the developed devices can only be implemented by the electronics industry if they exhibit a standard retention time of 10 years. Material engineering modifies not only target ferroelectric properties, but also the retention time. To understand how to maintain the sufficient retention, the physical mechanism behind it should be clarified. For this purpose, we have fabricated the capacitor memory cell with a high rate of retention loss. Comparing the device performance with the results of capacitance transient spectroscopy, operando hard X-ray photoelectron spectroscopy and in situ piezoresponse force microscopy, we have concluded that the retention loss is caused by the accumulation of the positively charged oxygen vacancies at the interfaces with capacitor electrodes. The redistribution of charges during long-term storage of information is fully defined by the domain structure in memory cell.

Robust in-plane polarization switching in epitaxial BiFeO3 films

The enrichment domain state and its stability consist an imperative part regarding to the high density non-volatile ferroelectric random access memory (FRAM) devices. In this work, epitaxial BiFeO3 (BFO) thin films were prepared by pulsed-leaser deposition (PLD) technology, and the morphology, local polarization state and piezoelectric response were characterized by scanning probe microscopy (SPM) and piezoresponse force microscopy (PFM). The robust in-plane domain dynamic process is observed under the external electric fields. Furthermore, good retention and repeatability are observed especially under high temperature. The new-developed domain patterns are nearly stable, which is quite different from the existed research. The formation of these domains is probably induced by the combination of built-in electric fields and distribution of tip fields, both of which affect the polarization dynamic process. All the obtained results pave a great way for application corresponding to the FRAM devices especially under high temperature circumstance.

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Enhanced ferroelectric polarization in epitaxial superconducting–ferroelectric heterostructure for non-volatile memory cell

We report the growth and polarization switching properties of epitaxial ferroelectric–superconducting heterostructure PbZr0.52Ti0.48O3 (PZT) (100 nm)/YBa2Cu3O7−δ (YBCO) (100 nm) thin films for non-volatile ferroelectric random access memory elements. The epitaxial nature of the heterostructure is verified using the reciprocal space mapping data with the superconducting phase transition temperature (Tc) of nearly 25 K far below the Tc of as-grown YBCO under the same condition. The significant remanent polarization (Pr) ∼ 45 µC/cm2 at 1 kHz can switch from one state to another using 1 μs pulse. The devices meet the basic criteria of memory elements, such as high resistance ∼10 GΩ at 8 V, a butterfly-like capacitance–voltage (C/V) loop, significant polarization, a sharp change in the displacement current, long-time charge retention, and small fatigue at room temperature.

Dielectric properties of Na and Pr doped SrBi4Ti4O15 ceramics

Bismuth Layer-Structured Ferroelectrics materials (BLSFs) are widely used for many applications like resonators, filters, sensors, actuators and are appropriate for non-volatile Ferroelectric Random Access Memory (FeRAM) applications. The additional advantage of BLSF materials is that in electrical applications in they work at high temperatures. In the case of resonator applications, piezoelectric components are used as inductors. Usually BLSF materials typically have higher Curie temperature than PZT materials, so instead of PZT materials, BLSFs are used for high temperature piezoelectric sensor applications. SrBi4Ti4O15 (SBT) compound is a member of Aurivillius family Bismuth layered structured Ferroelectrics. The dielectric properties of Sodium and Praseodymium doped SBT, Sr0.2Na0.4Pr0.4Bi4Ti4O15 (SNPBT), prepared by solid state double sintering method using planetary ball mill, and is studied in the present work. Single phase is confirmed from XRD and microstructure is studied from SEM. The temperature dependence of dielectric constant and dielectric loss are studied at different frequencies. The Curie temperature increased by doping SBT with Na1+and Pr3+. Thus this piezoelectric ceramic is suitable for high temperature sensor applications.

Temperature dependence of ferroelectricity and domain switching behavior in Pb(Zr0·3Ti0.7)O3 ferroelectric thin films

Abstract Owing to their high remanent polarization, fast switching behavior, and controllable preparation process, Pb(Zr0·3Ti0.7)O3 (PZT) thin films are considered to be one of the most promising materials for non-volatile ferroelectric random access memory (FRAM). In this work, high-quality PZT thin films with a preferred (111) orientation and smooth surface are obtained by the sol–gel method. The temperature dependence of the ferroelectricity and domain switching behavior is thoroughly investigated by piezoresponse force microscopy (PFM). The domain structure undergoes distinct 180° switching and reconstruction under an external DC voltage. Polarization hysteresis loop analysis of PZT reveals that the spontaneous polarization (Ps), remanent polarization (Pr), and coercive voltage (Vc) decrease as the temperature increases from 20 to 70 °C, in agreement with the Landau classical theory of phase transition. In addition, the PFM results show that the PZT films consistently exhibit good switching behavior as the temperature increases from 23.7 to 70 °C. An increase in the piezoelectric constant (Dmax) with increasing temperature is ascribed to more rapid domain wall movement and easier domain reorientation, as predicted by thermodynamic theory. This investigation suggests that the temperature strongly affects the ferroelectricity and switching behavior of PZT thin films and offers guidance for the design of FRAM.

