Applications In Memory Devices Research Articles

Phase transition behavior, dielectric and ferroelectric properties of (1-x)NaNbO3-xLiTaO3 ceramics

(1-x)NaNbO3-xLiTaO3 ceramics were synthesized by solid state reactions and the phase transition, dielectric and ferroelectric properties were studied. XRD and Raman results suggested that the anti-ferroelectric orthorhombic P phase (Pbma) gradually transformed into the ferroelectric orthorhombic Q phase (P21ma) when x ≥ 0.10, and the turning points of v1, v3 and v6 appeared when x = 0.15, which is probably due to the competition of two kinds of O phase with increasing x. The diffuse relaxor behavior of NN ceramics were obviously increased by LT addition, and the x = 0.10 sample possessed a comparable ΔTm with x = 0.05 samples. As a result, the x = 0.10 sample possesses a fast ferroelectric switching time of 1.75 ms and a high Pr of 27μC/cm2, due to the enhanced ferroelectric phase and relaxor diffuse behavior. In addition, the x = 0.10 sample also shows a well rectangular P-E loops, indicating its potential applications in ferroelectric memory devices.

Graphene quantum dots as shallow traps in a high-k polymer matrix for bipolar resistive switching

Graphene quantum dots (GQDs) have broad applications in electronic and photonic devices. As a typical zero-dimensional material, a GQD has a stronger quantum confinement effect than do two-dimensional graphene or one-dimensional graphene nanoribbons, and provide efficient charge trapping sites, which are useful in nonvolatile memory devices. Here, we report the fabrication of 30-nm-thick GQD-blended high-k polymer cyanoethylated pullulan (CEP) thin films sandwiched between an Al top electrode and an indium tin oxide (ITO) bottom electrode. Bipolar resistive switching behavior was observed with a low onset (−1.7 V) and offset (1.3 V) voltages. The resistive switching behavior originates from shallow traps that induce space-charge-limited current conduction. The morphology, crystallinity and photoluminescence of the GQDs were also studied by atomic force microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The CEP/GQD blended films may have applications in nonvolatile resistive memory devices.

Disorder is not always bad for charge-to-spin conversion in WTe2

The Wang group at Stanford University demonstrates disordered WTex films for efficient charge-to-spin conversion phenomena. The deposition of these films by sputtering and the charge-to-spin conversion resilience against disorder make them attractive for applications in new magnetic memory devices.

A study on microstructure and magnetic properties of nanostructured CoxNi1-xMn0.5Fe1.5O4(x=0,0.25,0.5,0.75,1) spinel ferrites

A low-temperature synthesis of novel nanostructured CoxNi1-xMn0.5Fe1.5O4(x=0,0.25,0.5,0.75,1) ferrites was carried out by sol-gel auto-combustion technique. The obtained nanostructured ferrites were investigated by employing the techniques of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometry (VSM). The XRD diffractograms of the prepared ferrites revealed the formation of a spinel phase with face centered cubic (fcc) structure belonging to Fd- m space group. The average lattice parameter ‘a’ of ferrites exhibited a rise versus a rise in Co2+ concentration in accordance with the Vegard’s law. The SEM investigation of NiMn0.5Fe1.5O4 powder revealed an existence of octahedral-shaped morphology of ferrite grains. The TEM investigation of NiMn0.5Fe1.5O4 powder showed nanostructures of ferrite particles with sizes consistent with the crystallite sizes as estimated by Debye-Scherer’s formula. An EDX spectrum of NiMn0.5Fe1.5O4 powder confirmed its elemental composition. The M-H hysteresis loops recorded by VSM at room temperature revealed a dependence of coercivity (Hc), maximum magnetization (Mmax) and retentivity (Mr) on Co2+concentration. Due to the shape dependence of M-H loops on Co2+ concentration in compounds enabled their candidature for applications in memory devices and magnetic sensors.

