Ferroelectric Memory Devices Research Articles

A computational study of hafnia-based ferroelectric memories: from ab initio via physical modeling to circuit models of ferroelectric device

The discovery of ferroelectric properties of binary oxides revitalized the interest in ferroelectrics and bridged the scaling gap between the state-of-the-art semiconductor technology and ferroelectric memories. However, before hitting the markets, the origin of ferroelectricity and in-depth studies of device characteristics are needed. Establishing a correlation between the performance of the device and underlying physical mechanisms is the first step toward understanding the device and engineering guidelines for a novel, superior device. Therefore, in this paper a holistic modeling approaches which lead to a better understanding of ferroelectric memories based on hafnium and zirconium oxide is addressed. Starting from describing the stabilization of the ferroelectric phase within the binary oxides via physical modeling the physical mechanisms of the ferroelectric devices are reviewed. Besides, limitations and modeling of the multilevel operation and switching kinetics of ultimately scaled devices as well as the necessity for Landau-Khalatnikov approach are discussed. Furthermore, a device-level model of ferroelectric memory devices that can be used to study the array implementation and their operational schemes are addressed. Finally, a circuit model of the ferroelectric memory device is presented and potential further applications of ferroelectric devices are outlined.

Ferroelectric transistors with monolayer molybdenum disulfide and ultra-thin aluminum-doped hafnium oxide

In this letter, we demonstrate ferroelectric memory devices with monolayer molybdenum disulfide (MoS2) as the channel material and aluminum (Al)-doped hafnium oxide (HfO2) as the ferroelectric gate dielectric. Metal-ferroelectric-metal capacitors with 16 nm thick Al-doped HfO2 are fabricated, and a remnant polarization of 3 μC/cm2 under a program/erase voltage of 5 V is observed. The capability of potential 10 years data retention was estimated using extrapolation of the experimental data. Ferroelectric transistors based on embedded ferroelectric HfO2 and MoS2 grown by chemical vapor deposition are fabricated. Clockwise hysteresis is observed at low program/erase voltages due to slow bulk traps located near the 2D/dielectric interface, while counterclockwise hysteresis is observed at high program/erase voltages due to ferroelectric polarization. In addition, the endurances of the devices are tested, and the effects associated with ferroelectric materials, such as the wake-up effect and polarization fatigue, are observed. Reliable writing/reading in MoS2/Al-doped HfO2 ferroelectric transistors over 2 × 104 cycles is achieved. This research can potentially lead to advances of two-dimensional (2D) materials in low-power logic and memory applications.

Nonvolatile ferroelectric domain wall memory.

Ferroelectric domain walls are atomically sharp topological defects that separate regions of uniform polarization. The discovery of electrical conductivity in specific types of walls gave rise to "domain wall nanoelectronics," a technology in which the wall (rather than the domain) stores information. This paradigm shift critically hinges on precise nanoengineering of reconfigurable domain walls. Using specially designed nanofabricated electrodes and scanning probe techniques, we demonstrate a prototype nonvolatile ferroelectric domain wall memory, scalable to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read out nondestructively at moderate voltages (<3 V), exhibits relatively high OFF-ON ratios (~103) with excellent endurance and retention characteristics, and has multilevel data storage capacity. Our work thus constitutes an important step toward integrated nanoscale ferroelectric domain wall memory devices.

Synthesis and characterization of novel Cu2O/PVDF nanocomposites for flexible ferroelectric organic electronic memory devices

Copper oxide nanoparticles loaded Poly(vinylidene fluoride) (PVDF) nanocomposites, at concentrations from 1 to 9 wt%, were developed by casting technique. The effect of the Cu2O nanoparticles on the structure, thermal, mechanical and dielectric properties of the PVDF were inspected. The scanning electron microscopy (FE-SEM) revealed the well dispersion of the Cu2O nanoparticles into the PVDF matrix up to 5 wt%. The X-ray diffraction (XRD) patterns and the infrared spectroscopy (FTIR) measurements implied that the inclusion of the Cu2O nanoparticles into the PVDF matrix leads to a transform of the non-polar semi-crystalline α-phase of the neat PVDF to highly crystallized polar β-phase. Moreover, an increase of the thermal stability and crystallinity of the PVDF after the incorporation of the Cu2O nanoparticles was achieved. The Cu2O/PVDF nanocomposites exhibited excellent cyclic mechanical property compared to the neat PVDF. The introduction of 5 wt% Cu2O nanoparticles into the PVDF composites resulted in remarkable increase of the dielectric constant by 13 times while the dielectric loss was very low. A device based on Al/5 wt% Cu2O doped PVDF/Pt structure is developed. This device shows ferroelectric hysteresis with butterfly like shape and exhibited remanent polarization of 11.2 μC/cm2 with coercive field of 49 MV/m. These superior properties enable the developed Cu2O/PVDF nanocomposite films to be a potential candidate in the field of ferroelectric organic electronic memory devices.

