Applications In Nonvolatile Memory Devices Research Articles

Resistance switching properties of Cu2S film by electrochemical deposition

Electric pulse-induced resistance (EPIR) effect in metal⿿oxide⿿metal devices is a fascinating phenomenon toward next generation universal nonvolatile memories. Herein, the cuprous sulfide (Cu2S) film was prepared by electrochemical deposition. Further, the resistive switching (RS) behaviors of Ag/Cu2S/Ag and Ag/Cu2S/Cu sandwich structures were observed and compared. Ag/Cu2S/Ag device shows a reversible, high ratio (⿥103) RS effect. However, Ag/Cu2S/Cu device displays a low operation voltage (⿤0.5V) and a more consistent high resistance. Our data suggest that the RS effect in Ag/Cu2S/Ag device results from formation or annihilation of conductive filaments. Moreover, a space charge barrier at the interface of Ag/Cu2S film is used to explain the RS effect in Ag/Cu2S/Cu device. This study is useful for exploring the sulfide materials and their applications in nonvolatile memory devices.

Photo-electron double regulated resistive switching memory behaviors of Ag/CuWO4/FTO device

In this work, the CuWO4 film based resistive switching memory capacitors were fabricated with hydrothermal and spin-coating approaches. The device exhibits excellent photo-electron double controlled resistive switching memory characteristics with OFF/ON resistance ratio of ~103. It is believed that the interface of CuWO4 and FTO is responsible for such a switching behavior and it can be described by the Schottky-like barriers model. This study is useful for exploring the multifunctional materials and their applications in photo-electron double controlled nonvolatile memory devices.

Resistive switching memory characteristics of single MoSe2 nanorods

Resistive switching memory effect in metal–oxide–metal structures is a fascinating phenomenon toward next generation universal nonvolatile memories. Herein, the MoSe2 nanorods were prepared by hydrothermal method. Further, a resistive switching memory device of single MoSe2 nanorods is demonstrated. The device presents stable resistive switching memory behaviors with on–off ratio (memory window) of ∼50 at room temperature. Moreover, the origin of switching behavior in these devices on the basis of formation and annihilation of conducting filaments is addressed. This study is useful for exploring the multifunctional materials and their applications in nonvolatile memory devices.

Electrical Characteristics of Charge Trap Flash Memory with a Composition Modulated (ZrO2)x(Al2O3)1-xFilm

This research proposes the use of a composition modulated (ZrO₂)_x(Al₂O₃)_1-x film as a charge trapping layer for charge trap flash memory; this is possible when the Zr (Al) atomic percent is controlled to form a variable bandgap as identified by the valence band offsets and electron energy loss spectrum measurements. Compared to memory devices with uniform compositional (ZrO₂)_0.1(Al₂O₃)_0.9 or a (ZrO₂)_0.92(Al₂O₃)_0.08 trapping layer, the memory device using the composition modulated (ZrO₂)_x(Al₂O₃)_1-x as the charge trapping layer exhibits a larger memory window (6.0 V) at the gate sweeping voltage of ±8 V, improved data retention, and significantly faster program/erase speed. Improvements of the memory characteristics are attributed to the special energy band alignments resulting from non-uniform distribution of elemental composition. These results indicate that the composition modulated (ZrO₂)_x(Al₂O₃)_1-x film is a promising candidate for future nonvolatile memory device applications.

Two quinoxaline derivatives designed from isomer chemistry for nonvolatile ternary memory device applications

In this paper, two quinoxaline-based small organic molecules, which are conjugated and mutual isomers in vertical and horizontal directions, were successfully synthesized. By exploring isomers with identical functional groups, the impact from changing in molecular composition on the observed different performance can be minimized. Their photoelectrical properties, thin-film morphology and the electrical memory performance were comparatively investigated. The devices based on these two small molecules both showed ternary WORM storage performance. Compared with vertical direction, horizontal molecule had a lower threshold voltage and higher current ratio. The results of AFM and XRD showed that horizontal molecule has better thin-film morphology and layered accumulation, which are conducive to the migration of carriers. We hope this study can provide a new reference to the future elaborate design and synthesis of novel small-molecule electrical memory materials.