Recent developments on 2D magnetic materials: challenges and opportunities

The emergence of two-dimensional (2D) magnetic materials exhibiting strong magnetization at ultrathin limits above room temperature is promising for miniaturization of devices beyond Moore’s law for future energy efficient nanoelectronic devices. Here, the current status, different mechanisms for the existence of magnetism, spin current injection, and other magnetic properties of monolayer to few layers of various 2D magnetic materials are reviewed. Some of the promising applications of these materials are spintronics devices such as spin valves, spin tunnel field-effect transistors, and spin-filtering magnetic tunnel junctions. Due to the tunable electronic properties of these 2D materials, it is quite interesting to inject the spin current with suitable ferromagnetic contacts. For instance, black phosphorus is a layered material with a small Schottky barrier height capable of injecting spin current. This review includes many recently explored 2D magnetic materials ranging from exfoliated 2D crystals to CVD grown materials from single to several layers, demonstrating tunable layer-dependent magnetic properties. We also explore some of the promising theoretical study based on 2D magnetic compounds such as 2D alkali-based chromium chalcogenides, which shows ferromagnetic as well as semiconducting behavior. The layer-dependent magnetic ordering has been observed in layered compounds like 1T-CrTe2, VSe2, CrI3, and Fe3GeTe2, which have great potential for the future applications in magnetic-based electronic devices. Finally, we emphasize the challenges, opportunities, and future directions of the 2D magnetic materials, where new discoveries might have outstanding impact in transformational scientific breakthroughs towards memory, spintronics, optoelectronics, and other multifunctional device applications.

Investigation on structure and stability of cerium doped (Ba0.5Sr0.5)(Fe1-xCex)O3-δ (x = 0–1.0) oxides by Rietveld refinement

Perovskite-type materials have been attracted due to their relevant applications in memory devices, tunable microwave displays, oxygen-permeable membranes, sensors, and capacitors. An attempt has been made here to analyze the structure of (Ba0.5Sr0.5)(Fe1-xCex)O3-ẟ (x = 0–1.0) oxides by Rietveld refinement under different space groups. It possesses cubic (space group Pm3m, Z = 1), cubic with orthorhombic, and orthorhombic (space group Pnma, Z = 4) structures for the composition x = 0, 0.10–0.60, and 0.80–1.0, respectively. The amount of cubic phase decreases from 100% to 24% with cerium content (x) = 0–0.60 and complete transformation to the orthorhombic phase occurred at x = 0.80. Moreover, single – dual - single structural phase transitions have been observed. The average bond length decreases with cerium content. The bond angles along O–B–O and B–O–B are deviated significantly due to the tilting and rotation of (Ce,Fe)O6 octahedra. The presence of distortion/oxygen vacancies makes the compound useful in industrial applications.

A Visible Light Modulated Resistive Switching Memory Behaviors in the Ta/BiFeO3/Carbon/BaTiO3/Si Device

A bipolar resistance switching memory effect is observed in the Ta/BiFeO3/carbon/BaTiO3/Si structure. The resistance switching (RS) behavior can be modulated by an optical signal. The HRS/LRS resistance ratio and threshold voltage change remarkably under white-light irradiation. The trap-based charges in the Schottky-like depletion layers excited by the white light are responsible for the white-light controlled RS memory behaviors. This research is useful in exploring multi-functional materials and applications in optical-controlled nonvolatile memory devices.

Correlation driven topological nodal ring ferromagnetic spin gapless semimetal: CsMnF4

The observation of in-plane ferromagnetism in layered magnetic materials in conjunction with the topological nodal-ring dispersion in a spin gapless semimetal with 100 % spin polarization has a fertile ground for novel physics, rich scientific significance and for the next-generation advanced spintronic and topological devices. Topological nodal ring spin gapless semimetals with large band gap in the other spin channel prevents the spin leakage and are excellent spintronic materials. On the basis of density functional theory (DFT), we have studied the layered magnetic perovskite, CsMnF4 which is predicted to be a ferromagnetic insulator though the fellow compounds like AMnF4 (A = Na, K, Rb) are anti-ferromagnetic in nature. DFT + U calculations reveal that this layered system undergoes a transition from an insulating to half-semimetallic nature with decreasing on-site Hubbard Coulomb interaction, U. For U = 2.5 eV, we observe the topological nature in the system with the emergence of four Mexican hat like dispersions associated with band-flipping. Also, we calculated the magneto-crystalline anisotropic energy with inclusion of spin–orbit coupling (SOC) and found that the system consists of in-plane ferromagnetism. Transport properties infer huge anisotropy of one order of magnitude between ’a’ and ’c’ axes. Interestingly, the estimated Fermi velocities are 2.66 × 105 and 2.24 × 105 m s−1 for Z(=0) and Z(=0.5) plane respectively and are comparable to that of graphene, which might fetch applications in high speed spin electronic devices. The topological phase observed is robust to SOC and the band-crossings associated with nodal rings could be preserved by additional symmetry as the time-reversal symmetry breaks in magnetic systems. The nearly charge compensation observed from Fermi surfaces might fetch memory device applications.

