Self-assembly-induced formation of high-density silicon oxide memristor nanostructures on graphene and metal electrodes

We report the direct formation of ordered memristor nanostructures on metal and graphene electrodes by a block copolymer self-assembly process. Optimized surface functionalization provides stacking structures of Si-containing block copolymer thin films to generate uniform memristor device structures. Both the silicon oxide film and nanodot memristors, which were formed by the plasma oxidation of the self-assembled block copolymer thin films, presented unipolar switching behaviors with appropriate set and reset voltages for resistive memory applications. This approach offers a very convenient pathway to fabricate ultrahigh-density resistive memory devices without relying on high-cost lithography and pattern-transfer processes.     

Device performance of ferroelectric/correlated oxide heterostructures for non-volatile memory applications

Ferroelectric field effect devices offer the possibility of non-volatile data storage. Attempts to integrate perovskite ferroelectric materials with silicon semiconductors, however, have been largely unsuccessful in creating non-volatile, nondestructive read memory elements because of difficulties in controlling the ferroelectric/semiconductor interface. Correlated oxide systems have been explored as alternative channel materials to form all-perovskite field effect devices. We examine a non-volatile memory using an electric-field-induced metal-insulator transition in PbZr(0.2)Ti(0.8)O(3)/La(1 - x)Sr(x)MnO(3) (PZT/LSMO), PZT/La(1 - x)Ca(x)MnO(3) (PZT/LCMO) and PZT/La(1 - x)Sr(x)CoO(3) (PZT/LSCO) devices. The performance of these devices in the areas of switching time and retention are discussed.     

Bond- versus site-centred ordering and possible ferroelectricity in manganites

Transition metal oxides with a perovskite-type structure constitute a large group of compounds with interesting properties. Among them are materials such as the prototypical ferroelectric system BaTiO(3), colossal magnetoresistance manganites and the high-T(c) superconductors. Hundreds of these compounds are magnetic, and hundreds of others are ferroelectric, but these properties very seldom coexist. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields. Here, we report on a possible way to avoid this incompatibility, and show that in charge-ordered and orbitally ordered perovskites it is possible to make use of the coupling between magnetic and charge ordering to obtain ferroelectric magnets. In particular, in manganites that are less than half doped there is a type of charge ordering that is intermediate between site-centred and bond-centred. Such a state breaks inversion symmetry and is predicted to be magnetic and ferroelectric.     

Solution-phase deposition and nanopatterning of GeSbSe phase-change materials

Chalcogenide films with reversible amorphous-crystalline phase transitions have been commercialized as optically rewritable data-storage media, and intensive effort is now focused on integrating them into electrically addressed non-volatile memory devices (phase-change random-access memory or PCRAM). Although optical data storage is accomplished by laser-induced heating of continuous films, electronic memory requires integration of discrete nanoscale phase-change material features with read/write electronics. Currently, phase-change films are most commonly deposited by sputter deposition, and patterned by conventional lithography. Metal chalcogenide films for transistor applications have recently been deposited by a low-temperature, solution-phase route. Here, we extend this methodology to prepare thin films and nanostructures of GeSbSe phase-change materials. We report the ready tuneability of phase-change properties in GeSbSe films through composition variation achieved by combining novel precursors in solution. Rapid, submicrosecond phase switching is observed by laser-pulse annealing. We also demonstrate that prepatterned holes can be filled to fabricate phase-change nanostructures from hundreds down to tens of nanometres in size, offering enhanced flexibility in fabricating PCRAM devices with reduced current requirements.     

