High-performance Memory Devices Research Articles

Fully-transparent graphene charge-trap memory device with large memory window and long-term retention

A fully-transparent graphene-based charge-trap memory device was realized by fabricating a graphene-channel field-effect transistor with high-k/low-k/high-k oxide stacks of Al2O3/AlOx/Al2O3 and indium-tin-oxide gate/source/drain electrodes on the polyethylene naphthalate substrate (i.e., ITO-gated AXA-gFET). The usage of low-k AlOx as a charge-trap layer allowed us to demonstrate a high-performance memory device, exhibiting a large memory window of ∼9.2 V and a tenacious retention of the memory window margin up to ∼57% after 10 years. Memory cells comprising the ITO-gated AXA-gFET arrays displayed a high transparency with the average optical transmittance of ∼83% in visible wavelength regions. These properties may move us a step closer to the practical application of graphene-based memories for future transparent electronics. In-depth analyses on the electrical characteristics and the mechanisms of the memory functions are presented.

Different Steric-Twist-Induced Ternary Memory Characteristics in Nonconjugated Copolymers with Pendant Naphthalene and 1,8-Naphthalimide Moieties.

Herein, novel random copolymers PMNN and PMNB were designed and synthesized, and the memory devices Al/PMNN and PMNB/ITO both exhibited ternary memory performance. The switching voltages of the OFF-ON1 and ON1-ON2 transitions for both memory devices are around -2.0 and -3.5 V, respectively, and the ON1/OFF, ON2/ON1 current ratios are both up to 103 . The observed tristable electrical conductivity switching could be attributed to field-induced conformational ordering of the naphthalene rings in the side chains, and subsequent charge trapping by 1,8-naphthalimide moieties. More interestingly, by adjusting the connection sites of 1,8-naphthalimide moieties to tune the steric-twist effect, different memory properties were achieved (PMNN showed nonvolatile write once, read many (WORM) memory behavior, whereas PMNB showed volatile static RAM (SRAM) memory behavior). This result will offer a guideline for the design of different high-performance multilevel memory devices by tuning the steric effects of the chemical moieties.

High-Performance Nonvolatile Organic Transistor Memory Using Quantum Dots-Based Floating Gate

A novel nonvolatile floating-gate transistor memory device using CdSe@ZnS quantum dots (QDs) embedded the insulating polymer as a charge-storage layer along with the rational design of device structure is presented. The core-shell structure CdSe@ZnS QDs can efficiently trap both holes and electrons under the applied writing/erasing operations, resulting in a considerable threshold voltage shifts (ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ) over 50 V and forming high-conductance (ON) and low-conductance (OFF) states at a gate voltage of 0 V. The value of threshold voltage shift is controlled by writing and erasing voltages, regardless with source-drain voltages. Furthermore, it exhibits a long retention time (the ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> can maintain 76% at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> s) and outstanding endurance characteristics (>500 cycles), demonstrating extraordinary stable and reliable memory property. Moreover, a thin layer of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was introduced as tunneling dielectric layer which is essential for the high-performance floating-gate transistor memory device. The nonvolatile organic transistor memory devices using QDs-based floating gate show great potential application for high-performance organic memory devices.

Enhanced resistive memory in Nb-doped BaTiO3 ferroelectric diodes

In this study, we report on enhanced resistive memory in BaTiO3-based ferroelectric diodes due to the doping of donors. A large ON/OFF current ratio of ∼2000, about two orders of magnitude higher than that of Au/BaTiO3/SrRuO3, is achieved in a Au/Nb:BaTiO3/SrRuO3 diode at room temperature. This can be ascribed to the enhanced ferroelectric-modulation on the potential barrier at the Nb:BaTiO3/SrRuO3 interface associated with the (NbTi4+5+)· donors, which gives rise to an efficient control of device transport between a bulk-limited current in the ON state and an interface-limited Schottky emission in the OFF state. In contrast, the resistance switching is suppressed in a Au/Fe:BaTiO3/SrRuO3 device since the (FeTi4+3+)′ acceptors suppress semiconducting character of the BaTiO3 thin film and make the polarization-modulation of the band diagram negligible. The present work facilitates the design of high-performance resistive memory devices based on ferroelectric diodes with controllable charged defects.