White-light-controlled resistive switching chearacteristics of TiO2/Cu2O composite nanorods array

TiO2/Cu2O composite nanorods array were grown on fluorine-doped tin oxide (FTO) substrate by hydrothermal process, and white-light-controlled resistive switching characteristics of Ag/[TiO2/Cu2O]/FTO structure were further investigated. The current–voltage characteristics of the composite nanorods array represent a good rectifying property and bipolar resistive switching behavior. Specially, the resistive switching behavior can be regulated by white-light illuminating at room temperature. This study is helpful for exploring the memory materials and their applications in nonvolatile light-controlled memory devices.

Non-Volatile Resistive Memory Switching in Pulsed Laser Deposited Rare-Earth Gallate-GdGaO3 Thin Films

Resistive random access memories (ReRAMs) have emerged as a potential memory candidate that has the capability of fulfilling the future high density requirements due to their simple design, excellent scalability, high speed storage capacity, low power consumption, and CMOS process-flow compatibility. With this motivation pulsed laser-deposited amorphous GdGaO3 (GGO) thin films sandwiched between two Pt-electrodes were investigated to explore their resistive switching (RS) behaviour. Pt/GGO/Pt memory cells showed a stable resistance ratio of ~104 between low resistance state (LRS) and high resistance state (HRS) and non-overlapping switching voltages (set voltage VON ~1.2˗1.4 V and reset voltage VOFF ~0.5˗0.7 V) with a small variation of 5˗10%. To confirm the reliability of the Pt/GGO/Pt memory cells, cyclic endurance up to 60 set/reset cycles and data retention up to 103 seconds were measured. Only a unipolar RS mechanism in Pt/GGO/Pt memory cells was observed due to a conductive filamentary (thermo-chemical) mechanism in which, the change in resistance state (LRS↔HRS) of the cell was found to occur from rupture of conductive filaments consisting of metallic Gd atoms and oxygen vacancies. The charge transport mechanisms of the cell in LRS and HRS states were found to be dominated by Ohmic conduction and trap controlled space–charge limited current, respectively. This work reveals the potential of rare-earth gallates for non-volatile memory device applications.

Room-temperature polarization switching and antiferromagnetic coupling in epitaxial (Ga,Fe)2O3/SrRuO3 heterostructures

Room-temperature reversible remnant polarization of gallium ferrite thin-films is reported as a multiferroic material with non-zero order parameters of polarization and magnetization. With the addition of Fe ions in Ga sites, Ga0.6Fe1.4O3 (GFO) thin films have been considered as potentially promising of multiferroicity. The b-axis oriented epitaxial GFO films were grown on SrRuO3(111)/SrTiO3(111). The six-fold symmetric in-plane epitaxy of the GFO films was confirmed using X-ray diffraction. The magnetic moment of the films was measured as a function of temperature and external magnetic field, which shows a room-temperature non-zero magnetization. Macroscopic and microscopic methods have been applied to demonstrate the polarization switching of the films. The remnant polarization is measured as 0.05 μC/cm2. Reduction of leaky behaviors of the GFO films owing to the conducting oxide of SrRuO3 will pave a way to take advantage of the room-temperature non-zero multi-orders for future non-volatile memory device applications.