Lattice dynamics and ferroelectric properties of the nitride perovskite LaWN3

Using first-principles calculations we examine the crystal structures and phase transitions of nitride perovskite ${\mathrm{LaWN}}_{3}$. Lattice dynamics calculations indicate that the ground-state structure belongs to space group $R3c$. Two competitive phase transition pathways are identified which are characterized by symmetry-adapted distortion modes. The results suggest that $R3c\phantom{\rule{4pt}{0ex}}{\mathrm{LaWN}}_{3}$ should be an excellent ferroelectric semiconductor, as its large spontaneous polarization of around 61 $\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$ is comparable to that of ${\mathrm{PbTiO}}_{3}$, and its band gap is about 1.72 eV. Ferroelectricity is found to result from the $B$-site instability driven by hybridization between $\mathrm{W}\ensuremath{-}5d$ and $\mathrm{N}\ensuremath{-}2p$ orbitals. These properties make ${\mathrm{LaWN}}_{3}$ an attractive candidate material for use in ferroelectric memory devices and photovoltaic cells.

Switching Kinetics in Nanoscale Hafnium Oxide Based Ferroelectric Field-Effect Transistors.

The recent discovery of ferroelectricity in thin hafnium oxide films has led to a resurgence of interest in ferroelectric memory devices. Although both experimental and theoretical studies on this new ferroelectric system have been undertaken, much remains to be unveiled regarding its domain landscape and switching kinetics. Here we demonstrate that the switching of single domains can be directly observed in ultrascaled ferroelectric field effect transistors. Using models of ferroelectric domain nucleation we explain the time, field and temperature dependence of polarization reversal. A simple stochastic model is proposed as well, relating nucleation processes to the observed statistical switching behavior. Our results suggest novel opportunities for hafnium oxide based ferroelectrics in nonvolatile memory devices.

Resistance of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 under high voltage microsecond pulse induced breakdown

Ferroelectric ceramics have been widely used in lots of fields, such as mechanical-electric transducer, ferroelectric memory, and energy storage devices. The dielectric breakdown process of ferroelectric ceramic has received much attention for years, due to the fact that this issue is critical in many electrical applications. Though great efforts have been made, the mechanism of dielectric breakdown is still under debate. The reason is that the electrical breakdown is a complex process related to electrical, thermal, and light effects. In the present work, we investigate the breakdown process of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3(PZT95/5) ceramic, which is a kind of typical ferroelectric ceramic working in the high voltage environments. The high voltage pulse generator is used in the breakdown experiments to apply a square pulsed voltage with an amplitude of 10 kV and a width of 7 s. The resistivity change in the breakdown process is recorded by the high-frequency oscillograph in nano-second. The results show that there are two different breakdown types for our sample, i.e. body-breakdown and flashover. To better understand the breakdown mechanism of the PZT95/5 ceramic, the formation of the conductive channel in ceramic in the process is investigated by comparing the resistivity development in body-breakdown and flashover processes. The development of the conductive channel formation can be divided into three steps in body-breakdown. In the first step that lasts for the first 40 ns of breakdown, the conductive channel starts forming, with the equivalent resistance sharply decreasing to about 105 in the mean time. Then, i.e. in the second step, conductive path grows into a stable one with the equivalent resistance decreasing to the magneitude of about 102 . The resistance decreases slowly to about 130 in the third step, which means that the conductive channel is completely formed. The channel formation of flashover can also be divided into three steps. The first step is similar to that of body-breakdown, with the equivalent resistance decreasing to about 105 in about 40 ns. In the second step of flashover, the conductive path keeps growing into a stable one with the equivalent resistance decreasing to 102 , but with a different resistance changing rate from that in body-breakdown, and the resistance decreases slowly to about 20 in the end. Different behavior between the body-breakdown and the surface flashover can be explained by different carrier densities on the conductive paths in the two breakdown processes. In the body-breakdown, the carrier density in the conductive channel is higher than that in the surface flashover, which improves the electron transfer and reduces the resistance. This may explain the reason why the channel formation in body-breakdown is faster than in flashover. This study is helpful for further materials design and applications.