Characteristics of MoS2 monolayer non-volatile memory field effect transistors affected by the ferroelectric properties of BiFeO3 thin films with Pt and SrRuO3 bottom electrodes grown on glass substrates

We investigated the characteristics of ferroelectric non-volatile random-access memory field effect transistors (MoS2-BFO NVRAM FETs) consisting of monolayer MoS2 channels, BiFeO3 (BFO) gate layers, and Pt and SrRuO3 bottom electrodes on glass substrates. As a result of the comparative experiments of the Pt and SrRuO3 bottom electrodes, it was confirmed that the BFO gate layer using the SrRuO3 bottom electrode showed a higher remanent polarization, faster switching time, and superior fatigue endurance than the BFO gate layer using the Pt bottom electrode. It is suggested that the good ferroelectric properties of the BFO gate dielectric layer on the SrRuO3 bottom electrode is resulting from its (001)-oriented growth. The MoS2 BFO NVRAM FET with a SrRuO3 bottom electrode exhibited a wider memory window than the Pt bottom electrode because the SrRuO3 bottom electrode had a higher remanent polarization of the BFO gate layer compared to the Pt bottom electrode.

Nonvolatile ferroelectric memory based on PbTiO3 gated single-layer MoS2 field-effect transistor

We fabricated ferroelectric non-volatile random access memory (FeRAM) based on a field effect transistor (FET) consisting of a monolayer MoS2 channel and a ferroelectric PbTiO3 (PTO) thin film of gate insulator. An epitaxial PTO thin film was deposited on a Nb-doped SrTiO3 (Nb:STO) substrate via pulsed laser deposition. A monolayer MoS2 sheet was exfoliated from a bulk crystal and transferred to the surface of the PTO/Nb:STO. Structural and surface properties of the PTO thin film were characterized by X-ray diffraction and atomic force microscopy, respectively. Raman spectroscopy analysis was performed to identify the single-layer MoS2 sheet on the PTO/Nb:STO. We obtained mobility value (327 cm2/V·s) of the MoS2 channel at room temperature. The MoS2-PTO FeRAM FET showed a wide memory window with 17 kΩ of resistance variation which was attributed to high remnant polarization of the epitaxially grown PTO thin film. According to the fatigue resistance test for the FeRAM FET, however, the resistance states gradually varied during the switching cycles of 109.

Investigation of the features of integrating nonvolatile FRAM elements with CMOS technology

The results of the studies of the features of integrating the elements of nonvolatile ferroelectric random access memory (FRAM) with CMOS technology are presented. The possibility and prospects of using their nanosized layers in the elements of nonvolatile FRAM are demonstrated.

Improved Polarization Retention of BiFeO3 Thin Films Using GdScO3 (110) Substrates**Supported by the National Basic Research Program of China under Grant No 2014CB921004, and the National Natural Science Foundation of China under Grant No 61225020.

Epitaxial ferroelectric thin films on single-crystal substrates generally show a preferred domain orientation in one direction over the other in demonstration of a poor polarization retention. This behavior will affect their application in nonvolatile ferroelectric random access memories where bipolar polarization states are used to store the logic 0 and 1 data. Here the retention characteristics of BiFeO3 thin films with SrRuO3 bottom electrodes on both GdScO3 (110) and SrTiO3 (100) substrates are studied and compared, and the results of piezoresponse force microscopy provide a long time retention property of the films on two substrates. It is found that bismuth ferrite thin films grown on GdScO3 substrates show no preferred domain variants in comparison with the preferred downward polarization orientation toward bottom electrodes on SrTiO3 substrates. The retention test from a positive-up domain to a negative-down domain using a signal generator and an oscilloscope coincidentally shows bistable polarization states on the GdScO3 substrate over a measuring time of 500 s, unlike the preferred domain orientation on SrTiO3, where more than 65% of upward domains disappear after 1 s. In addition, different sizes of domains have been written and read by using the scanning tip of piezoresponse force microscopy, where the polarization can stabilize over one month. This study paves one route to improve the polarization retention property through the optimization of the lattice-mismatched stresses between films and substrates.