(Invited) Atom Switch Technology for Low-Power Nonvolatile Logic Application

Resistive switching devices (ReRAM) represent a broad class of two-terminal continuously tunable resistors including memristors, phase change memory (PCM), valence change memory (VCM), and electrochemical metallization cells (ECM). Though these devices, especially PCM, are increasingly being commercialized by industry for use in next generation memories, they are also all actively studied for use as synaptic weights in next generation hardware-accelerated neuromorphic networks. We have previously investigated Electron Beam Induced Current Microscopy as a means of reliably characterizing resistive switches. In that investigation, we observed surprising electronic effects, such as internal secondary electron emission, in addition to more traditional electron-hole pair separation, and we broke those up into constituent currents based on their origin through Monte-Carlo modeling of the electron beam-matter interaction. Now, armed with a new understanding of the physics of EBIC imaging, we study the impact of manufacturing variations on resistive switches by continuously tuning the thickness of an Al2O3 interfacial barrier. Shifts in the apparent ratios of internal secondary electron emission from the top electrode to the bottom electrode and vice versa appear to indicate a continuous tuning of the apparent filament diameter as both a function of the injected current and the interfacial barrier thickness. This yields an apparent reduction in the current density, the primary effect of which is a reduction in the device damage from forming and a suppression of parasitic leakage currents imaged in devices without interfacial barriers. Figure 1

The atomic switch operation is demonstrated using a solid polymer electrolyte (SPE) as a matrix material. Similar to inorganic electrolyte-based atomic switches, SPE-based atomic switches exhibit not only bi-stable resistive switching but also quantized conductance. The high ionic conductivity of SPE enables us to directly observe filament growth behaviors even in micrometer-scaled devices, and to reveal the kinetic factors determining the filament growth processes. We also succeed in fabricating devices on a plastic substrate using an ink-jet technique, and to demonstrate stable resistive switching under substrate bending. All the results indicate that the SPE-based atomic switch has great potential for the development flexible switch/memory devices as well as new types of atomic-scale devices with high-speed operation and ultra-low power consumption.

Resistive switching (RS) devices based on Nb:SrTiO3 (NSTO) hold significant potential for next-generation data storage due to their tunable resistance, stability, and low power consumption. The majority of existing research studies have predominantly focused on metal/NSTO structures that utilize single-crystal NSTO. Although this configuration exhibits superior performance, it is costly and cannot be easily integrated. Another important point is that epitaxial structures can enhance device stability because of their excellent interfaces. However, epitaxial growth of metals on NSTO poses significant challenges due to inherently high structural mismatches, which has led to inconsistent results reported by various researchers employing the same metal/NSTO structure. In this study, NSTO films with different Nb doping amounts were prepared on YBa2Cu3O[Formula: see text][Formula: see text] (YBCO) layers to form NSTO/YBCO heterostructures. YBCO serves as both the electrode and the epitaxial heterolayer material for NSTO. The photogenerated voltage and RS properties of epitaxially grown NSTO/YBCO structures were investigated in detail. At a low Nb doping amount, NSTO cannot produce sufficient photogenerated carriers, while excessive Nb doping results in a low interface barrier height, causing the photogenerated open-circuit voltage of the NSTO/YBCO structure to initially increase and then decrease with the increase in Nb doping amount (the peak value is observed at a Nb doping concentration of 1%). On the other hand, the electrical conductivity of NSTO increases with Nb doping, leading to an increase in the photogenerated short-circuit current. Further, the RS characteristics of the NSTO/YBCO structure were investigated, and clockwise and counterclockwise RS behaviors under low and high bias voltages were observed, respectively. At low voltages, the clockwise RS effect is primarily attributed to the increased interfacial barrier resulting from the migration of oxygen vacancies in YBCO. Conversely, at high voltages, the migration of oxygen ions within NSTO leads to a reduction in the interfacial barrier, thereby inducing a counterclockwise RS effect. In addition, at low bias voltages, the resistance ratio between the high-resistance state and the low-resistance state under [Formula: see text] 1/[Formula: see text] 2 V bias voltages first increases and then decreases as the Nb doping amount increases, with the highest resistance ratio (>100) observed at 1% Nb doping. These findings provide novel insights into the RS mechanism, and it is envisaged that they will promote the application of conductive oxides in RS devices and the development of multilevel memory devices.