Nanoscale ferroelectric information storage based on scanning nonlinear dielectric microscopy

An investigation of ultrahigh-density ferroelectric data storage based on scanning nonlinear dielectric microscopy (SNDM) is described. For the purpose of obtaining fundamental knowledge on high-density ferroelectric data storage, several experiments on nanodomain formation in a lithium tantalate (LiTaO3) single crystal were conducted. Through domain engineering, a domain dot array with an areal density of 1.5 Tbit/inch2 was formed on congruent LiTaO3 (CLT). Sub-nanosecond (500 psec) domain switching speed also has been achieved. Next, actual information storage is demonstrated at a density of 1 Tbit/inch2. Finally, it is described that application of a very small dc offset voltage is very effective in accelerating the domain switching speed and in stabilizing the reversed nano-domain dots. Applying this offset application technique, we formed a smallest artificial nano-domain single dot of 5.1 nm in diameter and artificial nano-domain dot-array with a memory density of 10.1 Tbit/inch2 and a bit spacing of 8.0 nm, representing the highest memory density for rewritable data storage reported to date.     

Core-shell heterostructured phase change nanowire multistate memory

Phase-change memory, which switches reversibly between crystalline and amorphous phases, is promising for next generation data-storage devices. In this work, we present a novel, nonbinary data-storage device using core-shell nanowires to significantly enhance memory capacity by combining two phase-change materials with different electronic and thermal properties to engineer different onsets of amorphous-crystalline transitions. Electric-field induced sequential amorphous-crystalline transition in core-shell nanowires displays three distinct electronic states with high, low, and intermediate resistances, assigned as data "0", "1", and "2".     

Solid-state cooling line based on the electrocaloric effect

In this paper, the feasibility of using ferroelectric materials as a cooling device or solid-state refrigerator based on the electrocaloric effect has been considered. The electrocaloric responses of two different ferroelectric capacitors (PMN-25PT, PZN-4.5PT) are studied. In this study, the dynamics of temperature variation at the edges of a layered structure comprised of an electrocaloric material with heatconducting elements on its sides to an applied periodic electric field have been studied. Electrocaloric elements can generate directed heat flux as a thermal pump by alternative switching. A temperature reduction of 6° is obtained in an electric field of 1 KV/mm at a frequency of 1 Hz for PMN-25PT material after applying 80 cycles. It is shown that the type of applied electric pulses and ferroelectric material affect the results.     

Gate-Coupling-Enabled Robust Hysteresis for Nonvolatile Memory and Programmable Rectifier in Van der Waals Ferroelectric Heterojunctions

Ferroelectric field-effect transistors (FeFETs) are one of the most interesting ferroelectric devices; however, they, usually suffer from low interface quality. The recently discovered 2D layered ferroelectric materials, combining with the advantages of van der Waals heterostructures (vdWHs), may be promising to fabricate high-quality FeFETs with atomically thin thickness. Here, dual-gated 2D ferroelectric vdWHs are constructed using MoS₂, hexagonal boron nitride (h-BN), and CuInP₂S₆ (CIPS), which act as a high-performance nonvolatile memory and programmable rectifier. It is first noted that the insertion of h-BN and dual-gated coupling device configuration can significantly stabilize and effectively polarize ferroelectric CIPS. Through this design, the device shows a record-high performance with a large memory window, large on/off ratio (10⁷), ultralow programming state current (10⁻¹³ A), and long-time endurance (10⁴ s) as nonvolatile memory. As for programmable rectifier, a wide range of gate-tunable rectification behavior is observed. Moreover, the device exhibits a large rectification ratio (3 × 10⁵) with stable retention under the programming state. This demonstrates the promising potential of ferroelectric vdWHs for new multifunctional ferroelectric devices.     

Self-assembled nanoscale ferroelectrics

Multifunctional ferroelectric materials offer a wide range of useful properties, from switchable polarization that can be applied in memory devices to piezoelectric and pyroelectric properties used in actuators, transducers and thermal sensors. At the nanometer scale, however, material properties are expected to be different from those in bulk. Fundamental problems such as the super-paraelectric limit, the influence of the free surface, and of interfacial and bulk defects on ferroelectric switching, etc., arise when scaling down ferroelectrics to nanometer sizes. In order to study these size effects, fabrication methods of high quality nanoscale ferroelectric crystals have to be developed. The present paper briefly reviews self-patterning and self-assembly fabrication methods, including chemical routes, morphological instability of ultrathin films, microemulsion, and self-assembly lift-off, employed up to the date to fabricate ferroelectric structures with lateral sizes in the range of few tens of nanometers.     

Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures

Magnetoelectric coupling between magnetic and electrical properties presents valuable degrees of freedom for applications. The two most promising scenarios are magnetic-field sensors that could replace low-temperature superconducting quantum interference devices, and electric-write magnetic-read memory devices that combine the best of ferroelectric and magnetic random-access memory. The former scenario requires magnetically induced continuous and reversible changes in electrical polarization. These are commonly observed, but the coupling constants thus obtained are invalid for data-storage applications, where the more difficult to achieve and rarely studied magnetic response to an electric field is required. Here, we demonstrate electrically induced giant, sharp and persistent magnetic changes (up to 2.3 x 10(-7) s m(-1)) at a single epitaxial interface in ferromagnetic 40 nm La(0.67)Sr(0.33)MnO(3) films on 0.5 mm ferroelectric BaTiO(3) substrates. X-ray diffraction confirms strain coupling via ferroelastic non-180( composite function) BaTiO(3) domains. Our findings are valid over a wide range of temperatures including room temperature, and should inspire further study with single epitaxial interfaces.     

不挥发铁电存储器的最新发展

铁电材料具有自发极化并可由外电场反转，因此可以构成一种不挥发存储器．铁电薄膜与半导体集成，产生铁电随机存储器，并将成为存储器技术的主体．     

Mechanical and electrical properties of fluorine-containing niobate ferroelectric ceramics

The presence of fluorine impurity in alkali metal niobate ceramics sharply deteriorates both mechanical and electrical properties of these ferroelectric materials. This effect is related to the separation of a glassy phase and the formation of an inhomogeneous microstructure, these factors giving rise to local stresses in the structural elements. It is concluded that intact niobate ferroelectric articles can be made of technical-grade Nb₂O₅ only provided that the initial material contains a minimum amount of fluorine impurity (below 0.2 wt%).     

Thin film ferroelectrics for capacitor applications

Pulsed laser deposition (PLD) has been used to fabricate simple thin film capacitor structures with a variety of ferroelectric materials. Thin film capacitors using the conventional ferroelectric material BaxSr1-xTiO3(BSTO) have been made across the entire compositional series. Electrical characterization shows that in thin film form these ferroelectrics display Curie point behaviour which is largely independent of composition. This contrasts sharply with bulk behaviour. The thin film fabrication and characterization of relaxor ferroelectric ceramics, such as Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3-BaTiO3(PZN-BT), is also reported. © 1998 Chapman & Hall     

Phase-change materials for rewriteable data storage

Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. This review looks at the unique property combination that characterizes phase-change materials. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. The origin of the fast crystallization kinetics is also discussed.     

FeTRAM. An organic ferroelectric material based novel random access memory cell

Science and technology in the electronics area have always been driven by the development of materials with unique properties and their integration into novel device concepts with the ultimate goal to enable new functionalities in innovative circuit architectures. In particular, a shift in paradigm requires a synergistic approach that combines materials, devices and circuit aspects simultaneously. Here we report the experimental implementation of a novel nonvolatile memory cell that combines silicon nanowires with an organic ferroelectric polymer-PVDF-TrFE-into a new ferroelectric transistor architecture. Our new cell, the ferroelectric transistor random access memory (FeTRAM) exhibits similarities with state-of-the-art ferroelectric random access memories (FeRAMs) in that it utilizes a ferroelectric material to store information in a nonvolatile (NV) fashion but with the added advantage of allowing for nondestructive readout. This nondestructive readout is a result of information being stored in our cell using a ferroelectric transistor instead of a capacitor-the scheme commonly employed in conventional FeRAMs.     

Organic field effect transistors with ferroelectric hysteresis

The ferroelectric copolymer Poly(vinylidene fluoride trifluoroethylene) is used as insulating material for capacitor structures and organic field effect transistors. For capacitors, we find the typical hysteresis in the capacitance-voltage characteristic upon increasing the voltage scan window. A writing process with adequate electric fields causes shifts in the flatband voltage. Based on these results, we fabricate organic transistors with regioregular poly(3-hexylthiophene) as organic semiconductor. The transistors are constructed in bottom gate architecture with thin layers (100 nm) of spincoated copolymer as gate insulation. The drain source current of the transistor is reversible affected by the polarized gate, which gives opportunities for fabrication of organic nonvolatile memory elements.     