Hierarchically Self-Assembled Block Copolymer Blends for Templating Hollow Phase-Change Nanostructures with an Extremely Low Switching Current

Phase change memory (PCM) is one of the most promising candidates for next-generation nonvolatile memory devices because of its high speed, excellent reliability, and outstanding scalability. However, the high switching current of PCM devices has been a critical hurdle to realize low-power operation. Although one solution is to reduce the switching volume of the memory, the resolution limit of photolithography hinders further miniaturization of device dimensions. In this study, we employed unconventional self-assembly geometries obtained from blends of block copolymers (BCPs) to form ring-shaped hollow PCM nanostructures with an ultrasmall contact area between a phase-change material (Ge2Sb2Te5) and a heater (TiN) electrode. The high-density (approximately 0.1 terabits per square inch) PCM nanoring arrays showed extremely small switching current of 2–3 μA. Furthermore, the relatively small reset current of the ring-shaped PCM compared to the pillar-shaped devices is attributed to smaller switching volume, which is well supported by electro-thermal simulation results. This approach may also be extended to other nonvolatile memory device applications such as resistive switching memory and magnetic storage devices, where the control of nanoscale geometry can significantly affect device performances.

Memory effect by charging of ultra-small 2-nm laser-synthesized solution processable Si-nanoparticles embedded in Si-Al2 O3 -SiO2 structure

A memory structure containing ultra-small 2-nm laser-synthesized silicon nanoparticles is demonstrated. The Si-nanoparticles are embedded between an atomic layer deposited high-κ dielectric Al2O3 layer and a sputtered SiO2 layer. A memory effect due to charging of the Si nanoparticles is observed using high frequency C–V measurements. The shift of the threshold voltage obtained from the hysteresis measurements is around 3.3 V at 10/−10 V gate voltage sweeping. The analysis of the energy band diagram of the memory structure and the negative shift of the programmed C–V curve indicate that holes are tunneling from p-type Si via Fowler–Nordheim tunneling and are being trapped in the Si nanoparticles. In addition, the structures show good endurance characteristic (>105 program/erase cycles) and long retention time (>10 years), which make them promising for applications in non-volatile memory devices.

Influence of La-doping on structure and magnetic behaviors in BiFeO3

The series of polycrystalline Bi1−xLaxFeO3 (0⩽x⩽0.50) ceramic powders was successfully synthesized by sol–gel method in this paper. X-ray diffraction and Rietveld crystal structure refinement software (FULLPROF) were used as complementary method to study structure and hyperfine interactions of the samples. It was a clear indication of structural modification and symmetry changes brought about by La doping. The BFO is crystallized in rhombohedrally distorted perovskite structure with space group R3c for x⩽0.10. With an increase of La content, within the range x=0.10–0.20 a structural phase evolutes from rhombohedral to cubic Pm3¯m structure, and then transits to orthorhombic Pbnm structure on further increasing the La-doping to x=0.50. The magnetization of La-doped samples is improved with the increase of La content. The coercive field (Hc) increases remarkably with an increase in La doping and x=0.50 exhibits the highest Hc (20.2kOe at room temperature) reported so far in any rare earth doped BFO, which will have a great potential for practical application in non-volatile memory devices. Moreover, high-field magnetization and remanence (Mr) increase with La doping content due to the gradual change in spin cycloid structure.

Ferroelectricity in Strategically Synthesized Pb-free LiNbO3-type ZnSnO3 Nanostructure Arrayed Thick Films

ABSTRACTWe report the evidence of ferroelectricity from LN-type ZnSnO3 nanostructure arrayed thick films (10 - 20 µm) on Si with remanent polarization value as high as ≈ 30 µC/cm2 in nanowire arrays. A combined pulsed-laser deposition (PLD) technique and a solvothermal synthesis scheme was adopted to effectively synthesize the nanostructured samples assisted by conducting ZnO template-layers. The similar crystal symmetry and comparable lattice parameter between ZnO and LN-type ZnSnO3 facilitated the dense growth of high-quality ZnSnO3 nanostructure arrays in the form of one-dimensional vertical nanowires, nanorods and two-dimensional nanoflakes. The strategic synthesis method allowed controlled tunability of the morphology, crystallinity, and packing density of ZnSnO3 nanostructures, which in turn facilitated the measurement of ferroelectric (FE) properties using a simple sandwich-device geometry. Analyses of the FE properties in relation to the structures are presented and their potential for designing future Pb-free FE devices for non-volatile memory applications is discussed.

