Filament Formation Process Research Articles

Effect of ion irradiation on resistive switching in metal-oxide memristive nanostructures

The development of artificial intelligence systems is needed to solve many important challenges in neurobiology and neuroengineering for recreation of brain functions and efficient biorobotics. Here we propose a metal-oxide memristive device compatible with CMOS technology and suitable for hardware implementation of neuromorphic tasks. However, metal-oxide memristors have a significant drawback such as variation of resistive switching parameters due to the stochastic nature of filament formation in oxide material. In this work, we control the filament formation process by irradiation of oxide film surface with heavy ions. We have shown that the irradiation of oxide surface in the Au/SiO2/TiN memristive device with Xe+ ions (at energy of 5 keV) decreases the fluctuations of electroforming voltage, current-voltage characteristics during switching and increases the current ratio in the high/low resistance states. The ability to control internal parameters of the memristor can allow more efficiently using memristor as an element of neural networks and other neuromorphic circuits.

Computed tomography-guided additive manufacturing of Personalized Absorbable Gastrointestinal Stents for intestinal fistulae and perforations

Small bowel perforations and obstructions are relatively frequent surgical emergencies, are potentially life-threatening, and have multiple etiologies. In general, treatment requires urgent surgical repair or resection and at times can lead to further complications. Stents may be used to help with healing intestinal perforations but use is limited as currently available stents are non-absorbable, are manufactured in a narrow size range, and/or are limited to usage in locations that are accessible for endoscopic removal post-healing. The use of 3D-printed bioresorbable polymeric stents will provide patients with a stent that can prevent leakage, is tailored specifically to their geometry, and will be usable within the small bowel, which is not amenable to endoscopic stent placement. This work focused on the rapid manufacturing of gastrointestinal stents composed of a polycaprolactone-polydioxanone (PCL-PDO) composite. Dynamic Mechanical Analysis (DMA) tests were conducted to separately analyze the effects of composition, the filament formation process, and physiological temperature on the PCL-PDO material properties. The proposed stent design was then modeled using computer-aided design, and Finite Element Analysis (FEA) was used to simulate the effects of physiologically relevant forces on stent integrity. The presence of hydrolysable ester bonds was confirmed using FT-IR spectroscopy. In vitro studies were used to evaluate the biocompatibility of the polymer composite. Further analyses were conducted through stent placement in ex vivo pig intestines. PCL-PDO stents were then 3D-printed and placed in vivo in a pig model.

(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.

Direct wavefront measurements of filaments in the assisted-collapse regime.

Wavefront measurements are used to characterize the process of filament formation in air under assisted-collapse conditions. Direct wavefront evolution within the filament, measured for the first time, is used to characterize the role of energy reservoir in filament formation. This information provides new insights into the filament process and phase sensitive applications such as engineered plasma waveguides.

Fabrication and Characterization of Flexible Medical-Grade TPU Filament for Fused Deposition Modeling 3DP Technology.

The possibility of using additive manufacturing (AM) in the medicine area has created new opportunities in health care. This has contributed to a sharp increase in demand for 3D printers, their systems and materials that are adapted to strict medical requirements. We described herein a medical-grade thermoplastic polyurethane (S-TPU) which was developed and then formed into a filament for Fused Deposition Modeling (FDM) 3D printers during a melt-extrusion process. S-TPU consisting of aliphatic hexamethylene 1,6-diisocyanate (HDI), amorphous α,ω-dihydroxy(ethylene-butylene adipate) (PEBA) and 1,4 butandiol (BDO) as a chain extender, was synthesized without the use of a catalyst. The filament (F-TPU) properties were characterized by rheological, mechanical, physico-chemical and in vitro biological properties. The tests showed biocompatibility of the obtained filament as well as revealed no significant effect of the filament formation process on its properties. This study may contribute to expanding the range of medical-grade flexible filaments for standard low-budget FDM printers.