CH3 NH3 PbBr3 Perovskite Nanocrystals as Efficient Light-Harvesting Antenna for Fluorescence Resonance Energy Transfer.

Hybrid perovskites have created enormous research interest as a low-cost material for high-performance photovoltaic devices, light-emitting diodes, photodetectors, memory devices and sensors. Perovskite materials in nanocrystal form that display intense luminescence due to the quantum confinement effect were found to be particularly suitable for most of these applications. However, the potential use of perovskite nanocrystals as a light-harvesting antenna for possible applications in artificial photosynthesis systems is not yet explored. In the present work, we study the light-harvesting antenna properties of luminescent methylammonium lead bromide (CH3 NH3 PbBr3 )-based perovskite nanocrystals using fluorescent dyes (rhodamine B, rhodamine 101, and nile red) as energy acceptors. Our studies revealed that CH3 NH3 PbBr3 nanocrystals are an excellent light-harvesting antenna, and efficient fluorescence resonance energy transfer occurs from the nanocrystals to fluorescent dyes. Further, the energy transfer efficiency is found to be highly dependent on the number of anchoring groups and binding ability of the dyes to the surface of the nanocrystals. These observations may have significant implications for perovskite-based light-harvesting devices and their possible use in artificial photosynthesis systems.

Magnetic skyrmions on a two-lane racetrack

Magnetic skyrmions are particle-like textures in magnetization, characterized by a topological winding number. Nanometer-scale skyrmions have been observed at room temperature in magnetic multilayer structures. The combination of their small size, topological quantization and their efficient electric manipulation makes them interesting candidates for information carriers in high-performance memory devices. A skyrmion racetrack memory has been suggested, in which information is encoded in the distance between skyrmions moving in a one-dimensional nanostrip. Here, I propose an alternative design where skyrmions move in two (or more) parallel lanes and the information is stored in the lane number of each skyrmion. Such a multilane track can be constructed by controlling the height profile of the nanostrip. Repulsive skyrmion–skyrmion interactions in narrow nanostrips guarantee that skyrmions on different lanes cannot pass each other. Current pulses can be used to induce a lane change, and combining these elements provides a robust, efficient design for skyrmion-based storage devices.

Molecular beam epitaxial growth of oriented and uniform Ge2Sb2Te5 nanoparticles with compact dimensions

The scaling-down of phase change memory cell is critical to achieve high-performance and high-density memory devices. Herein, we report that Ge2Sb2Te5 nanoparticles along the [1 1 1] direction were synthesized without templates or etching in a molecular beam epitaxy system. Under non-stoichiometric Ge:Sb:Te beam ratio condition, the growth of high-density Ge2Sb2Te5 nanoparticles was achieved by Zn-doping. The average diameter of the nanoparticles is 8 nm, and the full width at half maximum of the size distribution is 2.7 nm. Our results suggest that the size and shape modifications of Ge2Sb2Te5 nanoparticles could be induced by Zn-doping which influences the nucleation in the growth process. In addition, the bonding states of Zn and Te verified by X-ray photoelectron spectroscopy proved that Zn atoms located in the Ge2Sb2Te5 matrix. This approach exemplified here can be applied to the sub-20 nm phase change memory devices and may also be extendable to be served in the design and development of more materials with phase transitions.

Manipulating the exchange bias effect of Pb(Zr0.52Ti0.48)O3/CoFe2O4/NiO heterostructural films by electric fields

The Pb(Zr0.52Ti0.48)O3/CoFe2O4/NiO heterostructural films with exchange bias (EB) effect have been prepared on Pt/Ti/SiO2/Si wafers using a sol-gel process, and reversible manipulation of EB effect by electric fields has been realized. Compared with the exchange bias field (Heb = −75 Oe) at as-grown state, the modulation gain of Heb by electric fields can reach 83% (Heb = −12.5 Oe) in the case of +5.0 V and 283% (Heb = −287.5 Oe) in the case of −5.0 V, respectively. Moreover, such electrically tunable EB effect is repeatable and has good endurance and retention. Through analyzing the energy band structures in different electric treatment states, we discuss the mechanism of such electric-field-tunable EB effect. Two factors, i.e., the filling (or releasing) of electrons into (or from) the defect levels produced by oxygen vacancies at positive (or negative) electric voltages, and the redistribution of electrons due to the ferroelectric polarization, both of which give rise to the variation of the strength of exchange interaction in the CFO layer, have been revealed to be responsible for the electric modulation of EB effect. This work provides a promising avenue for electrically manipulating the EB effect and developing high-performance memory and storage devices with low power consumption.