Electrical characteristics of gallium–indium–zinc oxide thin-film transistor non-volatile memory with Sm2O3 and SmTiO3 charge trapping layers

In this study, we report the structural and electrical characteristics of high-κ Sm2O3 and SmTiO3 charge trapping layers on an indium–gallium–zinc oxide (IGZO) thin-film transistor (TFT) for non-volatile memory device applications.

Memory characteristics and tunneling mechanism of Pt nano-crystals embedded in HfAlOx films for nonvolatile flash memory devices

A non-volatile flash memory device based on metal oxide semiconductor (MOS) capacitor structure has been fabricated using platinum nano-crystals(Pt–NCs) as storage units embedded in HfAlOx high-k tunneling layers. Its memory characteristics and tunneling mechanism are characterized by capacitance–voltage(C–V) and flat-band voltage-time(ΔVFB-T) measurements. A 6.5V flat-band voltage (memory window) corresponding to the stored charge density of 2.29 × 1013cm−2 and about 88% stored electron reserved after apply ±8V program or erase voltage for 105s at high frequency of 1 MHz was demonstrated. Investigation of leakage current–voltage(J–V) indicated that defects-enhanced Pool-Frenkel tunneling plays an important role in the tunneling mechanism for the storage charges. Hence, the Pt–NCs and HfAlOx based MOS structure has a promising application in non-volatile flash memory devices.

Fluorene based donor-acceptor polymer electrets for nonvolatile organic transistor memory device applications

Fluorene based donor-acceptor polyimides, including poly[bis-(4-aminophenyl)fluorene-hexanediamide] [PA(BAP F-AC)], poly[bis-(4-aminophenyl)fluorene-hexafluoroisopropylidenediphthalimide] [PI(BAPF-6FDA)], poly[bis-(4-aminophenyl)fluorene-oxydiphthalimide] [PI(BAPF-ODPA)], and poly[bis-(4-aminophenyl)fluorene-1,2,4,5-cyclohexanetetracarboxylic diimide] [PI(BAPF-HPMDA)], as charge storage layer (electret) are employed for nonvolatile memory device applications. The polyimides are consisted of electron-donating fluorene diamine moiety (BAPF) and neutral (AC and HPMDA) or electron-accepting (6FDA and ODPA) moieties, respectively. The memory characteristics of these devices can be tuned from the EORM (erase once and read many times) behavior [PA(BAPF-AC)], semi-flash [PI(BAPF-ODPA)], [PI(BAPF-HPMDA)], to a flash type memory [PI(BAPF-6FDA)]. The PI(BAPF-6FDA) devices show the largest memory window of 77 V and a long retention time over 104 s with a high Ion/Ioff current ratio of 108. This is attributed to the largest torsion angle of PI(BAPF-6FDA) stabilizing charge transfer (CT) complexes. The write-read-erase-read cycles were stably operated over 100 cycles. This work provides a new insight into the relationship between the CT effect and the nonvolatile memory behavior. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 602–614

Boost up carrier mobility for ferroelectric organic transistor memory via buffering interfacial polarization fluctuation.

Ferroelectric organic field-effect transistors (Fe-OFETs) have been attractive for a variety of non-volatile memory device applications. One of the critical issues of Fe-OFETs is the improvement of carrier mobility in semiconducting channels. In this article, we propose a novel interfacial buffering method that inserts an ultrathin poly(methyl methacrylate) (PMMA) between ferroelectric polymer and organic semiconductor layers. A high field-effect mobility (μFET) up to 4.6 cm2 V−1 s−1 is obtained. Subsequently, the programming process in our Fe-OFETs is mainly dominated by the switching between two ferroelectric polarizations rather than by the mobility-determined charge accumulation at the channel. Thus, the “reading” and “programming” speeds are significantly improved. Investigations show that the polarization fluctuation at semiconductor/insulator interfaces, which affect the charge transport in conducting channels, can be suppressed effectively using our method.

