Nonvolatile Memory Elements Research Articles

Redox-Gated Molecular Memory Devices Based on Dynamic Doping of Polythiophene

A solid state electrochemical cell resembling a field effect transistor will be described which can act as a nonvolatile memory element. “Write” and “Erase” pulses cause oxidation of a polythiophene to its conducting form, while the conductance is monitored by a “read” circuit. Raman and UV-Vis spectroelectrochemistry were used during device operation to investigate redox reactions which underlie memory operation. Important electrochemical concepts which control device operation include activated electron transfer, ion motion, and extended conjugation. In particular, internal ion transport creates ohmic potential losses which slow W/E operation, but can be significantly improved with higher mobility solid electrolytes. Applications seeking to augment existing silicon electronics with molecular components will be described.Recent references:(1) Yan, H.; Bergren, A. J.; McCreery, R.; Della Rocca, M. L.; Martin, P.; Lafarge, P.; Lacroix, J. C.; Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions; Proceedings of the National Academy of Sciences 2013, 110, 5326.(2) McCreery, R.; Yan, H.; Bergren, A. J.; A Critical Perspective on Molecular Electronic Junctions: There is Plenty of Room in the Middle; Phys. Chem. Chem. Phys. 2013, 15, 1065.(3) Sayed, S. Y.; Fereiro, J. A.; Yan, H.; McCreery, R. L.; Bergren, A. J.; Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime; Proceedings of the National Academy of Sciences 2012, 109, 11498.(4) Kumar, R.; Pillai, R. G.; Pekas, N.; Wu, Y.; McCreery, R. L.; Spatially Resolved Raman Spectroelectrochemistry of Solid-State Polythiophene/Viologen Memory Devices; Journal of the American Chemical Society 2012, 134, 14869.

Storage of Electrical Information in Metal–Organic‐Framework Memristors

Single crystals of a cyclodextrin-based metal-organic framework (MOF) infused with an ionic electrolyte and flanked by silver electrodes act as memristors. They can be electrically switched between low and high conductivity states that persist even in the absence of an applied voltage. In this way, these small blocks of nanoporous sugar function as a non-volatile RRAM memory elements that can be repeatedly read, erased, and re-written. These properties derive from ionic current within the MOF and the deposition of nanometer-thin passivating layers at the anode flanking the MOF crystal. The observed phenomena are crucially dependent on the sub-nanometer widths of the channels in the MOF, allowing the passage of only smaller ions. Conversely, with the electrolyte present but no MOF, there are no memristance or memory effects.

Resistive switching in metal-insulator-metal structures based on germanium oxide and stabilized zirconia

Bipolar resistive switching in metal-insulator-metal structures based on a double-layer insulator composed of a layer of yttria-stabilized zirconia (YSZ) containing 12 mol % Y2O3 and a layer of GeOx is studied. It is shown that the incorporation of an additional GeOx layer into the structure leads to a significant decrease in the variation of resistive switching parameters at both negative and positive voltages. Au/Zr/GeOx/YSZ/TiN structures exhibit a high stability of the resistance ratio in high-resistance and low-resistance states during cyclic switching. The studied structures can be used for designing next-generation nonvolatile memory elements.

Influence of oxygen content of room temperature TiO2−x deposited films for enhanced resistive switching memory performance

In this work, we demonstrate that TiO2−x based Resistive Random Access Memory devices can function without an initial electroforming process and a wide range of switching ratios could be achieved by controlling the oxygen content, the compliance current, the sweep bias amplitude, and the width of the voltage pulse applied on the memory cell. The influence of deposition ambient and more particularly of oxygen flux during thin film sputtering at room temperature to the resistive properties of titanium oxide will be discussed in detail. By controlling the density of oxygen vacancies into the dielectric matrix, we can also improve the repeatability and the operation of the device, in terms of distribution of the SET/RESET voltages. We propose that ultra high density of vacancies deteriorate the switching phenomenon, whereas high vacancy density results in better switching behavior. Moreover, we conclude that the oxygen vacancies density and distribution have a direct impact on the conducting filament diameter, in terms of sensitivity of the conducting paths (high OFF/ON ratio). By increasing the oxygen content, we reduce the size of vacancy based filaments, resulting in a more stable operation of our device. In addition, manipulation of population of oxygen ions into the Ti top electrode enables the creation of multilevel switching states. Switching speed, endurance, and retention performance reveals the excellent functionality of our device as a non-volatile memory element and conduction mechanism analysis demonstrates the manifestation of Poole-Frenkel emission in conjunction with trap-assisted tunneling, which is also deployed in order to interpret the gradual increase of current during SET process.

