Write-erase Cycles Research Articles

Finding optimal non-datapath caching strategies via network flow

Flash and non-volatile memory (NVM) devices have only a limited number of write-erase cycles. Consequently, when employed as caches, cache management policies may choose not to cache certain requested items in order to extend device lifespan. In this work, we propose a simple single-parameter utility function to model the trade-off between maximizing hit-rate and minimizing write-erase cycles for such caches, and study the problem of developing an off-line strategy for deciding whether to write a new item to cache, and if so which item already in the cache to replace. Our main result is, OPTm, an efficient, network flow based algorithm which finds optimal cache management policy under this new setting.

Preparation of Photoswitchable Polyacrylic Nanocomposite Fibers Containing Au Nanorods and Spiropyran: Optical and Plasmonic Properties

Photoswitchable nanofibers and nanocomposite fibers containing plasmonic nanoparticles have attracted a great deal of interest in optical and plasmonic devices. Herein, photoswitchable poly(methyl methacrylate-co-vinylimidazole-co-spiropyran ethyl acrylate) (MVSP) and its copolymer with butyl acrylate (MBVSP) were prepared via emulsion polymerization, and the corresponding nanofibers (MVSP@NF and MBVSP@NF) and nanocomposite fibers (MVSP/Au@NF and MBVSP/Au@NF) containing AuNRs were fabricated through electrospinning. FTIR and 1H NMR analyses confirmed the progress of the copolymerization reaction. The morphology of the prepared nanofibers containing AuNRs with an aspect ratio of 2.5 was identified by SEM and TEM techniques. The inclusion of vinylimidazole into the copolymer chains resulted in well-dispersed AuNRs. Photoisomerization studies revealed a higher photochromic efficiency for MBVSP@F (reflective intensity of 37.4%) with respect to MVSP@NF (reflective intensity of 62.5%) because of the greater flexibility of the chains. In addition, the presence of AuNRs in the nanocomposite fibers with high absorptivity intensified the photochromic properties for both samples. The polarization-dependent plasmonic band of AuNRs was switched between 650 and 634 nm through the photoisomerization of nonpolar SP to polar MC reversibly for MVSP/Au@NF. This displacement was just 4 nm for MBVSP/Au@NF, owing to the limited coupling between AuNRs and MC isomers. Besides, the capability of both nanocomposite fibers for reversible optical patterning was investigated by fast write-erase cycles. Enhanced photofatigue resistance in those fibers and the photomodulation of the plasmonic band of AuNRs using SP to MC isomerization revealed their promising potential for optical patterning and on-demand real-time plasmonic devices.

Optically Encodable and Erasable Multilevel Nonvolatile Flexible Memory Device Based on Metal-Organic Frameworks.

Multilevel and flexible nonvolatile memory (NVM) is a promising candidate for data storage in next-generation devices but its high bias and low mobility of conducting channels are often its drawbacks. In this study, we demonstrate a low bias of smaller than 0.1 V and a high-mobility graphene layer as a conducting channel for flexible optoelectronic NVM based on a composite thin film of indium-based MOF-derived InCl3 and 4,4-oxydiphthalic anhydride (odpta), Na[In3(odpt)2(OH)2(H2O)2](H2O)4, and reduced graphene oxide (rGO). The optoelectronic NVM device can be encoded and erased optically by ultraviolet (UV) light and visible light, respectively. Our device also achieves memory states over 192 (6-bit storage) distinct levels, which can emerge as mass data storage. It also shows an excellent endurance of write-erase cycles under irradiation with a laser of varying wavelengths, the mechanical stability of more than 1000 bending cycles, and stable retention for longer than 10 000 s. These results open an alternative route for developing low bias and innovative optoelectronic technologies.