Bipolar Resistive Switching Behavior in Sol-Gel MgTiNiOxMemory Device

High-resistance state (HRS) current has significant effect on the reliability and power consumption of resistive switching memories. Low HRS current is helpful for obtaining ultra-low power and for high ON/OFF ratio nonvolatile memory application. The reduced HRS current of a sol–gel magnesium titanate nickelate-based resistive random access memory by using nickel (II) acetylacetone as substitute for acetylacetone in magnesium titanate (MTO) was presented. Forming-free, high ON/OFF ratio of over $10^{6}$ , excellent current distribution and good retention at 85 °C were achieved. Moreover, the effect of nickel (Ni) on the surface roughness, operation voltage, switching cycles, HRS current, ON/OFF ratio, current distribution, and switching behavior was explored. These results indicate that the incorporation of Ni in sol-gel MTO is an effective way to achieve high-performance memory devices.

Graphene quantum dots as a highly efficient solution-processed charge trapping medium for organic nano-floating gate memory

A highly efficient solution-processible charge trapping medium is a prerequisite to developing high-performance organic nano-floating gate memory (NFGM) devices. Although several candidates for the charge trapping layer have been proposed for organic memory, a method for significantly increasing the density of stored charges in nanoscale layers remains a considerable challenge. Here, solution-processible graphene quantum dots (GQDs) were prepared by a modified thermal plasma jet method; the GQDs were mostly composed of carbon without any serious oxidation, which was confirmed by x-ray photoelectron spectroscopy. These GQDs have multiple energy levels because of their size distribution, and they can be effectively utilized as charge trapping media for organic NFGM applications. The NFGM device exhibited excellent reversible switching characteristics, with an on/off current ratio greater than 106, a stable retention time of 104 s and reliable cycling endurance over 100 cycles. In particular, we estimated that the GQDs layer trapped ∼7.2 × 1012 cm−2 charges per unit area, which is a much higher density than those of other solution-processible nanomaterials, suggesting that the GQDs layer holds promise as a highly efficient nanoscale charge trapping material.

Organic nonvolatile memory devices utilizing intrinsic charge-trapping phenomena in an n-type polymer semiconductor

Charge trapping is an undesirable phenomenon and a common challenge in the operation of n-channel organic field-effect transistors. Herein, we exploit charge trapping in an n-type semiconducting poly (naphthalene diimide-alt-biselenophene) (PNDIBS) as the key operational mechanism to develop high performance, nonvolatile, electronic memory devices. The PNDIBS-based field-effect transistor memory devices were programmed at 60 V and they showed excellent charge-trapping and de-trapping characteristics, which could be cycled more than 200 times with a current ratio of 103 between the two binary states. Programmed data could be retained for 103 s with a memory window of 28 V. This is a record performance for n-channel organic transistor with inherent charge-trapping capability without using external charge trapping agents. However, the memory device performance was greatly reduced, as expected, when the n-type polymer semiconductor was end-capped with phenyl groups to reduce the trap density. These results show that the trap density of n-type semiconducting polymers could be engineered to control the inherent charge-trapping capability and device performance for developing high-performance low-cost memory devices.

Memory operation devices based on light-illumination ambipolar carbon-nanotube thin-film-transistors

We report the memory operation behavior of a light illumination ambipolar single-walled carbon nanotube thin film field-effect transistors devices. In addition to the high electronic-performance, such an on/off transistor-switching ratio of 104 and an on-conductance of 18 μS, these memory devices have shown a high retention time of both hole and electron-trapping modes, reaching 2.8 × 104 s at room temperature. The memory characteristics confirm that light illumination and electrical field can act as an independent programming/erasing operation method. This could be a fundamental step toward achieving high performance and stable operating nanoelectronic memory devices.