Electroforming-Free and Multilevel Resistance Switching Properties in Al/TiOx/Cu Structure

Resistance random access memory (RRAM) in various oxide materials,especiallybinaryoxidematerialssuchasTiO2, 1 ZrO2, 2 HfO2, 3 and ZnO, 4 has attracted considerable attention as one of the promising applications in next-generation nonvolatile semiconductor memory devices. Although the resistance switching (RS) materials can be classified into several groups according to switching mechanisms, this study focuses on combined different mechanisms effects on the RS performance in Al/TiOx/Cu. As Cu is an active metal in the electrochemical metallization memory (ECM), 5 adopting Cu as electrode may induce Cu conductive filaments (CFs). Meanwhile, because of the higher oxidation potential of Al compared with Ti, 6 adopting Al onTiOx mayinducetheoxygenvacancy (Vo)relatedRSmechanisms, such as electronic switching related with the trapping and detrapping of carriers along the Vo channels. 7 Particularly, when RRAM devices is programmed, a filament with trap-controlled space charge limited current (SCLC) could be formed, while a metallic CFs could also be formed. Therefore, multilevel resistance states may be obtained by forming different filaments inside the device. However, research on combining these two mechanisms remains to be clarified. During the resistance switching process, the “electroforming” process is usually needed to obtain the resistance switching. However, this process often results in the random fluctuations of switching parameters and needs a much higher bias, which are generally unfavorable for the device fabrication and operation. 8 In this paper, the characteristics of multilevel resistance switching, electroforming-free and low Ireset (the maximum current level changes from LRS to HRS) were investigated through Al/TiOx/Cu structure. The relationship between electroforming-free and low Ireset will be elucidated in this paper, as well as the multilevel resistance switching mechanism.

Memory effect by carrier trapping into V3Si nanocrystals among SiO2 layers on multi-layered graphene layer.

We report the electrical characteristics and conduction mechanism of a resistive switching memory device consisting of V3Si nanocrystals embedded in the SiO2 layer on multi-layered graphene. The V3Si nanocrystals with average size of 5 nm were formed between the SiO2 layers by thin film deposition and post-annealing process at 800 degrees C for 5 s. The current values of high (HRS) and low resistance states (LRS) at 1 V were measured to be about 3.26 x 10(-9) A and 3.11 x 10(-8) A, respectively. The ratio of the HRS and LRS after applying sweeping bias of ± 6 V appeared to be about 9.54 at 1 V. The resistance switching could originate from the effect of carrier trap and emission into the V3Si nanocrystals via the tunneling, space charge limited current, and thermionic emission mechanisms controlled by the modulation of the Fermi level of the graphene layer. The V3Si nanocrystals memory device has a strong possibility for the application of nonvolatile memory devices.

First-principles theory, coarse-grained models, and simulations of ferroelectrics.

CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys.

Visible-light controlled ferroelectricity and magnetoelectric coupling in multiferroic BiCoO3nanoribbons

Multiferroic materials hold promising potential for multifunctional applications because they simultaneously possess ferromagnetism and ferroelectricity. Moreover, the magnetoelectric coupling in multiferroic materials is especially important for basic research, and it has significant applications in nonvolatile multiple-state memory devices. In this work, for the first time, we report that visible light can control the ferroelectricity and magnetoelectric coupling in BiCoO3 nanoribbons. The relative visible-light-induced improvement of the saturated ferroelectric polarization is more than 60%. The relative visible-light-induced enhancement of the magnetoelectric effect is about 10%, indicating that light offers additional degree of freedom in controlling the coupling between ferromagnetism and ferroelectricity.