Read Pulse Research Articles

Resistive switching in $$\hbox {BiFeO}_3$$ BiFeO 3 -based heterostructures due to ferroelectric modulation on interface Schottky barriers

Ferroelectric $$\hbox {BiFeO}_3$$ (BFO) thin films were deposited on (001) $$\hbox {SrTiO}_3$$ substrates buffered with $$\hbox {La}_{0.7}\hbox {Sr}_{0.3}\hbox {MnO}_3$$ (LSMO) electrodes. Bipolar resistive switching in Pt/BFO/LSMO heterostructures were observed with high stability and long retention. However, transport characteristics of Pt/BFO/LSMO is highly asymmetric and pronounced resistive switching can only be observed by applying negative reading pulses on the Pt top electrodes, i.e. when the Pt/BFO Schottky barrier is reverse-biased. This resistive switching is discussed in terms of a modulation on the Pt/BFO interface Schottky barrier by the polarization switching in ferroelectric BFO. Comparative studies on Pt/BFO/ $$\hbox {SrRuO}_3$$ and Pt/BFO/ $$\hbox {LaNiO}_3$$ heterostructures reveal that the work function of the electrode materials and the formation of Schottky barriers are significant to the observed resistive switching behaviors.

Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaOx interface.

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >105 cycles, and good data retention of >104 s with a good resistance ratio of >102 at 85°C are obtained using a Ti nanolayer to form a W/TiOx/TaOx/W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaOx/W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm2 is observed by transmission electron microscopy. The presence of oxygen-deficient TaOx nanofilament in a W/TiOx/TaOx/W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaOx interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.2.

Dramatic reduction of read disturb through pulse width control in spin torque random access memory

Magnetizations dynamic effect in low current read disturb region is studied both experimentally and theoretically. Dramatic read error rate reduction through read pulse width control is theoretically predicted and experimentally observed. The strong dependence of read error rate upon pulse width contrasts conventional energy barrier approach and can only be obtained considering detailed magnetization dynamics at long time thermal magnetization reversal region. Our study provides a design possibility for ultra-fast low current spin torque random access memory.

Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaO x interface.

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >105 cycles, and good data retention of >104 s with a good resistance ratio of >102 at 85°C are obtained using a Ti nanolayer to form a W/TiOx/TaOx/W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaOx/W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm2 is observed by transmission electron microscopy. The presence of oxygen-deficient TaOx nanofilament in a W/TiOx/TaOx/W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaOx interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.2.

Low operation voltage macromolecular composite memory assisted by graphene nanoflakes

The trend towards simple and low-cost processing is one of the most important for macromolecular memory development. Here, bistable memory devices using a solution-processable active material, a mixture of graphene nanoflakes (GNFs) and insulating poly(vinyl alcohol) (PVA), are investigated, which serve as the first example for the direct integration of as-prepared nanoscale graphene into macromolecular memory devices through a one-step low-temperature processing method. Bistable electrical switching behavior and nonvolatile rewritable memory effects are realized by using an indium–tin-oxide/GNF–PVA/silver (ITO/GNF–PVA/Ag) sandwich structure. The resulting device exhibits low operation voltages of +1.4 V (turn-on) and −1.3 V (turn-off), which is promising for memory cells with low power consumptions. The programmable ON- and OFF-states possess a retention time of over 104 s and endure up to 107 read pulses. The carrier transport in the OFF- and ON-states follows the typical trap-limited space charge limited current and Ohmic laws, respectively. The asymmetric electrical switch behavior is therefore attributed to conducting filaments formed in the PVA layer assisted by the charged GNFs that induce the transition of the conductivity. Our study provides a potential approach for integrating as-prepared graphene into macromolecular memory devices with excellent performances through a simple solution-process.

New Donor–Acceptor Oligoimides for High-Performance Nonvolatile Memory Devices

We report the synthesis, optoelectronic properties, and electrical switching memory characteristics of three new donor–acceptor oligoimides consisting of the electron-donating moieties (triphenylamine or carbazole) and electron-withdrawing phthalimide moieties. The influence of different donor (D)–acceptor (A) arrangements, including D-A-D and A-D-A structures, on the electrical properties was explored. Devices based on D-A-D oligoimides revealed a reversible nonvolatile negative-differential-resistance (NDR) characteristic and excellent stability during operation. Without applying voltage stress, the on and off states of the devices showed no obvious degradation for an operation time of 10 s and 108 read pulses. However, the devices prepared from the A-D-A oligoimide showed only the insulating properties. The different memory characteristic was probably because the terminal donor moieties in the D-A-D structure might facilitate the injection and transporting of the holes. Besides, the D-A-D oligoimide with triphenylamine groups exhibited an on/off ratio of 104, 2 orders of magnitude higher than that with carbazole groups. The mechanism related to electrical switching properties was elucidated through molecular simulation. Thus the significance of D-A-D structure on tuning memory characteristics for memory device applications was revealed.