Here, an optimized method for energy‐efficient and highly reliable multibit operation in a Au/Al2O3/HfO2/TiN resistive switching (RS) device is investigated. A thin Al2O3 layer inserted between the top electrode and the HfO2 RS layer in a capacitor‐like RS device plays the role of an electrical resistor and modulates the external bias‐dependent resistance variation behavior of the RS device. Compared to the case where a single HfO2 layer is used, the device with a Al2O3/HfO2 stacked layer shows slow and nonvolatile resistance state variation with a stepwise increase of the applied voltage, which can significantly reduce the occurrence frequency of the abnormal cases during the incremental‐step‐pulse‐programming (ISPP) and error‐check‐and‐correction (ECC) processes. This phenomenon reduces the number of ISPP/ECC sequences, which is advantageous for achieving energy‐efficient programming in the multibit operation of an RS device. To clarify the role of the Al2O3 thin film layer, a comparative study is performed with an Au/HfO2/TiN stacked device and a serially connected external load resistor. The observed behavior is consistent with that of the device with an Al2O3 layer. This study demonstrates that the simple insertion of an insulating layer in an RS device can facilitate reliable and energy‐efficient multibit operation for future nonvolatile memory applications.

Among the emerging memories, resistive switching (RS) based resistive random-access memories (RRAMs) are attracting lots of attention due to their simple metal–insulator–metal structures, low power consumption, long endurance and retention characteristics, low fabrication cost, ultrafast switching, and CMOS compatibility. In recent years, several oxides, chalcogenides, polymers and their composites have been explored for RS devices. Many of these studies show a high resistance ratio of the OFF and ON states with good RS characteristics. Some of these studies also show the realization of multifunctional RS devices such as the simultaneous switching of resistance and magnetic states. In order to scale up RS-based RRAMs, a detailed understanding of the occurrence of RS is very much desirable. In this review, we provide an overview of the current understanding, recent advances and future outlook of RS-based RRAM devices along with fundamental concepts of the different types of RS, and conventional as well as novel measurement techniques which are being used to characterize RS devices. Observations of RS in different materials are presented, and RS mechanisms, such as the valence change mechanism and electrochemical metallization memory, are discussed in detail. An overview of multifunctional RS devices and the main challenges faced in scaling up RS devices is also presented.

For the first time, Cu(AlTi)-alloy active-electrode coupled with RuTa inert-electrode is proposed to improve the reliability of nonvolatile Cu atom switch. The remarkably high rupture temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> >400 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C) of the nanometer-scale Cu bridge with the high thermal resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) is realized by the Cu-alloy without increasing programming current. The anti-Cu diffusive, RuTa-alloy improves the retention of the ON-state at 150 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C and endurance to >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cycles. The metallurgical prescription is a key to improve the reliability and switching performance of the conducting bridge.

Oxygen vacancies (VOs) are fundamental for oxide resistance switching (RS) devices, while the introduction of VOs in oxides during the fabrication processes has generally generated unfavorable defects which have limited the improvements in RS performance. Herein, a novel strategy, called ex situ VOs doping, has been proposed to increase the concentration of VOs to fabricate improved ZnO RS devices. The ex situ VOs doping process is similar to modulation doping of heterojunctions, where VOs are intentionally incorporated in a separate oxide material and then migrated toward the target materials. Specifically, HfO2/ZnO architecture with a thin oxygen-deficient HfO2 capping layer is prepared, where VOs in the HfO2 layer facilely migrated toward the underlying ZnO under applying bias. Theoretical calculations and atomic binding energy analysis have verified the effective ex situ VOs doping of ZnO in the HfO2/ZnO architecture. Benefiting from the dominant migration of VOs along grain boundaries, the accumulated VOs around grain boundaries at HfO2/ZnO interface provided preferential nucleation sites for deterministic formation of conductive filaments. Accordingly, improved ZnO RS devices have been demonstrated in terms of increased RS window and enhanced uniformity, stability, and endurance. The ex situ doping of VOs using a heterojunction architecture enables multi-dimensional regulation of the formation of conductive filaments, which will find potential applications in achieving improved oxide RS devices in the future.