Molecular filament formation and filament–cloud interaction: Hints from Nobeyama 45 m telescope observations

Abstract We present Nobeyama 45 m telescope C18O, 13CO, and 12CO(1–0) mapping observations towards an interstellar filament in the Taurus molecular cloud. We investigate the gas velocity structure along the filament and in its surrounding parent cloud. The filament is detected in the optically thin C18O emission as a single velocity component with a ∼1 pc long, ∼0.06 pc wide structure. The C18O emission traces dust column densities larger than ∼5 × 1021 cm−2. The line-of-sight (LOS) velocity fluctuates along the filament crest with an average amplitude of ∼0.2 km s−1. The 13CO and 12CO integrated intensity maps show spatially extended emission around the elongated filament. We identify three extended structures with LOS velocities redshifted and blueshifted with respect to the average velocity of the filament identified in C18O. Based on combined analyses of velocity-integrated channel maps and intensity variations of the optically thick 12CO spectra on and off the filament, we propose a three-dimensional structure of the cloud surrounding the filament. We further suggest a multi-interaction scenario where sheet-like extended structures interact, in space and time, with the filament and are responsible for its compression and/or disruption, playing an important role in the star formation history of the filament. We also identify, towards the same field, a very faint filament showing a velocity field compatible with the filament formation process proposed by Inoue et al. (2018, PASJ, 70, S53), where a filament is formed due to convergence of a flow of matter generated by the bending of the ambient magnetic field structure induced by an interstellar shock compression.

Stochastic calculus of protein filament formation under spatial confinement

The growth of filamentous aggregates from precursor proteins is a process of central importance to both normal and aberrant biology, for instance as the driver of devastating human disorders such as Alzheimer's and Parkinson's diseases. The conventional theoretical framework for describing this class of phenomena in bulk is based upon the mean-field limit of the law of mass action, which implicitly assumes deterministic dynamics. However, protein filament formation processes under spatial confinement, such as in microdroplets or in the cellular environment, show intrinsic variability due to the molecular noise associated with small-volume effects. To account for this effect, in this paper we introduce a stochastic differential equation approach for investigating protein filament formation processes under spatial confinement. Using this framework, we study the statistical properties of stochastic aggregation curves, as well as the distribution of reaction lag-times. Moreover, we establish the gradual breakdown of the correlation between lag-time and normalized growth rate under spatial confinement. Our results establish the key role of spatial confinement in determining the onset of stochasticity in protein filament formation and offer a formalism for studying protein aggregation kinetics in small volumes in terms of the kinetic parameters describing the aggregation dynamics in bulk.

Spectroscopic Indications of Tunnel Barrier Charging as the Switching Mechanism in Memristive Devices

AbstractResistive random access memory is a promising, energy‐efficient, low‐power “storage class memory” technology that has the potential to replace both flash storage and on‐chip dynamic memory. While the most widely employed systems exhibit filamentary resistive switching, interface‐type switching systems based on a tunable tunnel barrier are of increasing interest. They suffer less from the variability induced by the stochastic filament formation process and the choice of the tunnel barrier thickness offers the possibility to adapt the memory device current to the given circuit requirements. Heterostructures consisting of a yttria‐stabilized zirconia (YSZ) tunnel barrier and a praseodymium calcium manganite (PCMO) layer are employed. Instead of spatially localized filaments, the resistive switching process occurs underneath the whole electrode. By employing a combination of electrical measurements, in operando hard X‐ray photoelectron spectroscopy and electron energy loss spectroscopy, it is revealed that an exchange of oxygen ions between PCMO and YSZ causes an electrostatic modulation of the effective height of the YSZ tunnel barrier and is thereby the underlying mechanism for resistive switching in these devices.

(Invited) Fabrication and Characterization of Copper-Silica-Metal Resistive Switching Devices