Resistive switching characteristics of polyimides derived from 2,2′‐aryl substituents tetracarboxylic dianhydrides

Abstract2,2′‐Position aryl‐substituted tetracarboxylic dianhydrides including 2,2′‐bis(biphenyl)‐4,4′,5,5′‐biphenyl tetracarboxylic dianhydride and 2,2′‐bis[4‐(naphthalen‐1‐yl)phenyl)]‐4,4′,5,5′‐biphenyl tetracarboxylic dianhydride were synthesized. A new series of aromatic polyimides (PIs) were synthesized via a two‐step procedure from 3,3′,4,4′‐biphenyl tetracarboxylic dianhydride and the newly synthesized tetracarboxylic dianhydrides monomers reacting with 2,2′‐bis[4′‐(3″,4″,5″‐trifluorophenyl)phenyl]‐4,4′‐biphenyl diamine. The resulting polymers exhibited excellent organosolubility and thermal properties associated with Tg at 264 °C and high initial thermal decomposition temperatures (T5%) exceeding 500 °C in argon. Moreover, the fabricated sandwich structured memory devices of Al/PI‐a/ITO was determined to present a flash‐type memory behaviour, while Al/PI‐b/ITO and Al/PI‐c/ITO exhibited write‐once read‐many‐times memory capability with different threshold voltages. In addition, Al/polymer/ITO devices showed high stability under a constant stress or continuous read pulse voltage of − 1.0 V. Copyright © 2011 Society of Chemical Industry

Flash memory effects based on styrene/maleimiade copolymers with pendant azobenzene chromophores

Two styrene/maleimiade copolymers with pendant azobenzene chromophores, poly(styrene-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PS-DP) and poly((4-vinyl-benzyl)-9H-carbazole-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP), were synthesized. The polymeric memory devices based on each of the two polymer films (ITO/Polymer/Al) show similar rewritable flash memory behaviors but different transition voltages. By introduction of carbazole groups in the polymer side chains, the voltage difference from OFF to ON state of ITO/PVCz-DP/Al is reduced obviously in comparison with that of ITO/PS-DP/Al, which is beneficial to the protection of devices. Both ITO/PS-DP/Al and ITO/PVCz-DP/Al show high stability under a constant stress or continuous read pulses voltage of 1.0V. The memory mechanism is governed by space-charge limited conduction (SCLC) on the basis of the I–V curves of these fabricated memory devices. With excellent flash memory characteristics and simple processability, the memory devices fabricated with these two styrene/maleimiade copolymers have potential applications for the future electronic memory devices.

New random copolymers with pendant carbazole donor and 1,3,4-oxadiazole acceptor for high performance memory device applications

New non-conjugated random copolymers containing pendent electron-donating 9-(4-vinylphenyl)carbazole (VPK) and electron-accepting 2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (OXD) or 2-(4-vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD) were successfully synthesized by nitroxide-mediated free radical polymerization (NMRP) method. The prepared random copolymers are denoted as P(VPKxOXDy) or P(VPKxBOXDy) with three different electron donor/acceptor (x/y) ratios of 8/2, 5/5, and 2/8. The electrical switching behavior based on the ITO/polymer/Al device configuration could be tuned through the donor/acceptor ratio or acceptor trapping ability. Both experimental and theoretical results indicated that the charge transfer between the pendant donor and acceptor was relatively weak without significant orbital hybridization. In addition, the low-lying HOMO energy level of OXD or BOXD units as compared to VPK units created the trapping environment. Therefore, distinct electrical current–voltage (I–V) characteristics changed between the diode, the volatile memory, and the insulator depending on the relative donor/acceptor ratios of 10/0, 8/2, and (5/5, 2/8 and 0/10), respectively. The memory device based on P(VPK8OXD2) or P(VPK8BOXD2) copolymers exhibited volatile static random access memory (SRAM) behavior with an ON/OFF current ratio of approximately 104–105, up to 107 read pulses, and retention time of more than 1 h. The unstable ON state in the device was due to the shallow trapped holes with spontaneously back transferring of charge carriers when the electric field was removed and thus exhibited a volatile nature. The slightly lower HOMO level of OXD moieties than that of BOXD led to the P(VPK8OXD2) device storing the charge for a longer period of time. The present study suggested the high performance polymer memory devices could be achieved by changing the donor/acceptor ratio or chemical structure.