The Resistive RAM memories are among the most promising technologies opening new perspectives beyond the current FLASH memory characteristics [1]. Several options compatible with a CMOS integration exist imposing constraints in term of electrode materials and dielectric layers. Indeed, many options are tested since a few years and the actual literature shows a wide dispersion of very encouraging results. In the same time the theoretical effort to simulate and to model the RRAM essential features has rapidly grown. By sustaining the technological efforts, we need to provide sound basis in order to pursue the RRAM development by decreasing the trial error period. Nevertheless, the origin of the mechanisms at the heart of a RRAM is sufficiently complex to resist to both semi-classical and empirical approaches inherited from the microelectronics. One of the main reason is that these devices mix two types of motions: the electronic current and the ionic rearrangement mainly for the dielectric, which can also interfere in turn with the electrode. Therefore, the atomistic simulation appears as the main alternative to analyze the situation without doing too strong assumptions. Our contribution is based onto our previous works for a Ti/HfO2 OxRRAM [2-4] and a Cu/Al2O3 CBRAM [5-6] plus a new and original mechanism for the HfO2 dielectric breakdown. Depending onto the electrode work function and its oxygen solubility, we show that the Forming step is mainly governed by the available charge states for the oxygen vacancy and the oxygen interstitial. For the Ti electrode and HfO2 dielectric, this leads to only one scenario where several oxygen rails leave the dielectric toward the reactive electrode. Thereafter we propose a Forming scenario for a Ti/HfO2 OxRRAM up to obtaining a conductive filament through the dielectric, see Fig. 1. This filamentary mode is obtained by basing our thinking onto equilibrium thermodynamics in between Ti oxides and Hf oxides. This model is then used to evaluate the activation energies implied in the SET and RESET steps: in order to move an oxygen inward and outward the conductive filament. In parallel, the density of states is then analyzed to evaluate relative change of filament conductivity. Then in a second part, we show that a Cu/Al2O3 CBRAM shares the same principles for the oxygen movements but in a more complex way. By analyzing the activation energies for Cu, O and Al, we show that oxygen movements need to be combined with the copper interstitial. A copper based electrode reacts with the dielectric by a combined motion: copper ions eventually end their run in substitutional oxygen sites. This movement can also be accompanied by a slight reorganization of some aluminum atoms which is usually not possible with HfO2 where Hf atoms stay at rest due to their low diffusivity. In conclusion we draw a possible track to follow in order to combine both types of mechanisms in between OxRRAM and CBRAM. Bibliography [1] M. Ueki, K. Takeuchi, T. Yamamoto, A. Tanabe, N. Ikarashi, M. Saitoh, T. Nagumo, H. Sunamura, M. Narihiro, K. Uejima, K. Masuzaki, N. Furutake, S. Saito, Y. Yabe, A. Mitsuiki, K. Takeda, T. Hase, and Y. Hayashi, in VLSI Technology (VLSI Technology), 2015 Symposium on (2015) pp. T108-T109[2] K.-H. Xue, B. Traore, P. Blaise, L. R. C. Fonseca, E. Vianello, G. Molas, B. D. Salvo, G. Ghibaudo, B. Magyari-Köpe, and Y. Nishi, IEEE Transactions on Electron Devices 61, 1394 (2014). [3] B. Traore, P. Blaise, E. Vianello, E. Jalaguier, G. Molas, J. Nodin, L. Perniola, B. De Salvo, and Y. Nishi, in IEEE IRPS (2014) pp. 5E.2.1-5E.2.5.[4] B. Traore, P. Blaise, E. Vianello, L. Perniola, B. De Salvo, and Y. Nishi, Electron Devices, IEEE Transactions on Electron Devices 63, 360 (2016). [5] J. Guy, G. Molas, P. Blaise, A. Roule. G. LeCarval, V. Delaye, A. Toffoli, G. Ghibaudo, F. Clermidy, B. de Salvo, L. Perniola, in IEEE Transactions on Electron Devices 62, pp. 3482-3489, (2015). [6] C. Nail, P. Blaise, G. Molas, SISC proceedings (2015) Figure 1