CMOS compatible nanoscale nonvolatile resistance switching memory

We report studies on a nanoscale resistance switching memory structure based on planar silicon that is fully compatible with CMOS technology in terms of both materials and processing techniques employed. These two-terminal resistance switching devices show excellent scaling potential well beyond 10 Gb/cm2 and exhibit high yield (99%), fast programming speed (5 ns), high on/off ratio (10(3)), long endurance (10(6)), retention time (5 months), and multibit capability. These key performance metrics compare favorably with other emerging nonvolatile memory techniques. Furthermore, both diode-like (rectifying) and resistor-like (nonrectifying) behaviors can be obtained in the device switching characteristics in a controlled fashion. These results suggest that the CMOS compatible, nanoscale Si-based resistance switching devices may be well suited for ultrahigh-density memory applications.     

Experimental realization of optical lumped nanocircuits at infrared wavelengths

The integration of radiofrequency electronic methodologies on micro- as well as nanoscale platforms is crucial for information processing and data-storage technologies. In electronics, radiofrequency signals are controlled and manipulated by 'lumped' circuit elements, such as resistors, inductors and capacitors. In earlier work, we theoretically proposed that optical nanostructures, when properly designed and judiciously arranged, could behave as nanoscale lumped circuit elements--but at optical frequencies. Here, for the first time we experimentally demonstrate a two-dimensional optical nanocircuit at mid-infrared wavelengths. With the guidance of circuit theory, we design and fabricate arrays of Si3N4 nanorods with specific deep subwavelength cross-sections, quantitatively evaluate their equivalent impedance as lumped circuit elements in the mid-infrared regime, and by Fourier transform infrared spectroscopy show that these nanostructures can indeed function as two-dimensional optical lumped circuit elements. We further show that the connections among nanocircuit elements, in particular whether they are in series or in parallel combination, can be controlled by the polarization of impinging optical signals, realizing the notion of 'stereo-circuitry' in metatronics-metamaterials-inspired optical circuitry.     

Ag/Bi4Ti3O12/p-Si异质结的制备及其C-V特性研究

为制备符合铁电存储器件要求的高质量铁电薄膜,采用溶胶-凝胶(Sol-Gel)工艺,制备了Si基Bi4Ti3O12铁电薄膜及MFS结构的Ag/Bi4Ti3O12/P-Si异质结,对Bi4Ti3O12薄膜的相结构特征及异质结的C-V特性进行了测试与分析.XRD图谱显示,Si基Bi4Ti3O12薄膜具有沿c-轴择优取向生长的趋势,而Ag/Bi4Ti3O12/p-Si异质结顺时针回滞的C-V特性曲线则表明,该异质结可实现电极化存储.此外,对该异质结C-V特性曲线的非对称及向负偏压方向偏移的产生原因也进行了分析.在此基础上,为提高铁电薄膜的铁电性能及改善其C-V特性提出了合理的结构设想.     

Corona poling of highly (001)/(100)-oriented lead zirconate titanate thin films

Corona poling is an interesting non-contact poling process for ferroelectric materials which, has in the past, been extensively applied to ferroelectric polymers, but not to other types of ferroelectric materials. Here, it has been investigated as an alternative technique to the conventional direct-contact poling of ferroelectric thin films.

Contact-poling and corona poling techniques were applied to highly (001)/(100) oriented lead zirconate titanate thin films of composition 52% Zr, 48% Ti (PZT52/48). Different poling voltages and durations were used to pole the samples, and the effects on the piezoelectric coefficients e_31,f and d_33,f were explored. From the magnitudes of piezoelectric coefficients and suppression of e_31,f relative to d_33,f, it was concluded that corona poling is a more suitable technique for poling PZT52/48 thin films than direct poling, since piezoelectric coefficients were higher and sample damage was less for corona poling when compared with contact-poling.