Green Production of Lead-free Bzt-bct Thin Films for Applications in Mems Devices

This paper reports the production of Lead-free (1-x)(BaZr0.2Ti0.8O3)-x(Ba0.7Ca0.3O3) ferro/piezoelectric thin films by means of repeated chemical solution deposition routes. Two different kinds of precursor solutions were compared: i) a literature standard precursor solution involving the use of 2-methoxyethanol (2-MOE) which is a toxic and carcinogenic solvent and ii) a “greener” precursor solution based on 1-methoxy-2-propanol (PGME), a non-toxic and non-carcinogenic solvent. Morphology, crystal structure and ferroelectric behavior of the films is also reported.

Optical erasable bipolar resistive switching on TiO2 film in nanoscale

Controlling the resistive switching parameters by using external fields is of great importance in the application of the resistive memory devices. In this work, we investigate the effect of light illumination and oxygen atmosphere on the resistive switching of the anatase phase polycrystalline TiO2 thin film using the in-situ conductive atomic force microscopy and Kelvin probe force microscopy. A forming free bipolar resistive switching effect with ON/OFF ratio over 104 was achieved in nanoscale uniformly on the film. The conduction characteristics can be changed from the non-rectifying behavior in vacuum to the Schottky rectifying behavior in oxygen gas with nearly the same ON/OFF ratio. The reversible surface potential shift accompanied by the resistance switching was also observed. Furthermore, the resistance states and the surface potential contrast can be erased by 395 nm near-UV illumination, demonstrating a good optical erasing performance of the device. Combining the surface potential shift and the response to the illumination and oxygen gas, a charge trapping/detrapping mechanism was confirmed to account for the observed switching behavior. These results provide a new way to fabricate the controllable and optical erasable resistive switching devices in nanoscale.

Spin-Orbit Torque Switching of a High-Quality Perpendicularly Magnetized Ferrimagnetic Heusler Mn3Ge Film.

Current-induced spin-orbit torque (SOT) switching of magnetization has attracted great interest due to its potential application in magnetic memory devices, which offer low-energy consumption and high-speed writing. However, most of the SOT studies on perpendicularly magnetized anisotropy (PMA) magnets have been limited to heterostructures with interfacial PMA and poor thermal stability. Here, we experimentally demonstrate a SOT magnetization switching for a ferrimagnetic D022-Mn3Ge film with high bulk PMA and robust thermal stability factor under a critical current density of 6.6 × 1011 A m-2 through the spin Hall effect of an adjacent capping Pt and a buffer Cr layer. A large effective damping-like SOT efficiency of 2.37 mT/1010 A m-2 is determined using harmonic measurements in the structure. The effect of the double-spin source layers and the negative-exchange interaction of the ferrimagnet may explain the large SOT efficiency and the manifested magnetization switching of Mn3Ge. Our findings demonstrate that D022-Mn3Ge is a promising candidate for application in high-density SOT magnetic random-access memory devices.

Transport properties of B/P doped graphene nanoribbon field-effect transistor

In this work, we have studied the transport properties of a 2.55 nm graphene nanoribbon field-effect transistor by the first principle study. The gate voltage (Vgate) shows good control of the drain current (Ids), the Ids increases linearly with the increase of the positive bias voltage (Vbias) and the current density reaches 49 μA/nm. Under the negative Vbias, the device appears the negative differential resistance (NDR) effect and it shows the dependence of the doping positions. We have analyzed the transport mechanism by the band matching relation between two electrodes and the transmission spectra. The Molecular Projected Self-consistent Hamiltonian (MPSH) indicates the electron distribution in the scattering region and the transmission eigenvalues show the transmission probabilities of the electrons. The device shows the potential for applications in memory devices, oscillators and fast switching devices.