ITRS [1] has identified Tunnel-FETs as the most promising transistors to reduce the inverse subthreshold swing with respect to MOSFETs and thus allow for an aggressive VDD scaling, and encouraging experimental data have been reported [2]. Interface states, however, are a serious concern for III-V FETs [3], because, by acting as stepping stones for the tunneling through the gap, they may have a large impact on IDS versus VGS(I-V) characteristics of Tunnel-FETs.In this work we present a comprehensive computational study employing a full quantum transport model to investigate the effects of interface traps in nanowire InAs Tunnel-FETs and MOSFETs by varying the trap energy level, its volume and position, as well as the working temperature [4, 5]. To this purpose, we introduced a description of interface traps in a simulator based on the Non-equilibrium Green’s functions formalism, using an 8 bands k.p Hamiltonian and accounting for phonon-scattering [6]. In our model, traps are treated as 0-D electrically active states that can affect the I-V curves of the transistors both by modifying the device electrostatics and by actively participating to the carrier transport.Our 3-D self-consistent simulations show that: even a single trap can deteriorate the inverse subthreshold slope of a nanowire InAs Tunnel-FET; shallow traps have the largest impact on subthreshold slopes; and the inelastic phonon-assisted tunneling through interface traps results in a temperature dependence of the otherwise temperature-independent Tunnel-FET characteristics.Further, we considered Tunnel FETs or MOSFETs with a spatially random distribution of traps. The physical mechanisms behind the effects of traps in either tunnel FETs or MOSFETs are compared and, furthermore, traps are also investigated as a possible source of device variability [7]. We show that, in MOSFETs, an aggressive oxide thickness scaling can effectively counteract the degradation of the inverse subthreshold slope possibly produced by interface traps. Tunnel FETs are instead more vulnerable to traps, which are probably the main hindrance to the experimental realization of tunnel FETs with an inverse subthreshold slope better than 60 mV/decade.[1] International Technology Roadmap for Semiconductors: 2011 Update. http://www.itrs.net, 2011.[2] G. Dewey, et al., “Fabrication, characterization, and physics of III–V heterojunction tunneling Field Effect Transistors (H-TFET) for steep sub-threshold swing,” in IEEE IEDM Technical Digest, pp. 33.6.1–33.6.4, 2011.[3] L. Lin and J. Robertson, “Defect states at III-V semiconductor oxide interfaces,” Applied Physics Letters, vol. 98, no. 8, p. 082903, 2011.[4] MG. Pala and D. Esseni, “Interface Traps in InAs Nanowire Tunnel-FETs and MOSFETs—Part I: Model Description and Single Trap Analysis in Tunnel-FETs,” Electron Devices, IEEE Transactions on, vol. 60, pp. 2795 - 2801, 2013.[5] D. Esseni and M.G. Pala, “Interface Traps in InAs Nanowire Tunnel FETs and MOSFETs—Part II: Comparative Analysis and Trap-Induced Variability,” Electron Devices, IEEE Transactions on, vol. 60, pp. 2802 - 2807, 2013.[6] F. Conzatti, et al., “Strain-induced performance improvements in InAs nanowire tunnel FETs,” Electron Devices, IEEE Transactions on, vol. 59, pp. 2085–2092, 2012.[7] F. Conzatti, M.G. Pala, and D. Esseni, “Surface-roughness-induced variability in nanowire InAs tunnel FETs,” IEEE Electron Device Letters, vol. 33, pp. 806–808, 2012.

We developed a one-transistor one-resistor cell composed of one TiO2-based resistive switching (RS) device and one ZnO-based thin-film transistor (TFT). We study the electric characteristics of each component individually, and their interplay when both work together. We explored the direct control of bipolar RS devices, using our TFTs to drive current in both directions. We also report striking power implications when we swap the terminals of the RS device. The target of our work is the introduction of RS devices in large-area electronic (LAE) circuits. In this context, RS devices can be beneficial regarding functionality and energy consumption, when compared to other ways to introduce memory cells in LAE circuits.