Switching in metal-oxide-metal devices has been studied since the late 1960s. A number of mechanisms have been identified to explain the change in resistance in such structures, including the formation of metallic filaments which bridge the insulating layer and provide a low resistance pathway between the electrodes. The low power “cationic” switching of copper-silica-metal devices, where the metal is a relatively inert conductor (e.g., tungsten), has gained attention in recent times as these materials are already present in the back-end-of-line (BEOL) of integrated circuits, making device integration simpler and less expensive than in the case of more “exotic” resistive switching systems. This technology is a variant of programmable metallization cell (PMC) or conductive-bridging random access memory (CBRAM). The switching mechanism involves the transport of copper ions in the oxide and their subsequent reduction to form the conducting metallic pathway. The oxidizable electrode provides ions into the reaction environment to maintain the supply of copper for the growing filament. The switching process is bipolar, in that a bias opposite to that used for writing is required to reverse the filament formation process to return the device to its high resistance state. The copper-silica-metal structures exhibit similar switching characteristics to previously-commercialized devices based on higher chalcogenide glasses, albeit at somewhat lower switching speed for the same programming voltage, while possessing inherently greater CMOS compatibility. Devices have been fabricated using either physical vapor deposition (PVD) or chemical vapor deposition (CVD) of the silica under conditions that allow the film to have a slightly lower density than a typical deposited oxide to promote ion transport and facilitate filament formation. A forming step is required to form a switchable region in the oxide and following this, sub-µs switching at low voltage and current is readily attainable. In this talk we will report on the operational characteristics of these copper-silica-metal devices and the effect of different processing approaches on individual structures as well as crossbar arrays. We will also show the results of impedance spectroscopy analysis as they pertain to the physical structure of the devices. The applications of this technology range from active and passive non-volatile memories to neuromorphic elements and physical unclonable functions (PUFs) for physical and cyber security and these uses will be highlighted in the talk.

Field-induced nucleation in threshold switching characteristics of electrochemical metallization devices

In this research, we investigate electrically driven threshold switching (TS) characteristics in electrochemical metallization cells by adopting the field-induced nucleation theory. For this aim, Ag/HfO2 and Ag/TiO2 based TS devices are prepared and examined. First, we carry out the field driven turn-on process to form Ag filaments created as a consequence of sequential nucleation of Ag ions from the bottom electrode. During the filament formation process, it is observed that the prepared devices show switching time exponential in voltage and temperature with different nucleation barrier energies (W0), which confirms the field-induced nucleation theory. Furthermore, we find that the device with higher W0 shows faster dissolution speed. This implies that the slow turn-off speed of the TS device can be improved by finding a material system with a higher W0 value.

A physically based model for resistive memories including a detailed temperature and variability description

A new model to account for variability in resistive memories is presented. It is included in a previous general current model that considers the main physical mechanisms involved in the conductive filament formation and disruption processes that lead to different resistive states. The validity of the model has been proved for different technologies of metal-insulator-metal bipolar resistive memories. The model can be implemented in Verilog-A for circuit simulation purposes.

Dynamics of the current filament formation and its steady-state characteristics in chalcogenide based PCM

In the phase-change memory (PCM) crystallization occurs in the high-current filament which forms during switching to the conductive state. In the present paper we conduct a numerical modeling of the current filament formation dynamics in thin chalcogenide films using an electronic-thermal model based on negative-U centers tunnel ionization and Joule heating. The key role of inhomogeneities in the filament formation process is shown. Steady-state filament parameters were obtained from the analysis of the stationary heat conduction equation. The filament formation dynamics and the steady-state filament radius and temperature could be controlled by material parameters and contact resistance. Consequently it is possible to control the size of the region wherein crystallization occurs. A good agreement with numerous experimental data leads to the conclusion that thermal effects play a significant role in CGS conduction and high-current filament formation while switching.

(Invited) Hard X-Ray Photoelectron Spectroscopic Study on High-k Dielectrics Based Resistive Random Access Memor

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials and advanced to the commercial manufacturing stage. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. These are many resistivity changing mechanisms for the oxide based ReRAM structures such as the insulator–metal transition in perovskite oxides and conductive-bridging (CBRAM). In our research, we focus on the CBRAM with nano-electrolyte reaction, whose advantage is the low forming voltage. The CBRAM with nano-electrolyte reaction comprises the generation and rupture of a metal filament using a metal such as Ag and Cu acting as a fast mobile ion in oxides. Hafnium oxide (HfO2), which is used as a high-k gate insulator for advanced complementary metal-oxide-semiconductor (CMOS) technologies, has shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials.1) To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important. Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. HX-PES was performed at the SPring-8 BL15XU undulator beamline. The incident X-ray energy and the total energy resolution were 5.95 keV and 240 meV, respectively. We have demonstrated resistance switching using HfO2 film with a Cu top electrode for nonvolatile memory applications, and revealed the Cu diffusion into the HfO2 layer during the conductive filament formation process. Resistive switching was clearly observed in the Cu/HfO2/Pt structure by performing current-voltage measurements. The current step from a high resistive state to a low resistive state was of the order of 103-104, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing HX-PES under bias operation. The application of a bias to the structure reduced the Cu2O state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. These results also provide evidence that the resistance switching of the Cu/HfO2/Pt structure originates in a solid electrolyte reaction containing Cu ions. HX-PES also revealed the top or bottom electrode dependences of interface reduction or oxidization, and ion migration behaviors.2-5) In the presentation, we will show the details of the correlation between the switching mechanism and the interface reaction in the electrode/high-k dielectrics based ReRAM structure.6) We are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8. The HX-PES measurements were performed under the approval of the NIMS Beamline Station (Proposal Nos. 2009A 4600, 2010B 4600, 2011A 4611, 2011B 4613, and 2012A 4613).