Van der Waals Epitaxy of Horizontally Orientated Bismuth Iodide/Silicon Heterostructure for Nonvolatile Resistive‐Switching Memory with Multistate Data Storage

AbstractThe film quality of insulators significantly affects performance of resistive random‐access memories (RRAMs), particularly in current leakage and degradation. In this study, a facile and practical method is employed to achieve the van der Waals epitaxy of bismuth iodide (BiI3) on silicon by using a self‐assembled monolayer of octadecyltrichlorosilane (OTS) as a buffer layer. The BiI3layer is compact and has high crystallinity, a pinhole‐free, and compact surface; every BiI3crystal is horizontally aligned with OTS–Si substrate. The RRAMs with the Si++/OTS/BiI3/Au structure exhibit excellent resistive switching properties with a very high on/off ratio of 109, long‐term stability for data retention, high endurance in write–erase cycles, and multistate information storage capacity. The crystal orientation, anisotropic carrier transport, morphology, deposition rate, and roughness considerably influence the resistive switching results. These are thoroughly investigated by analyzing the current–voltage characteristics at various temperatures, scanning electron microscope, atomic force microscope, X‐ray photoemission spectroscopy, and X‐ray diffraction patterns. It is proposed that the resistive switching mechanisms is caused by the iodine ion migration, which changes the valence charge of bismuth. This leaves a partially formed conductive metallic bismuth filament in the BiI3layer under the electrical field that enables multistate data storage.

Excitation wavelength as logic operator.

Multiple molecular logic gates were harvested on a single synthesized material, (E)-2-(2-hydroxy-3-methoxybenzylideneamino)phenol (MBAP), by combining excitation wavelength dependent multi-channel fluorescence outputs and the same chemical inputs. Interestingly, the effortless switching of logic behavior was achieved by simply tweaking the excitation wavelength and sometimes the emission wavelengths with no alteration of chemical inputs and the main device molecule, MBAP. Additionally, new generation purely optically driven memory units were designed on the same system supporting an almost infinite number of write-erase cycles since inter-conversion of memory states was completely free from chemical interferences and impurity issues. Two-way memory functions ("erase-read-write-read" and "write-read-erase-read") worked simultaneously on the same system and could be accessed by simple optical switching between two excitation and emission wavelengths. Our optically switchable device might outperform traditional multifunctional logic gates and memory devices that generally employ chemical triggers to switch functionality and memory states. These optically switchable multifunctional molecular logic gates and memory systems might drive smart devices in the near future with high energy efficiency, extended life span, structural and functional simplicity, exclusive reversibility and enhanced data storage density.

Fabrication of Robust Spatially Resolved Photochromic Patterns on Cellulose Papers by Covalent Printing for Anticounterfeiting Applications

Despite its millennial age, cellulose paper remains the preferred material for domestic and professional printings, covering applications from simple office paperwork to fiducial solutions such as bills, passports, and head letters. The creation of robust photochromic patterns on cellulose papers for anticounterfeiting applications is an important and still partially unaddressed challenge. In this contribution, we report the covalent printing of dibenzothienylethenes as photochromic compounds through a spatially controlled light-mediated thiol–X ligations. Photophysical and theoretical studies provide evidence for a reversible photochromism behavior, not affected significantly by the polar environment of the cellulose matrix, and demonstrates a high fatigue resistance over 18 successive write–erase cycles. The strong coloration–discoloration switch can be easily followed by a direct naked-eye readout.

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend.

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write-erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.

Dynamics of creation photoinduced birefringence on (PAH/PAZO)n layer-by-layer films: Analysis of consecutive cycles

Reproducibility and reliability of data is an important subject in the development of organic devices for photonics applications. This work reports the analysis of successive photoinduced birefringence creation curves in layer-by-layer films of poly(allylamine hydrochloride) (PAH) and poly{1-(4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido)-1,2-ethanediyl, sodium salt} (PAZO) with different number of bilayers. The birefringence creation or writing curves are described by two processes: a faster one referring the contribution of trans–cis–trans photoisomerization cycles to the birefringence; and a slower one associated to the contribution of motion of the polymer chain to the birefringence. As the number of write–erase cycles increases, the characteristic times of these processes decreases, respectively, to values of 18 and 212s independently of the number of bilayers of films while for the magnitudes the fast process prevailed relatively to the slow, by 70% against 30%. The observed behavior is explained by the thermal treatment given by the laser beam in the irradiated area with increase of free volume which contributes for the chromophore mobility. This conclusion was achieved by measuring the surface temperature during and after irradiation and analyzing by optical microscopy the film surface where an increase of holes and aggregation as a result of irradiation was observed. Infrared spectra of films after and before irradiation showed changes in the CC absorbance indicating aggregation of azobenzene groups while changes in the protonated and deprotonated carboxylic acid groups are consistent with ionization degree diminishing which is explained by the removal of water molecules by heating caused by laser. The results presented in this paper indicates that an increase in the number of write–erase cycles contributes to reliable and reproducible birefringence characteristics of PAH/PAZO films – a good new from point of view of possible applications.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons.

Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 106 write–erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump–pump–probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.

Rewritable Polymer Brush Micropatterns Grafted by Triazolinedione Click Chemistry.

Triazolinedione (TAD) click reactions were combined with microcontact chemistry to print, erase, and reprint polymer brushes on surfaces. By patterning substrates with a TAD-tagged atom-transfer radical polymerization initiator (ATRP-TAD) and subsequent surface initiated ATRP, it was possible to graft micropatterned polymer brushes from both alkene- and indole-functionalized substrates. As a result of the dynamic nature of the Alder-ene adduct of TAD and indole at elevated temperatures, the polymer pattern could be erased while the regenerated indole substrate could be reused to print new patterns. To demonstrate the robustness of the methodology, the write-erase cycle was repeated four times.

Interfacial phase-change memory

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

A High Performance Co-design of 26 nm 64 Gb MLC NAND Flash Memory using the Dedicated NAND Flash Controller

It is progressing as new advents and remarkable developments of mobile device every year. On the upper line reason, NAND FLASH large density memory demands which can be stored into portable devices have been dramatically increasing. Therefore, the cell size of the NAND Flash memory has been scaled down by merely 50% and has been doubling density each per year. [1] However, side effects have arisen the cell distribution and reliability characteristics related to coupling interference, channel disturbance, floating gate electron retention, write-erase cycling owing to shrinking around 20㎚ technology. Also, FLASH controller to manage shrink effect leads to speed and current issues. In this paper, It will be introduced to solve cycling, retention and fail bit problems of sub-deep micron shrink such as Virtual negative read used in moving read, randomization. The characteristics of retention, cycling and program performance have 3 K per 1 year and 12.7 MB/s respectively. And device size is 179.32 ㎟ (16.79 ㎜ × 10.68 ㎜) in 3 metal 26 ㎚ CMOS.

Potential fluctuations in phase change memory materials

Long-range potential fluctuations have been quantified in amorphous and crystallised thin films of a family of Ge–Sb–Te (GST) chalcogenide glasses. Among the compositions studied, the width of the potential fluctuations is the smallest for amorphous Ge2Sb2Te5. This is also the most robust material in terms of the number of write–erase cycles for GST films when used as phase change memory materials. A plausible explanation for this observation is given and a criterion for selecting suitable compositions in optical memory devices is proposed.

Resistive Switches and Memories from Silicon Oxide

Because of its excellent dielectric properties, silicon oxide (SiO(x)) has long been used and considered as a passive, insulating component in the construction of electronic devices. In contrast, here we demonstrate resistive switches and memories that use SiO(x) as the sole active material and can be implemented in entirely metal-free embodiments. Through cross-sectional transmission electron microscopy, we determine that the switching takes place through the voltage-driven formation and modification of silicon (Si) nanocrystals (NCs) embedded in the SiO(x) matrix, with SiO(x) itself also serving as the source of the formation of this Si pathway. The small sizes of the Si NCs (d ∼ 5 nm) suggest that scaling to ultrasmall domains could be feasible. Meanwhile, the switch also shows robust nonvolatile properties, high ON/OFF ratios (>10(5)), fast switching (sub-100-ns), and good endurance (10(4) write-erase cycles). These properties in a SiO(x)-based material composition showcase its potentials in constructing memory or logic devices that are fully CMOS compatible.

Unipolar Switching Characteristics of Nonvolatile Memory Devices Based on Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Thin Films

This paper describes the fabrication and electric characteristics of nonvolatile memory devices from a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin film sandwiched between two Al electrodes. These devices have unipolar switching behavior. The on and off voltages are about 2–3 and 0.5 V, respectively. The on/off current ratio of the device is 104–109. Also, the write–erase cycle test was operated 27 times and the retention time was up to 6 h. These characteristics were affected by several factors, such as the formation of current paths due to the redox behavior of PEDOT:PSS chain, the application of the compliance current and the existence of the native thin oxide layer, Al2O3, on top of the Al bottom electrode.