Low field depoling phenomena in soft lead zirconate titanate ferroelectrics

Reduction in polarisation of ferroelectric materials due to repeated electrical cycling is a major problem in ferroelectric non-volatile memory devices. There is a large amount of data addressing this issue at high electric field strengths under bipolar loading conditions, however the effect of field cycling at low electric field strengths (< Ec) has not been fully investigated. This paper addresses the effects of repeated cycling of soft lead zirconate titanate using electrical pulses at fields well below the coercive field strength of the material. It is shown that this mode of loading diminishes the macroscopic polarisation and mechanical response of the material. The origins of this behaviour are found to be a statistical non-non-reversible switching processes that does not result in classical fatigue related mechanical microstructural damage or defect agglomeration and domain pinning. Instead the process is fully recoverable and attributed to local changes in switching energy and clustering of switched ferroelectric cells.

Improved fatigue property of hafnium substitute lead zirconate titanate deposited by pulse laser deposition

Lead hafnate titanate (PHT) thin film was deposited by pulse laser deposition, characterized for the usage as a ferroelectric capacitor in view of Ferroelectric Random Access Memory (FeRAM) devices, and compared with a lead zirconate titanate (PZT) reference, which was fabricated in the same conditions. Results indicate that the 126 nm-thickness PHT thin film with a low temperature self-buffer layer has a single (111)-orientation. The following electric measurements were performed to investigate the possibility of this thin film to use in the FeRAM applications by contrast to a PZT reference. This PHT sample demonstrates a good ferroelectric property with the remanent polarization of 51 μC/cm2. The remanent polarization with no obvious degradation has been observed after 2 × 109 fatigue reversals, which indicate that the PHT thin film a promising candidate in view of FeRAM applications.

Comprehension of the Electric Polarization as a Function of Low Temperature

Polarization response to warming plays an increasingly important role in a number of ferroelectric memory devices. This paper reports on the theoretical explanation of the relationship between polarization and temperature. According to the Fermi–Dirac distribution, the basic property of electric polarization response to temperature in magnetoelectric multiferroic materials is theoretically analyzed. The polarization in magnetoelectric multiferroic materials can be calculated by low temperature using a phenomenological theory suggested in this paper. Simulation results revealed that the numerically calculated results are in good agreement with experimental results of some inhomogeneous multiferroic materials. Numerical simulations have been performed to investigate the influences of both electric and magnetic fields on the polarization in magnetoelectric multiferroic materials. Furthermore, polarization behavior of magnetoelectric multiferroic materials can be predicted by low temperature, electric field and magnetic induction using only one function. The calculations offer an insight into the understanding of the effects of heating and magnetoelectric field on electrical properties of multiferroic materials and offer a potential to use similar methods to analyze electrical properties of other memory devices.

Conformal BaTiO3 Films with High Piezoelectric Coupling through an Optimized Hydrothermal Synthesis.

Two-dimensional (2D) ferroelectric films have vast applications due to their dielectric, ferroelectric, and piezoelectric properties that meet the requirements of sensors, nonvolatile ferroelectric random access memory (NVFeRAM) devices, and micro-electromechanical systems (MEMS). However, the small surface area of these 2D ferroelectric films has limited their ability to achieve higher memory storage density in NVFeRAM devices and more sensitive sensors and transducer. Thus, conformally deposited ferroelectric films have been actively studied for these applications in order to create three-dimensional (3D) structures, which lead to a larger surface area. Most of the current methods developed for the conformal deposition of ferroelectric films, such as metal-organic chemical vapor deposition (MOCVD) and plasma-enhanced vapor deposition (PECVD), are limited by high temperatures and unstable and toxic organic precursors. In this paper, an innovative fabrication method for barium titanate (BaTiO3) textured films with 3D architectures is introduced to alleviate these issues. This fabrication method is based on converting conformally grown rutile TiO2 nanowire arrays into BaTiO3 textured films using a simple two-step hydrothermal process which allows for thickness-controlled growth of conformal films on patterned silicon wafers coated with fluorine-doped tin oxide (FTO). Moreover, the processing parameters have been optimized to achieve a high piezoelectric coupling coefficient of 100 pm/V. This high piezoelectric response along with high relative dielectric constant (εr = 1600) of the conformally grown textured BaTiO3 films demonstrates their potential application in sensors, NVFeRAM, and MEMS.