TrmBL2 from Pyrococcus furiosus Interacts Both with Double-Stranded and Single-Stranded DNA

In many hyperthermophilic archaea the DNA binding protein TrmBL2 or one of its homologues is abundantly expressed. TrmBL2 is thought to play a significant role in modulating the chromatin architecture in combination with the archaeal histone proteins and Alba. However, its precise physiological role is poorly understood. It has been previously shown that upon binding TrmBL2 covers double-stranded DNA, which leads to the formation of a thick and fibrous filament. Here we investigated the filament formation process as well as the stabilization of DNA by TrmBL2 from Pyroccocus furiosus in detail. We used magnetic tweezers that allow to monitor changes of the DNA mechanical properties upon TrmBL2 binding on the single-molecule level. Extended filaments formed in a cooperative manner and were considerably stiffer than bare double-stranded DNA. Unlike Alba, TrmBL2 did not form DNA cross-bridges. The protein was found to bind double- and single-stranded DNA with similar affinities. In mechanical disruption experiments of DNA hairpins this led to stabilization of both, the double- (before disruption) and the single-stranded (after disruption) DNA forms. Combined, these findings suggest that the biological function of TrmBL2 is not limited to modulating genome architecture and acting as a global repressor but that the protein acts additionally as a stabilizer of DNA secondary structure.

Kinetic factors determining conducting filament formation in solid polymer electrolyte based planar devices.

Resistive switching characteristics and conducting filament formation dynamics in solid polymer electrolyte (SPE) based planar-type atomic switches, with opposing active Ag and inert Pt electrodes, have been investigated by optimizing the device configuration and experimental parameters such as the gap distance between the electrodes, the salt inclusion in the polymer matrix, and the compliance current applied in current-voltage measurements. The high ionic conductivities of SPE enabled us to make scanning electron microscopy observations of the filament formation processes in the sub-micrometer to micrometer ranges. It was found that switching behaviour and filament growth morphology depend strongly on several kinetic factors, such as the redox reaction rate at the electrode-polymer interfaces, ion mobility in the polymer matrix, electric field strength, and the reduction sites for precipitation. Different filament formations, resulting from unidirectional and dendritic growth behaviours, can be controlled by tuning specified parameters, which in turn improves the stability and performance of SPE-based devices.

Modelling of oxygen vacancy aggregates in monoclinic HfO2: can they contribute to conductive filament formation?

Formation of metal rich conductive filaments and their rearrangements determine the switching characteristics in HfO2 based resistive random access memory (RRAM) devices. The initiation of a filament formation process may occur either via aggregation of pre-existing vacancies randomly distributed in the oxide or via generation of new oxygen vacancies close to the pre-existing ones. We evaluate the feasibility of vacancy aggregation processes by calculating the structures and binding energies of oxygen vacancy aggregates consisting of 2, 3 and 4 vacancies in bulk monoclinic (m)-HfO2 using density functional theory (DFT). We demonstrate that formation of neutral oxygen vacancy aggregates is accompanied by small energy gain, which depends on the size and shape of the aggregate. In the most strongly bound configurations, vacancies are unscreened by Hf cations and form voids within the crystal, with the larger aggregates having larger binding energy per vacancy (−0.11 to −0.18 eV). The negatively charged di-vacancy was found to have similar binding energies to the neutral one, while the positively charged di-vacancy was found to be unstable. Thus aggregation process of either neutral or negatively charged oxygen vacancies is energetically feasible.