High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Cobalt Ferrite Nanoparticles.

Nano-floating gate memory (NFGM) devices are transistor-type memory devices that use nanostructured materials as charge trap sites. They have recently attracted a great deal of attention due to their excellent performance, capability for multilevel programming, and suitability as platforms for integrated circuits. Herein, novel NFGM devices have been fabricated using semiconducting cobalt ferrite (CoFe2O4) nanoparticles (NPs) as charge trap sites and pentacene as a p-type semiconductor. Monodisperse CoFe2O4 NPs with different diameters have been synthesized by thermal decomposition and embedded in NFGM devices. The particle size effects on the memory performance have been investigated in terms of energy levels and particle-particle interactions. CoFe2O4 NP-based memory devices exhibit a large memory window (≈73.84 V), a high read current on/off ratio (read I(on)/I(off)) of ≈2.98 × 10(3), and excellent data retention. Fast switching behaviors are observed due to the exceptional charge trapping/release capability of CoFe2O4 NPs surrounded by the oleate layer, which acts as an alternative tunneling dielectric layer and simplifies the device fabrication process. Furthermore, the NFGM devices show excellent thermal stability, and flexible memory devices fabricated on plastic substrates exhibit remarkable mechanical and electrical stability. This study demonstrates a viable means of fabricating highly flexible, high-performance organic memory devices.

Modeling, Fabrication, and Reliability of Through Vias in Polycrystalline Silicon Panels

Silicon interposers with through-silicon vias (TSVs) have been developed in single-crystalline silicon wafer to address the high I/O density requirements between high performance logic, memory, graphic, and other devices. However, single-crystalline silicon interposers suffer from many shortcomings such as high cost, low electrical performance, and reliability. To overcome these shortcomings of traditional silicon interposers, an entirely different approach using polycrystalline silicon panels with polymer liners and through-package-vias (TPVs) is proposed by Georgia Tech Packaging Research Center. This paper, for the first time, focuses on the reliability of TPVs in polycrystalline silicon interposers fabricated from panels. Mechanical simulations were carried out that show lower stresses in TPVs in polycrystalline silicon lined with thick polymer liners, compared with TSVs in traditional single-crystalline silicon with thin SiO2 layers. TPVs were fabricated for thermal cycling tests, resistance monitoring, and scanning electron microscope imaging. The reliability characterization results showed good mechanical reliability of TPVs in polycrystalline silicon panels.

Nonvolatile memories using the electrets of conjugated rod-coil block copolymer and its nanocomposite with single wall carbon nanotubes

Self-assembled conjugated rod-coil block copolymer and its nanocomposites with SWCNT could be used as the charge storage layer for high-performance OFET memory devices.

The steric effect of α- and β-substituted anthraquinone units on high performance polymeric memory devices

New anthraquinone-substituted triarylamine-based high-performance polymeric memory devices provide a strategy for tailoring the memory characteristics through structure modulation.

Graphene-graphene oxide floating gate transistor memory.

A novel transparent, flexible, graphene channel floating-gate transistor memory (FGTM) device is fabricated using a graphene oxide (GO) charge trapping layer on a plastic substrate. The GO layer, which bears ammonium groups (NH3+), is prepared at the interface between the crosslinked PVP (cPVP) tunneling dielectric and the Al2 O3 blocking dielectric layers. Important design rules are proposed for a high-performance graphene memory device: (i) precise doping of the graphene channel, and (ii) chemical functionalization of the GO charge trapping layer. How to control memory characteristics by graphene doping is systematically explained, and the optimal conditions for the best performance of the memory devices are found. Note that precise control over the doping of the graphene channel maximizes the conductance difference at a zero gate voltage, which reduces the device power consumption. The proposed optimization via graphene doping can be applied to any graphene channel transistor-type memory device. Additionally, the positively charged GO (GO-NH3+) interacts electrostatically with hydroxyl groups of both UV-treated Al2 O3 and PVP layers, which enhances the interfacial adhesion, and thus the mechanical stability of the device during bending. The resulting graphene-graphene oxide FGTMs exhibit excellent memory characteristics, including a large memory window (11.7 V), fast switching speed (1 μs), cyclic endurance (200 cycles), stable retention (10(5) s), and good mechanical stability (1000 cycles).