Ba-DOPED ZnO MATERIALS: A DFT SIMULATION TO INVESTIGATE THE DOPING EFFECT ON FERROELECTRICITY

ZnO is a semiconductor material largely employed in the development of several electronic and optical devices due to its unique electronic, optical, piezo-, ferroelectric and structural properties. This study evaluates the properties of Ba-doped wurtzite-ZnO using quantum mechanical simulations based on the Density Functional Theory (DFT) allied to hybrid functional B3LYP. The Ba-doping caused increase in lattice parameters and slight distortions at the unit cell angle in a wurtzite structure. In addition, the doping process presented decrease in the band-gap (Eg) at low percentages suggesting band-gap engineering. For low doping amounts, the wavelength characteristic was observed in the visible range; whereas, for middle and high doping amounts, the wavelength belongs to the Ultraviolet range. The Ba atoms also influence the ferroelectric property, which is improved linearly with the doping amount, except for doping at 100% or wurtzite-BaO. The ferroelectric results indicate the ZnO:Ba is an strong option to replace perovskite materials in ferroelectric and flash-type memory devices.

Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory.

Enhancing the device performance of organic memory devices while providing high optical transparency and mechanical flexibility requires an optimized combination of functional materials and smart device architecture design. However, it remains a great challenge to realize fully functional transparent and mechanically durable nonvolatile memory because of the limitations of conventional rigid, opaque metal electrodes. Here, we demonstrate ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer. The strong crystallographic interaction between PVDF-TrFE and graphene results in the orientation of the crystals with distinct symmetry, which is favorable for polarization switching upon the electric field. The epitaxial growth of PVDF-TrFE on a graphene layer thus provides excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices. Furthermore, a fully transparent and flexible array of ferroelectric field effect transistors was successfully realized by adopting transparent poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] semiconducting polymer.

Proposal of a ferroelectric multi‐bit memory structure for reliable operation at sub‐100 nm scale

Although ferroelectric materials are attractive due to their non-volatility originating from their spontaneous polarisation, advances in integrated density of these materials are required. To overcome the poor integration density compared with silicon integrated circuits, the concept of multi-bit memory has arisen. Previous ferroelectric multi-bit memory devices were developed through the realisation of a framework of two laterally neighbouring capacitors, in which both capacitors have different thicknesses for individual switching in different voltage ranges. However, a reliability issue with regard to limiting the additional scale down of these materials to the sub-100 nm level arose due to self-crosstalk, which was defined as a protrusion of the electric fringing field when a logic state was written in a multi-bit memory device. Here, self-crosstalk by simulating an electric field distribution in a multi-bit memory cell based on an actual sample fabricated with a ferroelectric polymer, after which they suggest a new three-dimensional structure of the ferroelectric multi-bit memory device to eliminate self-crosstalk.

Dielectric, Polarization and Strain Response of Enhanced Complex Ceramics: The Study through Pb(Zr0.52Ti0.48)O3-SrBi2Ta2O9

Electrical properties of the (1-x)PZT-xSBT ceramics (0.1≤x≤0.3) were investigated in comparison with pure PZT and SBT ceramics. It was found that the dielectric constant increased with small addition of SBT (x = 0.1) into PZT. The hysteresis loop measurement demonstrated that the PZT-SBT ceramics showed larger remanent polarization than pure PZT and less coercive filed than pure SBT ceramic. Strain behavior showed a symmetric butterfly curve in the compositions with 0≤x≤0.3 but the low strain obtained was found in pure SBT. This research suggested that 0.9PZT-0.1SBT ceramic is a promising material to be further utilized in ferroelectric memory devices.