Properties of interstellar filaments as derived from Herschel, Planck, and molecular line observations

AbstractRecent Herschel and Planck observations of submillimeter dust emission revealed the omnipresence of filamentary structures in the interstellar medium (ISM). The ubiquity of filaments in quiescent clouds as well as in star-forming regions indicates that the formation of filamentary structures is a natural product of the physics at play in the magnetized turbulent cold ISM. An analysis of more than 270 filaments observed with Herschel in 8 regions of the Gould Belt, shows that interstellar filaments are characterized by a narrow distribution of central width sharply peaked at ~0.1 pc, while they span a wide column density range. Molecular line observations of a sample of these filaments show evidence of an increase in the velocity dispersion of dense filaments with column density, suggesting an evolution in mass per unit length due to accretion of surrounding material onto these star-forming filaments. The analyses of Planck dust polarization observations show that both the mean magnetic field and its fluctuations along the filaments are different from those of their surrounding clouds. This points to a coupling between the matter and the $\vec{B}$-field in the filament formation process. These observational results, derived from dust and gas tracers in total and polarized intensity, set strong constraints on our understanding of the formation and evolution of filaments in the ISM. They provide important clues on the initial conditions of the star formation process along interstellar filaments.

Implications of Lower Zero-Field Activation Energy of Dielectric in Al2O3/HfO2 Bi-Layer Dielectric RRAM Forming Process

Resistive memory offers many favorable properties such as simple fabrication, higher retention, and excellent endurance for future non-volatile memory applications [1]. However, some of the physical mechanisms of these memories are not well understood, one of such phenomenon is oxygen vacancy generation which leads to the creation of the filament [2]. Filament formation process has been studied extensively for single layer dielectrics [2-4], but very few studies on filament formation in bilayer dielectric has been conducted so far. Previously, it has been reported that addition of Al2O3 into HfO2 RRAM provides numerous advantages [5], one critical advantage being reduction in the forming voltage [6]. In this experiment, we studied the vacancy generation and forming properties of single layer HfO2 and Al2O3/HfO2 bilayer dielectric RRAM.Metal-insulator-metal (MIM) capacitors with area of 100 µm x 100 µm were fabricated with either HfO2, or bilayer Al2O3/HfO2 as insulator. Tungsten was chosen as electrodes so that interfacial oxide growth can be prevented. Fig. 1 shows the forming characteristics of 5.3 nm HfO2 dielectric and 5.3 nm HfO2 + 1.1 nm Al2O3 dielectric, equivalent electric field is calculated as voltage per EOT. A gradual rise in leakage current leading to breakdown at ~5.5V was observed in only HfO2, whereas a two-step rise in leakage current was observed in Al2O3/HfO2. A fast rise was observed up to ~2V, followed by a gradual rise leading to breakdown at ~5.1V. Fig. 2 shows the forming voltage with single HfO2 and bilayer HfO2/Al2O3 dielectrics. Inset in Fig. 2 shows reset characteristics of single layer HfO2 and bilayer dielectric. It was observed that bilayer dielectric not only provides large Ion/Ioff ratio due to the incorporation of Al2O3 layer but also offers lower forming voltage. Forming voltage uniformity is also better in bilayer dielectric as shown in Fig. 2. All these observations make Al2O3/HfO2 bilayer dielectric a superior candidate over single layer HfO2for future RRAM applications.To understand the origin of lower forming voltage with bi-layer dielectric, oxygen vacancy generation property of dielectric was studied. Vacancy generation in the dielectric is governed by the equation (1), Where G represents the vacancy generation rate, G0 represents the effective vibration frequency, b represents the bond polarization factor between metal and oxygen atom, and F is the effective electric field in the oxide [6]. Ea for Al2O3 and HfO2 was found to be 1.8 eV and 4.6 eV respectively; b for Al2O3 and HfO2 was found to be 19 eA0 and 56 eA0 respectively; and value of G0 were assumed to be 2 x 1013 Hz and 7 x 1013 Hz for Al2O3 and HfO2 respectively [4,7]. Simulation showed that vacancy generation is a faster process in Al2O3 because of lower Ea which dominates the exponential term of equation (1). Fig. 3 shows the electric field and corresponding vacancy generation rate in individual layers of bilayer dielectric. It has been reported that dielectric have excess of vacancies when value of G approaches 0 [4]. Electric field in Al2O3 layer is higher than in HfO2 leading to excess number of vacancies in Al2O3 at ~2V, whereas breakdown of entire bi-layer stack occurs at ~5.4 V. Comparing with single layer HfO2 dielectric, 2V corresponds to an electric field of 3.77 MV/cm and a generation rate of ~10-30 vacancies per second, which is significantly low. Hence at ~2V, there is no additional vacancy generated in 5.3 nm HfO2 dielectric, whereas entire Al2O3 layer have excess of vacancies in case of 1.1 nm Al2O3 + 5.3 nm HfO2. Since every vacancy have higher electric field nearby it [4], hence presence of excess vacancies leads to faster vacancy generation in case of bilayer dielectric.We show that the incorporation of thin Al2O3 layer on HfO2 dielectric lower the forming voltage as well as large Ion/Ioff ratio. It was found that the reduction in forming voltage with bi-layer dielectric is attributed to (i) low-k of Al2O3 leads to higher electric field, and (ii) lower zero-field activation energy (Ea) of Al2O3 leads to a faster filament formation.[1] H. –S. P. Wong, Proceedings of IEEE, vol. 100,no.6, pp.1951-1970 (2012).[2] L. Vandelli, IEEE International Memory Workshop (IMW), pp.1-4 (2011).[3] S. Yu, Functional Metal Oxide Nanostructures. New York: Springer-Verlag, (2011).[4] L. Vandelli, IEEE Transactions on Electron Devices, 60(5), 1754-1762 (2013).[5] S. Yu, International Symposium on VLSI Technology, Systems and Applications (VLSI-TSA), pp. 1-2 (2011).[6] L. Goux, Symposium on VLSI Technology (VLSIT), pp. 159-160 (2012).[7] S. Davis, Journal of Physics: Condense Matter, vol. 23, 495401 (2011).