Study of ferroelectric properties of spray pyrolysis deposited cesium nitrate films

Cesium nitrate (CsNO 3) films were prepared by ultrasonic spray pyrolysis technique at different substrate temperatures ( T s) and their ferroelectric and switching properties were examined. The morphology of the deposited films was studied using FESEM. The ferroelectric properties were optimized based on remanent polarization as a function of substrate temperature. The switching response was studied using pulse width technique. The switching parameters such as effective dimensionality, switching time, and nucleation rate were extracted using finite grain model (FGM). The width of reading pulse shows significant effect on switching transients.

Molecular Monolayer Nonvolatile Memory with Tunable Molecules

An unforgettable monolayer: Dialkylthiolate-tethered metal complex self-assembled monolayers (see picture; thickness 3 nm) provide stable and reproducible molecular monolayer nonvolatile memory (MMNVM) characterized by its hysteretic current–voltage (I–V) properties and retention time for write–multiple read–erase–multiple read pulse cycles. This is the first known voltage-driven MMNVM using only a molecular monolayer.

Frequency-shifted light storage via stimulated Brillouin scattering in optical fibers

A method for storing optical data pulse sequences, frequency shifted with respect to the original data pulse frequency, is theoretically described and experimentally demonstrated. Data pulses are converted into long-living acoustic waves via stimulated Brillouin scattering in optical fiber by counterpropagating write pulses of one frequency, and later they are retrieved by read pulses at a different frequency giving rise to frequency-shifted stored pulses. The shifted frequency is governed by the phase-matching condition between the read pulse and the acoustic wave, which can be satisfied using birefringent fibers. The converted frequency is +/-52 GHz and is tuned by applying strain to the fiber with a slope coefficient of +/-1.8 MHz/micro epsilon, and conversion efficiency can be as high as 13% for the storage time of 8-25 ns.

Optimized Quantitative DEPT and Quantitative POMMIE Experiments for 13C NMR

In quantitative analysis, inverse gated (1)H decoupled (13)C NMR provides higher resolution than (1)H NMR. However, due to the lower sensitivity and longer relaxation time, (13)C NMR experiment takes much longer time to obtain a spectrum with adequate signal-to-noise ratio. The sensitivity can be enhanced with DEPT and INEPT approaches by transferring polarization from (1)H (I) to (13)C (S), but since the enhancements depend on coupling constants ( (1) J SI) and spin systems (SI, SI 2, SI 3), the enhancements for different spin systems are not uniform and quantitative analyses are seriously affected. To overcome these problems, Henderson proposed a quantitative DEPT (Q-DEPT) method by cycling selected read pulse angles and polarization-transfer delays (Henderson, T. J. J. Am. Chem. Soc. 2004, 126, 3682-3683), and satisfactory results for SI system are achieved. However, the optimization is incomplete for the SI 2 and SI 3 systems. Here, we present an improved version of Q-DEPT (Q-DEPT (+)) and a quantitative POMMIE (Q-POMMIE) where the cyclic delays and read pulse phases are applied. The improved methods prove to be suitable for all spin systems over a large J-coupling range (90-230 Hz), and the (13)C signals are nearly equally enhanced with standard deviation less than 5%.

Thermally stable polymer memory devices based on a π-conjugated triad

Polymer memory devices based on a π-conjugated poly [9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] covalently bridged, Disperse Red 1 (a dye) triad were fabricated. The devices exhibit electrical bistability in the I-V characteristics and can be used to perform read-write-erase memory functions. The memory devices exhibit good performance with an on/off current ratio up to 103 and stable on and off states under a constant voltage stress and read pulses. Furthermore, memory retention tests show that it is possible to preserve both states at 150°C under ambient atmosphere for about 1h when using Cu as the top electrode.

Nuclear quadrupole resonance exchange spectroscopy with shaped radiofrequency pulses

Recent work on the nuclear quadrupole resonance (NQR) investigation of molecular dynamics in the solid state has relied on 2D methods. We report our studies of dynamic processes by 1D shaped pulse NQR spectroscopy. Significant advantages include considerably shorter experimental duration, clear definition of the exchange time window, and avoidance of off-resonance effects. The reorientation of the Cl3C˜ group in polycrystalline chloral hydrate [Cl3C–CH(OH)2] is considered as a test case. This may be modelled as a three-site exchange process. An analysis of the generalised Bloch–McConnell equation is performed to formulate the kinetic matrix. The present approach involves simultaneous excitation of the sites that undergo chemical exchange by employing a suitably modulated shaped RF pulse, followed by a mixing time, and finally a suitable read pulse for signal detection. The experimental signal intensities are plotted against the mixing time to extract the kinetic parameters, i.e. the exchange rate and the spin-lattice relaxation rate. Variable temperature measurements are carried out to determine the activation parameters. Short experiment times are possible in our 1D mode, enabling a large number of runs to be readily performed as a function of mixing time and temperature. The kinetic and activation parameters obtained in the case of chloral hydrate are in good agreement with recent literature values.