Interfacial resistive switching properties of Sr2TiO4/SrTiO3 heterojunction thin films prepared via sol-gel process

Layered perovskite SrO(SrTiO3)n (n = 1) and conventional perovskite SrTiO3 were fabricated as a heterojunction thin-film device. The Sr2TiO4/SrTiO3 heterostructure exhibited markedly improved resistive switching (RS) properties over single-phase Sr2TiO4 and pure strontium titanate. The other characteristics of the heterostructure, including stabilisation, high-pressure performance, and fatigue resistance, were investigated. The results showed a switch ratio of over 102, indicating a good RS effect. Various characterisation methods, such as X-ray diffraction, field emission scanning electron microscopy, and atomic force microscopy, were applied to analyse the crystal phase and surface morphology of the Sr2TiO4/SrTiO3 heterojunction thin film. The interfacial conduction mechanism was explained with an appropriate diagram. This study is the first to apply a first-principles analysis based on the density functional theory to elaborate the experimental results at the micro-level. The research provides a deeper understanding of the role of specific defects due to the heterostructure, which are strongly related to interface contact, and promotes the performance of Ruddlesden–Popper-phase strontium titanate, which has potential applications in resistive random access memory devices.

Aluminum Substitution in Ni-Co Based Spinel Ferrite Nanoparticles by Sol–Gel Auto-Combustion Method

In this research work, aluminum substituted Ni-Co ferrite nanoparticles have been produced by a simple and cost-effective method, i.e., sol–gel auto-combustion. Synthesized nanoparticles were annealed in a muffle furnace at 600°C for 3 h before characterization. The x-ray diffraction patterns revealed that the ferrite nanoparticles grew preferentially along the (311) plane and exhibit face centered cubic structure. The crystallite size of nanoparticles (14 to 17 nm) was estimated by Scherrer’s relation. The effect of aluminum substitution on structural parameters of ferrite nanoparticles, such as lattice constant and stacking faults, have been studied. Structural analysis revealed that the lattice constant of the nanoparticles decreases as a function of aluminum content. The Fourier transform infrared spectroscopy confirmed the spinal ferrite crystal structure of synthesized aluminum substituted Ni-Co ferrite nanoparticles. The surface morphology observed through scanning electron microscopy depicts the growth and distribution of nanograins with uniform size with in the samples. Dielectric properties investigated through impedance analyzer spectroscopy revealed that aluminum substituted Ni-Co ferrite nanoparticles demonstrated the high conductivity along with potential dielectric properties. These aluminum substituted Ni-Co ferrite nanoparticles would have possible applications in high storage memory and microwave devices.

Electrically programmable magnetoresistance in text{AlO}_{x}-based magnetic tunnel junctions

We report spin-dependent transport properties and I–V hysteresis characteristics in an text{AlO}_{x}-based magnetic tunnel junction (MTJ). The bipolar resistive switching and the magnetoresistances measured at high resistance state (HRS) and low resistance state (LRS) yield four distinctive resistive states in a single device. The temperature dependence of resistance at LRS suggests that the resistive switching is not triggered by the metal filaments within the text{AlO}_{x} layer. The role played by oxygen vacancies in text{AlO}_{x} is the key to determine the resistive state. Our study reveals the possibility of controlling the multiple resistive states in a single text{AlO}_{x}-based MTJ by the interplay of both electric and magnetic fields, thus providing potential applications for future multi-bit memory devices.

Hysteresis Behavior of the Donor-Acceptor-Type Ambipolar Semiconductor for Non-Volatile Memory Applications.