A 3.4-pJ FeRAM-Enabled D Flip-Flop in 0.13-$\mu \hbox{m}$ CMOS for Nonvolatile Processing in Digital Systems

In order to realize a digital system with no distinction between “on” and “off,” the computational state must be stored in nonvolatile memory elements. If the energy cost and time cost of managing the computational state in nonvolatile memory can be lowered to the microsecond and picojoule-per-bit level, such a system could operate from unreliable harvested energy, never requiring a reboot. This work presents a nonvolatile D-flip-flop (NVDFF) designed in 0.13- μm CMOS that retains state in ferroelectric capacitors during sporadic power loss. The NVDFF is integrated into an ASIC design flow, and a test-case nonvolatile FIR filter with an accompanying power management unit automatically saves and restores the state based on the status of a one-bit indicator of energy availability. Correct operation has been verified over power-cycle intervals from 4.8 μs to 1 day. The round-trip save-restore energy is 3.4 pJ per NVDFF. Also presented are statistical measurements across 21 \thinspace000 NVDFFs to validate the capability of the circuit to achieve the requisite 10-ppm failure rate for embedded system applications.

Resistance Switching in Metal Oxide Thin Films and its Memory Application

We explore the unipolar resistance switching effect in sandwich structures based on Nb, Ta and Zr oxide thin films. The structures were fabricated by pulsed laser deposition and low temperature anodic oxidation methods. After electroforming process memory cells demonstrate reproducible switching between low and high resistance states with a resistance ratio around 102. Nonvolatile resistance storage was traced within 40 days. The low-temperature anodic oxidation of Nb was found to be suitable to fabricate flexible nonvolatile memory elements. The parameters of resistive switching are not degraded after 100000 flexing.

Thermal effects in magnetoelectric memories with stress-mediated switching

Heterostructures with magneto-electro-elastic coupling (e.g. multiferroics) are of paramount importance for developing new sensors, actuators and memories. With the progressive miniaturization of these systems it is necessary to take into account possible thermal effects, which may influence the normal operating regime. As a paradigmatic example we consider a recently introduced non-volatile memory element composed of a magnetostrictive nanoparticle embedded in a piezoelectric matrix. The distributions of the physical fields in this matrix/inclusion configuration are determined by means of the Eshelby theory, the magnetization dynamics is studied through the Landau–Lifshitz–Gilbert formalism, and the statistical mechanics is introduced with the Langevin and Fokker–Planck methodologies. As result of the combination of such techniques we determine the switching time between the states of the memory, the error probability and the energy dissipation of the writing process. They depend on the ratio kBT/v where T is the absolute temperature and v is the volume of the magnetoelastic particle.

Multilevel magnetization switching by electric field in c-axis oriented polycrystalline Z-type hexaferrite

Direct and converse magnetoelectric (ME) effects, namely, magnetic-field (B) induced electric polarization (P) and electric-field (E) induced magnetization (M), respectively, were investigated at room temperature for c-axis oriented polycrystalline specimens of a Z-type hexaferrite, Sr3Co2Fe24O41. The B profile of the linear ME coefficient obtained from the converse effect well coincides with that obtained from the direct effect. Furthermore, M-E curves show a substantial hysteretic behavior, which allows reversal and multilevel switching of M by applying pulsed E. The present results demonstrate the feasibility of nonvolatile memory elements by using the ME Z-type hexaferrite at room temperature.

A room-temperature non-volatile CNT-based molecular memory cell

Recent experiments with a carbon nanotube (CNT) system confirmed that the innertube can oscillate back-and-forth even under a room-temperature excitation. This demonstration of relative motion suggests that it is now feasible to build a CNT-based molecular memory cell (MC), and the key to bring the concept to reality is the precision control of the moving tube for sustained and reliable read/write (RW) operations. Here, we show that by using a 2-section outertube design, we are able to suitably recalibrate the system energetics and obtain the designed performance characteristics of a MC. Further, the resulting energy modification enables the MC to operate as a non-volatile memory element at room temperatures. Our paper explores a fundamental understanding of a MC and its response at the molecular level to roadmap a novel approach in memory technologies that can be harnessed to overcome the miniaturization limit and memory volatility in memory technologies.