Two-Dimensional Coding for Probe Recording on Magnetic Patterned Media

The effect of intertrack intersymbol interference in a magnetic patterned medium is studied. A 2-D channel code is proposed, dedicated to perpendicularly magnetized media without soft underlayer, which exhibit read pulses showing overshoot. Read pulse shapes were investigated using a magnetic-force microscope tip scanning the patterned medium row by row. To test different codes, a bit-detection simulation program was developed to generate large amounts of data on which bit error rates can be measured. Application of the 2-D channel code, which implies recording of particular dot positions with fixed bits (ones, zeros) resulted in the elimination of 2-D worst case bit patterns and a subsequent reduction of detected-bit errors. The accompanying redundancy of 22% is inevitable for this type of 2-D code

Multiple quantum correlated spectroscopy revamped by asymmetric z-gradient echo detection signal intensity as a function of the read pulse flip angle as verified by heteronuclear H1∕P31 experiments

Heteronuclear multiple quantum (n=+/-0 and n=+/-2) correlated spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) experiments were performed on the spins 31P and 1H in a H3PO4 solution in order to determine the optimum flip angle for the read pulse. It has been shown that for the negative quantum signals, the maximum signals appear at beta=0, and for the positive quantum signals, the maximum signals appear at beta=pi. The CRAZED signals were compared to the single quantum signals in two-pulse two-gradient experiments. It is found that the CRAZED signals can also be distinguished into gradient echoes and spin echoes. The gradient-echo-type CRAZED signal requires beta=0 and the spin-echo-type CRAZED signal requires beta=pi for maximum echo intensities, in the same way as in single quantum experiments.

One In, One Out

PHYSICS The successful development of optical-based quantum information processing and quantum cryptography will require the ability to store and retrieve known numbers of photons in a medium of choice. Despite significant progress in techniques to store single photons within a cloud of rubidium or cesium atoms, the overall efficiency of the storage and retrieval process in such systems has been limited by low retrieval efficiencies and relatively high noise levels. Laurat et al. show that the retrieval efficiency of single excitations stored in an ensemble of cold cesium atoms can be increased by careful optimization of the experimental parameters. The authors found that by increasing the number of photons in each read pulse to approximately 107 and increasing the optical depth of the atomic ensemble, they could raise retrieval efficiency to ∼50%, with a concurrent order-of-magnitude reduction in two-photon emission events. They argue that such an improvement bodes well for long-distance quantum communication. — ISO Opt. Express 14 , 6912 (2006).

Relevance of the pulsed capacitance–voltage measurement technique for the optimization of SrBi 2Ta 2O 9/high-k stack combination to be used in FeFET devices

The ferroelectric field effect transistor (FeFET) with metal–ferroelectric–insulator–semiconductor (MFIS) structure has attracted a lot of interest recently as a nonvolatile memory device. For best performances of FeFET with ferroelectric/high-k gate stacks, the deposition techniques as well as the materials for both the ferroelectric layer and the high-k buffer layer must be carefully selected. In this work, 120 nm SrBi 2Ta 2O 9 (SBT) films were deposited on Al 2O 3 or HfO 2 buffer layers by metal organic chemical vapor deposition (MOCVD) or metal organic decomposition (MOD) techniques. The different SBT/high-k stack combinations were optimized considering not only the remnant polarization of the SBT film, extracted from high-frequency capacitance–voltage (CV) measurements, but also considering the electron trapping–detrapping delay accessed by pulsed CV measurements, which gives insight into the electrical response to very fast program and read pulses. Based on these criteria, the MOCVD SBT/HfO 2 gate stack was found to be the best choice, resulting in a memory window of 0.45 V measured for a gate voltage scan range of only ±5 V. The memory switching occurs at rather low voltage due to the use of thinner SBT than typically reported. This optimized stack combination also exhibited low electron trap density and low leakage.

Tuning of electrical bistability in organic devices through electrochemical potential of metal contacts

The effect of different electrode combinations on conductance switching devices has been studied. A number of base electrodes were chosen while the top electrode was kept the same. With a suitable electrode combination, we could observe a large conductance switching (On/Off ratio = 10 7 ). Such a high ratio as compared to other electrode combinations has been discussed in terms of reversible electrochemical changes of charge transfer complexes and change in conduction mechanism. The switching devices showed an associated memory effect for data-storage applications. We could write or erase a state and read it for many cycles. The high On/Off ratio was maintained at the read pulse of write-read-erase-read sequence and also during long-time retention test. The results showed a novel route to select right electrode combination for efficient conductance-switching devices.