Hexagonal boron nitride (h-BN) is an attractive insulating material for nanoelectronic devices due to its high reliability as dielectric and excellent compatibility with other two dimensional (2D) materials (e.g. graphene, MoS2). Multilayer h-BN stacks have been readily grown on Cu and Pt substrates via chemical vapor deposition (CVD) approach, confirming its potential for wafer scale integration. However, the growth of h-BN on other substrates needs to be also achieved in order to expand the use of this material. Recently, the CVD growth of monolayer h-BN on Fe substrates was reported, but it just focused on material structure characterization. Here we present the first fabrication of electronic devices using multilayer h-BN dielectric stacks grown on Fe foils. We fabricate and characterize resistive switching (RS) devices based on Au/Ag/h-BN/Fe nanojunctions, and observe the coexistence of both volatile and non-volatile RS depending on the electrode to which the bias is applied. The characteristics measured agree well with those simulated via SIM2RRAM software and QPC modeling, and the cycle-to-cycle variability is slightly lower than that of transition metal oxide based RS devices.

Hafnium oxide-based resistive switching (RS) devices are promising candidates for next-generation nonvolatile memories because of their ability to integrate into silicon electronics. However, irreversible and unidirectional movements of oxygen vacancies in functional oxide in memristors limit the improvement of ON/OFF ratio and endurance simultaneously as well as the highest working temperature up to 300 °C. Here, we design varistructure RS devices through oxygen vacancy engineering and dynamical redox of Ta. The ON/OFF ratio and the endurance are largely co-improved due to dynamical redox of Ta at the interfaces. Compared to all current RS memories, it cumulatively demonstrates high endurance (105), retention (104 s), ON/OFF ratio (104–106), operation speed (75 ns), and low write voltage (2 V). Remarkably, the RS devices remain stable up to 300 °C.

Mechanism of ferroelectric resistive switching in Bi0.9La0.1FeO3 thin films

The achievement of a reproducible and controlled deposition of partitioned Cu2O/CuO thin films by techniques compatible with ULSI processing like reactive magnetron sputtering has been reported as an outstanding challenge in the literature. This phase partitioning underlies their performance as reversible resistive memory switching devices in advanced microelectronic applications of the future. They are currently fabricated by thermal oxidation and chemical methods. We have used a combination of an understanding from plasma chemistry, thermo-kinetics of ions, and rf power variation during deposition to successfully identify a processing window for preparing partitioned Cu2O/CuO films. The production of a core rich Cu2O and surface rich Cu2O/CuO mixture necessary for oxygen migration during resistive switching is confirmed by XRD peaks, Fourier transform infra red Cu (I)-O vibrational modes, XPS Cu 2P3/2 and O 1S peak fitting, and a comparison of satellite peak ratio's in Cu 2P3/2 fitted peaks. We are proposing based on the findings reported in this paper that an XPS satellite peak intensity(Is) to main peak intensity ratio (Im) ≤ 0.45 as an indicator of a core rich Cu2O and surface rich Cu2O/CuO formation in our prepared films. CuO is solely responsible for the satellite peaks. This is explained on the basis that plasma dissociation of oxygen will be limited to the predominant formation of Cu2O under certain plasma deposition conditions we have identified in this paper, which also results in a core-rim phase partitioning. The deposited films also followed a Volmer-Weber columnar growth mode, which could facilitate oxygen vacancy migration and conductive filaments at the columnar interfaces. This is further confirmed by optical transmittance and band-gap measurements using spectrophotometry. This development is expected to impact on the early adoption of copper oxide based resistive memory electronic switching devices in advanced electronic devices of the future. The relative abundance of copper is another major complementary driver for the interest in copper oxide films.

A detailed understanding of the conductance quantization and resistive switching phenomena in redox‐based memories is crucial for realizing atomic‐scale memory devices and for finding the adequate design principles on which they can be based. Here, the emergence of quantized conductance states and their correlation with resistive switching characteristics in polymer‐based atomic switches are investigated using combinations of current–voltage measurements and first‐principles density functional theory (DFT) simulations. Various conductance states, including integer and half‐integer multiples of a single atomic point contact and fractional conductance variations, are observed in an Ag/polyethylene oxide/Pt device under sweeping of bias voltage. Moreover, highly controllable and reproducible quantized conductance behaviors by tuning the voltage sweep rate and the sweep voltage range, suggesting well‐controlled formation of the atomic point contact, are demonstrated. The device also exhibits longer retention times for higher conductance states. The DFT simulations reveal the transmission eigenstate of geometrically optimized atomic point contact structures and the impact of the atomic configurations and structural stability on the conductance state, which also explains their resistive switching behaviors. The well‐defined, multiple quantized conductance states observed in these polymer‐based atomic switches show promise for the development of new multilevel memory devices.