The influence of shape anisotropy on vortex nucleation in Pacman-like nanomagnets

In this paper we explore magnetic properties of Permalloy Pacman-like (PL) nanomagnets in external in-plain magnetic field. PL nanomagnets represent unique magnetic systems with broken symmetry, which are perspective as non-volatile memory elements. In these nanomagnets both bits, chirality and polarity of a single vortex state, can be easily read and written by in-plane magnetic field only. In the experimental part of this work we show that namely chirality of the∼1-μm large PL nanomagnet can be red easily by magnetic force microscopy method. The easy bit reading is enabled due to coupling of the polarity magnetization vector to the magnetic charges located at the surface of the PL missing sector. Using micromagnetic simulations we show the influence of spatial anisotropy on vortex nucleation and annihilation fields in the PL nanomagnets. Angular dependence of the vortex nucleation field is analysed in detail for PL nanomagnets of different diameter, thickness, and missing-sector dimensions. Best control of the ground state can be achieved for diameters not exceeding 100nm, thicknesses from 40 to 45nm, and for the missing sector angles from 30 to 60°.

Control over the interface properties of carbon nanotube-based optoelectronic memory devices

Optoelectronic nonvolatile memory elements based on polymer-coated carbon nanotube devices can serve as building blocks in programmable circuits. Although essential for improving the circuit performances, the details of the charge trapping mechanism in these mixed organic/inorganic optoelectronic devices are not fully elucidated. The detailed mechanism was investigated by intercalating layers of a hydrophobic organic dielectric (parylene) at different interfaces in the device structure. A thin parylene layer separating the SiO2/nanotube interface from the photosensitive polymer coating is presented as an optimized solution in terms of charging, stability, and robustness.

Antihysteresis of the electrical resistivity of graphene on a ferroelectric Pb(Zr x Ti1 − x )O3 substrate

A quantitative model is developed to explain the antihysteretic behavior of the electrical resistivity of graphene on a ferroelectric Pb(Zr x Ti1 − x )O3 substrate as a function of the gate voltage. The model takes into account the trapping of the electrons from the graphene layer by the states related to the graphene-ferroelectric interface. The finite energy gap of impurity states is also taken into account, which makes it possible to describe the well-known experimental dependences, including an increase and the subsequent saturation of a “memory window” with the switching gate voltage. The obtained estimates can be important for creating next-generation nonvolatile memory elements, which use the two stable values of electrical resistivity (one of them is attributed to logical “0” and the other, to “1”) that result from the antihysteresis effect.

Photonic non-volatile memories using phase change materials

We propose an all-photonic, non-volatile memory, and processing element based on phase-change thin-films deposited onto nanophotonic waveguides. Using photonic microring resonators partially covered with Ge2Sb2Te5 (GST) multi-level memory operation in integrated photonic circuits can be achieved. GST provides a dramatic change in refractive index upon transition from the amorphous to crystalline state, which is exploited to reversibly control both the extinction ratio and resonance wavelength of the microcavity with an additional gating port in analogy to optical transistors. Our analysis shows excellent sensitivity to the degree of crystallization inside the GST, thus providing the basis for non-von Neumann neuromorphic computing.

Combined nanomechanical and nanomagnetic analysis of magnetoelectric memories

Magneto-electro-elastic and multiferroic materials can be combined in appealing nanostructures characterized by the coexistence and coupling of electric, magnetic, and mechanical phases with potential applications in novel multifunctional devices. Here, we derive a theory for nonvolatile room-temperature memory elements composed of magnetostrictive nanoparticles embedded in a piezoelectric matrix: two stable orthogonal magnetization states are obtained by the competition of anisotropy and external magnetic polarization. The innovative nontoggle switching between the states is modeled by a thorough combination of the nanomechanical Eshelby approach with the nanomagnetic Landau-Lifshitz-Gilbert formalism, yielding a robust picture of the dynamical behavior and allowing the improvement of the energetic efficiency.