Digital memory versatility of fully π-conjugated donor-acceptor hybrid polymers.

The fully π-conjugated donor-acceptor hybrid polymers Fl-TPA, Fl-TPA-TCNE, and Fl-TPA-TCNQ, which are composed of fluorene (Fl), triphenylamine (TPA), dimethylphenylamine, alkyne, alkyne-tetracyanoethylene (TCNE) adduct, and alkyne-7,7,8,8-tetracyanoquinodimethane (TCNQ) adduct, were synthesized. These polymers are completely amorphous in the solid film state and thermally stable up to 291-409 °C. Their molecular orbital levels and band gaps vary with their compositions. The TCNE and TCNQ units, despite their electron-acceptor characteristics, were found to enhance the π-conjugation lengths of Fl-TPA-TCNE and Fl-TPA-TCNQ (i.e., to produce red shifts in their absorption spectra and significant reductions in their band gaps). These changes are reflected in the electrical digital memory behavior of the polymers. Moreover, the TCNE and TCNQ units were found to diversify the digital memory modes and to widen the active polymer layer thickness window. In devices with aluminum top and bottom electrodes, the Fl-TPA polymer exhibits stable unipolar permanent memory behavior with high reliability. The Fl-TPA-TCNE and Fl-TPA-TCNQ devices exhibit stable unipolar permanent memory behavior as well as dynamic random access memory behavior with excellent reliability. These polymer devices were found to operate by either hole injection or hole injection along with electron injection, depending on the polymer composition. Overall, this study demonstrated that the incorporation of π-conjugated cyano moieties, which control both the π-conjugation length and electron-accepting power, is a sound approach for the design and synthesis of high-performance digital memory polymers. The TCNE and TCNQ polymers synthesized in this study are highly suitable active materials for the low-cost mass production of high-performance, polarity-free, programmable, volatile, and permanent memory devices that can be operated with very low power consumption, high ON/OFF current ratios, and high reliability.

High-Performance Memory Device Using Graphene Oxide Flakes Sandwiched Polymethylmethacrylate Layers

Organic bistable devices (OBDs) using graphene oxide (GO) flakes sandwiched polymethylmethacrylate (PMMA) films were fabricated. These devices exhibited two accessible conducting states, that is, a low-conductivity (OFF) state and a high-conductivity (ON) state. The devices can be switched to ON state under a negative electrical sweep; it can also be reset to the initial OFF state by a reverse (positive) electrical sweep. Detailed I-V measurements have shown that in ITO/PMMA/GO/PMMA/Al sandwiches the resistive switching originates from the formation and rupture of conducting filaments. The ON/OFF ratio of the OBDs was approximately 5 x 10(3), reproducibility of more than 10(5) cycles, and retention time of 10(4) s. These properties show that the device is promising for high-density, low-cost memory application.

A Novel Defect-Engineering-Based Implementation for High-Performance Multilevel Data Storage in Resistive Switching Memory

A novel strategy based on defect engineering is proposed for high-performance multibit data storage in oxide-based resistive random access memory (RRAM). Key innovations are: 1) material-oriented cell engineering for desired modification of physical locations of generated oxygen vacancies in resistive switching layer; and 2) operation scheme to control the amount of oxygen vacancy generated in the conducting filament regions during switching. Proper doping approach is applied to suppress the formation of oxygen vacancy clusters due to the avalanching effect in the forming and set processes. Gradual resistive switching process is observed in the devices with proper doping at the proper switching operation modes. Multilevels of resistance states are measured by the optimized dc or ac switching mode. Excellent memory performance with four-level data storage (good resistance uniformity under pulse switching, retention >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s at 150°C, and endurance >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cycles) is successfully demonstrated in hafnium oxide-based RRAM devices, indicating the viability of the proposed engineering design strategy. The proposed methodology helps to understand the mechanism of multilevel switching and provides guidelines for the design of high-performance multibit resistive switching memory devices.