Effect of temperature on the electrical properties of nanocrystalline CaTi1−x Fe x O3−Δ perovskite

Calcium titanate nanopowder doped with high content (1 M%) of iron was synthesized using simple sol–gel method. The four obtained samples were post-sintered at 100, 500, 900, and 1250 °C to get soft agglomerates of nanocrystalline perovskites, which were deagglomerated by wet milling process using ethanol as a medium. The general stoichiometric formula for iron-doped CaTiO3 was symbolized as CaTi1−x Fe x O3−δ . The crystallographic structure of perovskites was found to be orthorhombic for all the four samples. The real and imaginary parts of the impedance values of sintered samples as a function of frequency were obtained using complex impedance spectroscopy. All the instrumental parameters were calibrated before initiating the measurements. The frequency range was set between 0.01 Hz and 1 MHz, and AC bias voltage was maintained at 50 mV. The objective of this work was to find the economical relevance of pioneering materials for electronic applications such as hydrogen production in fuel cells, solar plant, ferroelectric memory devices, microwave resonators, and sensible membranes as they are cheap and readily available, and can be prepared using controllable synthesis in a commercial laboratory.

Piezoresponse force microscopy study on ferroelectric polarization of ferroelectric polymer thin films with various structural configurations

Ferroelectric polymer-based memory devices have attracted much attention due to their potential in low-cost flexible memories. However, bad retention property of recorded logic states limited their applications. Though mechanisms of retention degradation in ferroelectric memories are complicated and still an open question, depolarization in ferroelectric polymer layer was regarded as the main influencing factor. Here we reported our piezoresponse force microscopy (PFM) study of retention property of polarization states on various ferroelectric polymer based structures. PFM results indicated that, as for ferroelectric/semiconductor structure and ferroelectric/insulator/semiconductor structure with thin insulating layer, both positive and negative polarization states could retain for a relatively long time. Mechanisms of good retention of polarization states were discussed. The discrepancy in bad retention of logic states and good polarization retention of ferroelectric layer was also analyzed.

Fabrication of self-aligned, nanoscale, complex oxide varactors

Applications in ferroelectric random access memory and superparaelectric devices require the fabrication of ferroelectric capacitors at the nanoscale that exhibit extremely small leakage currents. To systematically study the material-size dependence of ferroelectric varactor performance, arrays of parallel-plate structures have been fabricated with nanoscale dielectric diameters. Electron beam lithography and inductively coupled plasma dry etching have been used to fabricate arrays of ferroelectric varactors using top electrodes as a self-aligned etch mask. Parallel-plate test structures using RF-sputtered Ba0.6Sr0.4TiO3 thin-films were used to optimize the fabrication process. Varactors with diameters down to 20 nm were successfully fabricated. Current-voltage (I-V) characteristics were measured to evaluate the significance of etch-damage and fabrication quality by ensuring low leakage currents through the structures.

Unconventional resistive switching behavior in ferroelectric tunnel junctions.

We investigate an unconventional resistive switching (RS) behavior in La0.67Sr0.33MnO3/BaTiO3/metal (LSMO/BTO) ferroelectric tunnel junctions (FTJs), which is dominated by the variation of the barrier potential profile modulated by the migration of oxygen vacancies in the p-LSMO/n-BTO junction. The LSMO/BTO/Co junction exhibits a remarkable self-rectifying effect ascribed to the high-density interface state at the BTO/Co interface, in contrast to the symmetric conductivity when the top metal electrode is inert Pt. The effects of ferroelectric polarization on the RS behavior are also emphasized. Our work builds a bridge between FTJs and resistive random access memory devices.

Hybrid dual gate ferroelectric memory for multilevel information storage

Here, we report hybrid organic/inorganic ferroelectric memory with multilevel information storage using transparent p-type SnO semiconductor and ferroelectric P(VDF-TrFE) polymer. The dual gate devices include a top ferroelectric field-effect transistor (FeFET) and a bottom thin-film transistor (TFT). The devices are all fabricated at low temperatures (∼200°C), and demonstrate excellent performance with high hole mobility of 2.7cm2V−1s−1, large memory window of ∼18V, and a low sub-threshold swing ∼−4Vdec−1. The channel conductance of the bottom-TFT and the top-FeFET can be controlled independently by the bottom and top gates, respectively. The results demonstrate multilevel nonvolatile information storage using ferroelectric memory devices with good retention characteristics.