Non-volatile memory and IR radiation modulators based upon graphene-on-ferroelectric substrate. A review

I present a review of both experimental and theoretical studies performed during the recent three years, which deal with the physical properties and possible applications of graphene placed on ferroelectric (organic or Pb(ZrxTi1-x)O3 (PZT) ceramic) substrates. The system 'graphene-on-ferroelectric substrate' has unique advantages in comparison with the graphene deposited on SiO2 or on dielectrics with high dielectric permittivity. In particular, one can obtain high (~ 1012 cm-2) carrier concentrations in the doped graphene-on-ferroelectric structures for moderate (of the order of 1 V) gate voltages. The existence of a hysteresis (or anti-hysteresis) in the dependence of electrical resistance of graphene channel on the gate voltage facilitates creating bistable systems for different applications. The use of ferroelectric substrates has already enabled developing of robust elements of non-volatile memory of a new generation. These elements operate for more than 105 switching cycles and store information for more than 103 s. Such systems can be characterised theoretically by ultrafast switching rates (~ 10-100 fs). A theoretical analysis has also demonstrated that the structures 'graphene-on-PZT ferroelectric substrate' would result in developing efficient and fast small-sized modulators of mid-IR and near-IR radiations for different optoelectronic applications.

Mechanical memory sees the light

A nanomechanical beam coupled to an optical cavity can be operated as a non-volatile memory element.

Dynamic manipulation of nanomechanical resonators in the high-amplitude regime and non-volatile mechanical memory operation

The ability to control mechanical motion with optical forces has made it possible to cool mechanical resonators to their quantum ground states. The same techniques can also be used to amplify rather than reduce the mechanical motion of such systems. Here, we study nanomechanical resonators that are slightly buckled and therefore have two stable configurations, denoted 'buckled up' and 'buckled down', when they are at rest. The motion of these resonators can be described by a double-well potential with a large central energy barrier between the two stable configurations. We demonstrate the high-amplitude operation of a buckled resonator coupled to an optical cavity by using a highly efficient process to generate enough phonons in the resonator to overcome the energy barrier in the double-well potential. This allows us to observe the first evidence for nanomechanical slow-down and a zero-frequency singularity predicted by theorists. We also demonstrate a non-volatile mechanical memory element in which bits are written and reset by using optomechanical backaction to direct the relaxation of a resonator in the high-amplitude regime to a specific stable configuration.

Bistable out-of-plane stress-mismatched thermally actuated bilayer devices with large deflection

In this paper, we explore microfabricated bistable actuators released as thin films from a silicon wafer. The actuators are based on a serpentine design where two cantilevers are coupled at the tips by a thin-film bar. These devices are parameterized by two lengths: cantilever length and the length of the coupling bar. These two dimensions are systematically varied to study the effect of design parameters on bistability. The three-dimensional devices have extremely large deflection (hundreds of microns rather than tens of microns for most planar microactuators of similar size) and are thermally actuated out of the plane of the wafer by applying a bias across either the left or right side of the serpentine. The bistability of these devices is evaluated using electron and optical microscopy. Potential applications include non-volatile mechanical memory, optical shutters, and reconfigurable antenna elements.

Magnetic crossover effect in Nickel nanowire arrays

A crossover effect in the magnetic reversal mechanism within arrays of Nickel nanowires whose diameter varies from 15 to 100 nm is observed around 50 nm. Hysteresis loops and FMR measurements confirm that nanowire diameter controls effectively the nanowire easy axis as well as the magnetization reversal mechanism. This might be very interesting for spintronic devices based on current-induced domain motion such as non-volatile magnetic memory elements (MRAM) and low Ohmic loss devices.

Engineering carbon chains from mechanically stretched graphene-based materials

The electrical response of graphene-based materials can be tailored under mechanical stress. We report different switching behaviors that take place in mechanically deformed graphene nanoribbons prior to the breakage of the junction. By performing tight-binding molecular dynamics, the study of structural changes of graphene nanoribbons with different widths is achieved, revealing that carbon chains are the ultimate bridges before the junction breaks. The electronic and transport calculations show that binary on/off states can be switched prior to and during breakage depending on the atomic details of the nanoribbon. This work supports the interpretation of recent experiments on nonvolatile memory element engineering based on graphene break junctions.