The tunable bistable and multistable memory effect in polymer nanowires

Tunable bistable and multistable resistance switching in conducting polymer nanowires hasbeen reported. These wires show reproducible switching transition under severalREAD–WRITE–ERASE cycles. The switching is observed at low temperature and theON/OFF resistance ratio for the voltage biased switching transition was found to be more than103. Current biasedmeasurements show lower ON/OFF ratio and some of the nanowires exhibit a multistable switching transition incurrent biased measurements. The threshold voltage for switching and theON/OFF resistance ratio can be tuned by changing doping concentration of the nanowires.

Application of interference microscopy to the study of hologram build-up in LiNbO3 crystals

Interference microscopy was applied to direct microscopic observation of temporal evolution of phase holograms in LiNbO3:Fe photorefractive crystals. First a hologram was recorded in the sample, and diffraction efficiency was monitored during hologram build-up using inactinic laser light. Thus kinetics of hologram build-up could be determined. The initial hologram was erased using white light. Then a series of write-erase cycles were performed with increasing exposure times. Holograms were observed by interference microscope after each exposure. The time elapsed between the exposure and the microscopic observation was negligible compared to the relaxation time of the hologram. The obtained temporal evolution of the grating profile gives a deeper insight into the physical mechanism of hologram formation in photorefractive materials than simple diffraction efficiency measurements. A congruently grown sample of LiNbO3 doped with 10−3mol/mol Fe in melting was studied by this method. Sample thickness was set to 300μm to allow correct microscopic observation. Plane-wave holograms were recorded in the samples using an Ar–ion laser at λ=488.0nm of grating constants of 3, 6.5 and 8.8μm.

Visualization using Scanning Nonlinear Dielectric Microscopy of Charges Stored in Flash memory

By applying Scanning Nonlinear Dielectric Microscopy (SNDM), we succeeded in clarifying the position of the electrons/holes in the gate SiO2-Si3N4-SiO2 (ONO) film of the Metal-ONO-Semiconductor type flash memories, After the write-erase cycling operation, the electrons were found in the Si3N4 part of the ONO film. The holes, on the other hand, were found in the Si3N4 film as well as at the bottom of the SiO2 film. This indicates that the electrons and holes are apparently neutralized but exist separately. We also succeeded in clarifying that electrons exist in the poly-Si layer of the floating gate of a flash memory. We confirmed that SNDM is one of the most useful methods for observing the charge in flash memories.

Organic CuTCNQ integrated in complementary metal oxide semiconductor copper back end-of-line for nonvolatile memories

Nanowires of the organometallic semiconductor CuTCNQ were grown from TCNQ vapor in 250nm diameter vias of a Cu back end-of-line process. Corresponding prototypes of cross-point Cu∕CuTCNQ nanowire/Al memories exhibited nonvolatile bistable conductive switching for several ten write-erase cycles with switching currents below 10μA and current densities 1000 times higher than for large-area devices. Scaling of memory elements was also investigated.

Electron-Beam Detection of Bits Reversibly Recorded on Epitaxial InSe/GaSe/Si Phase-Change Diodes

We demonstrate a data read-back scheme based on electron-beam induced current in a data storage device that utilizes thermal recording onto a phase-change medium. The phase-change medium is part of a heterojunction diode whose local charge-collection efficiency depends on the crystalline or amorphous state of a bit. Current gains up to 65 at 2 keV electron beam energy have been demonstrated using InSe/GaSe/Si epitaxial diodes. Fifteen write–erase cycles are obtained without loss of signal contrast by using a protective cap layer and short write pulses. 100 write–erase cycles have been achieved with some loss of contrast. Erasure times for the bits are longer than in similar polycrystalline In–Se media films. Possible reasons for the long erasure times are discussed in terms of a nucleation- or growth-dominated recrystallization. Prospects for extension to smaller bit sizes using electron-beam writing are considered.