Donor–acceptor-type organic semiconductor molecules are of great interest for potential organic field-effect transistor applications with ambipolar characteristics and non-volatile memory applications. Here, we synthesized an organic semiconductor, PDPPT-TT, and directly utilized it in both field-effect transistor and non-volatile memory applications. As-synthesized PDPPT-TT was simply spin-coated on a substrate for the device fabrications. The PDPPT-TT based field-effect transistor showed ambipolar electrical transfer characteristics. Furthermore, a gold nanoparticle-embedded dielectric layer was used as a charge trapping layer for the non-volatile memory device applications. The non-volatile memory device showed clear memory window formation as applied gate voltage increases, and electrical stability was evaluated by performing retention and cycling tests. In summary, we demonstrate that a donor–acceptor-type organic semiconductor molecule shows great potential for ambipolar field-effect transistors and non-volatile memory device applications as an important class of materials.

A study of enhanced structural, microstructural and dielectric behaviour of aliovalent ions doped BaZr0.05Ti0.95O3 ceramic

The lead-free ceramics are considered to be the best substitutes for lead based ceramics which have tremendous harmful effects regarding environmental pollution. In this concern, the lead-free ceramics have been widely attracted by research communities. In the present study we have prepared lead free BaZr0.05Ti0.95O3perovskite ceramics with various doping concentrations of Gd3+ ions by the conventional solid-state reaction method. The perovskite phase formation was studied by x-ray diffraction which indicate the transformation of crystal symmetry from orthorhombic to tetragonal structure for the Gd3+ ions doped samples. The scanning electron microscopy studies revealed the modification in grain size on doping Gd ions also the energy dispersive X-ray spectra have been obtained to study the compositional variations. The dielectric and loss studies have been performed in the large range of temperature and frequency. The low dielectric loss and high dielectric constant suggests possible applications in memory devices and ceramic capacitors. Temperature coefficient of capacitance plots have also been plotted.

Optically Controlled Ferroelectric Nanodomains for Logic-in-Memory Photonic Devices With Simplified Structures

We report visible-light-induced ferroelectric nanodomain reversal in conductive van der Waals (vdW) ferroelectric $\alpha $ -In2Se3. We show its promising application in two-terminal optoelectronic memory devices. Compared to other vdW material-based optoelectronic memories, such a novel working prototype allows for the device operation confined within a single material channel bridging its two electrodes, which greatly reduces the complexities of device construction. In addition, using $\alpha $ -In2Se3 memory devices, we also demonstrate the universal OR and AND optical logic gates for logic-in-memory application. Our results provide a new avenue to design simplified structures of vdW material-based optoelectronic memories for dense device integration and next-generation computation.

Borophene-Functionalized Magnetic Nanoparticles: Synthesis and Memory Device Application

Borophene has been proposed to be an energetic and promising material for electronic, optical, and thermal domains, but these applications have not been still achieved due to the metallic feature a...

Flexophotovoltaic Effect in Potassium Sodium Niobate/Poly(Vinylidene Fluoride-Trifluoroethylene) Nanocomposite.

Flexoelectricity is an electromechanical coupling effect in which electric polarization is generated by a strain gradient. In this investigation, a potassium sodium niobite/poly(vinylidene fluoride‐trifluoroethylene) (KNN/PVDF‐TrFE)‐based nanocomposite is fabricated, and the flexoelectric effect is used to enhance the photovoltaic current (I pv) in the nanocomposite. It is found that both a pyroelectric current and photovoltaic current can be generated simultaneously in a light illumination process. However, the photovoltaic current (I pv) in this process contributes ≈85% of the total current. When assessing the effect of flexoelectricity with a curvature of 1/20, the I pv of the curved KNN/PVDF‐TrFE (20%) (K/P‐20) composite increased by ≈13.9% compared to that of the flat K/P‐20 nanocomposite. Similarly, at a curvature of 1/20, the I pv of the K/P‐20 nanocomposite is 71.6% higher than that of the PVDF‐TrFE film. However, the photovoltaic effect induced by flexoelectricity is much higher than the increased polarization from flexoelectricity, so this effect is called as the flexophotovoltaic effect. Furthermore, the calculated energy conversion efficiency of the K/P‐20 film is 0.017%, which is comparable to the previous research result. This investigation shows great promise for PVDF‐based nanocomposites in ferroelectric memory device applications.