(Invited) Photoelectron Spectroscopic Study on High-k Dielectrics Based Nanoionics-Type ReRAM Structure under Bias Operation

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. Hafnium oxide (HfO2), which is used as a high-k gate insulator, has also shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials [1]. For the oxide based ReRAM with the conductive filament formation model, two mechanisms of resistance switching have been proposed. One is the nanoionics model, which comprises the generation and rupture of a metal filament using a metal such as Cu acting as a fast mobile ion in oxides. The other model is that of oxygen vacancy nucleation at the metal/oxide interface. To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important.Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation to examine the electronic structure of Pt or Cu/HfO2 ReRAM structures in an operating device. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. The interface electronic states of 10-nm-thick Pt or Cu top electrodes/HfO2/Pt structures were measured with HX-PES in the SPring-8 BL15XU undulator beamline. The incident X-ray energy was 5.95 keV. In the case of the Pt/HfO2 interface, applying a forward bias increased the Pt–O bonding peak as shown in Fig 1, indicating evidence of Pt electrode oxidization and oxygen vacancy formation around the interface [2]. In contrast, the application of a bias to the Cu/HfO2 interface reduced the copper oxide bonding state, providing evidence of oxygen reduction and Cu diffusion into the HfO2 layer [3,4]. We achieved direct observation of oxygen migration at the metal/HfO2 interface under device operation, which is the key to controlling the electrical properties of oxide based ReRAM. Based on these results, we demonstrated the control of the switching voltage and the initial conductive filament formation process of the Ta−Nb binary oxide ((TaxNb1−x)2O5) ReRAM structure using a combinatorial method [5]. The relationship between the interface structure and the electrical properties also will be discussed in detail at the presentation.

(Invited) Photoelectron Spectroscopic Study on High-k Dielectrics Based Nanoionics-Type ReRAM Structure under Bias Operation

We have demonstrated resistance switching using HfO2 film with a Cu top electrode for nonvolatile memory applications, and revealed the Cu diffusion into the HfO2 layer during the filament formation process. Resistive switching was clearly observed in the Cu/HfO2/Pt structure by performing current-voltage measurements. The current step from a high resistive state to a low resistive state was of the order of 103-104 Ω, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing hard x-ray photoelectron spectroscopy under bias operation. The application of a bias to the structure reduced the Cu2O state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. These results also provide evidence that the resistance switching of the Cu/HfO2/Pt structure originates in a solid electrolyte (nanoionics model) containing Cu ions.