The dynamics of mobile oxygen vacancies depend on electric field and temperature, and this is key to controlling interfacial resistive switching in BiFeO${}_{3}$ memristive devices, which are interesting especially for neuromorphic computing. The authors use impedance spectroscopy and quasistatic state measurements to reveal the dynamics of resistance changes in such devices, and relate these changes to the redistribution of oxygen vacancies via modeling. This work will also impact the use of other oxides with mobile oxygen vacancies in similar devices.

The forming process, which corresponds to the activation of the switching filament in Resistive Random Access Memory (RRAM) arrays, has a strong impact on the cells' performances. In this paper we characterize and compare different pulse forming techniques in terms of forming time, yield and cell-to-cell variability on 4 kbits RRAM arrays. Moreover, post-forming modeling during Reset operation of correctly working and over formed cells has been performed. An incremental form and verify technique, based on a sequence of trapezoidal waveforms with increasing voltages followed by a verify operation that terminates when the expected switching behavior has been achieved, showed the best results. This procedure narrows the post-forming current distribution whereas reducing the Reset switching voltage and the operative current. These advantages materialize in a better control of the cell-to-cell variability and in an overall time and energy saving at the system level. (C) 2015 Elsevier Ltd. All rights reserved.

Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the "OFF" state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability.

Graphene based electron field emitter arrays consisting of cone-shaped silicon tips, a thin Al2O3 tunnel barrier, and graphene top electrode are fabricated. Due to the monolayered graphene top electrode, the electrons are able to tunnel through the Al2O3 layer and emit into the vacuum. The temperature behavior of the tunnel leakage current as well as the emission current is characterized.

The intercell variability of the initial state and the impact of dc and pulse forming on intercell variability as well as on intracell variability in TiN/HfO 2 /Ti/TiN 1 transistor - 1 resistor (1T-1R) devices in 4-kb memory arrays were investigated. Nearly 78% of devices on particular arrays were dc formed with a wordline (WL) voltage V WL = 1.4 V and a bitline (BL) voltage V BL = 2.3 V, whereas 22% of devices were not formed due to the combined effect of the extrinsic process-induced intercell variability of the initial state and the intrinsic intercell variability after dc forming. Furthermore, pulse-induced forming with pulsewidths on the order of 10 μs (V WL = 1.4 V and V BL = 3.5 V) caused for 86% of devices a low-resistance state. Using a retry algorithm, we achieve 100% of formed devices. To assess and confirm the nature of the variability during forming operation and during cycling, the quantum point-contact model was considered. The modeling results demonstrate a relationship between the forming and the device performance. The cells requiring high energy for the forming operation, due to impurities in the HfO 2 deposition during array processing, are those subject to poor switching performance, larger variability, and faster wear out. Devices formed by a pulse-retry algorithm show: 1) shorter endurance and 2) higher variability during cycling.

In this work, the bipolar resistive switching (RS) characteristics of polycrystalline hafnium oxide are studied in embedded 1 transistor-1 resistor (1T1R) resistive random access memory (RRAM) devices. The HfO2 films with varied thickness from 15 to 25 nm were grown by the atomic vapor deposition method at 400 °C. The HfO2 films are polycrystalline, as evaluated by applying atomic force microscopy and x-ray diffraction. In addition, current–voltage characteristics of the 1T1R devices were investigated. The forming voltages of 15 nm thick HfO2 films are about 4 V and increase gradually with increasing film thickness. A reproducible resistance switching behavior was observed with a high resistance ratio of ∼20 during DC cycling of 100 times. The observed SET and RESET voltages are about 1.2 and 1.6 V, respectively, indicating that the 1T1R RRAM device can be operated below 2 V. The current–voltage characteristics are discussed in the frame of the quantum point contact model.

In this work, cells behavior during forming is monitored through an incremental pulse and verify algorithm on 4kbit RRAM arrays. This technique allows recognising different cell behaviors in terms of read-verify current oscillation: the impact of these oscillations on reliability and cell-to-cell variability has been investigated during 1k endurance cycles and 100k pulse stress under a variety of cycling conditions. Conductance histograms for the post-forming current reveal the nanosized nature of the filamentary paths across the dielectric film.

The reliability and performance characterization of each non-volatile memory technology requires the thorough investigation of dedicated array test structures that mimic the real operations of a fully functional integrated product. This makes no exception also for emerging non-volatile memories like the Resistive Random Access Memory (RRAM) concept. An extensive electrical characterization activity performed on test vehicles manufactured in a CMOS backend-of-line process allowed the first glance estimation of operation modes and reliability threats typical of this technology. In this paper, it is provided a review of the most important issues like forming instabilities, optimal set/reset operation finding, and read disturb to provide a guideline either for a further technology optimization or an efficient algorithms co-design to handle these reliability/performance threats.

The design and the manufacturing of ReRAM test structures allow deeper insight in the performance of the FORMING, RESET, and SET operations at array level, providing details on the process induced variability of the technology, and on the potential sources of failures. Test structures allow also demonstrating the integration capability of the ReRAM technology using a CMOS-compatible process ramping up such non-volatile memory to a maturity level.

In this study, direct experimental materials science evidence of the important theoretical prediction for resistive random access memory (RRAM) technologies that a critical amount of oxygen vacancies is needed to establish stable resistive switching in metal-oxide-metal samples is presented. In detail, a novel in-operando hard X-ray photoelectron spectroscopy technique is applied to non-destructively investigates the influence of the current compliance and direct current voltage sweep cycles on the Ti/HfO2 interface chemistry and physics of resistive switching Ti/HfO2/TiN cells. These studies indeed confirm that current compliance is a critical parameter to control the amount of oxygen vacancies in the conducting filaments in the oxide layer during the RRAM cell operation to achieve stable switching. Furthermore, clear carbon segregation towards the Ti/HfO2 interface under electrical stress is visible. Since carbon impurities impact the oxygen vacancy defect population under resistive switching, this dynamic carbon segregation to the Ti/HfO2 interface is suspected to negatively influence RRAM device endurance. Therefore, these results indicate that the RRAM materials engineering needs to include all impurities in the dielectric layer in order to achieve reliable device performance.

The conducting filament forming voltage of stoichiometric hafnium oxide based resistive switching layers increases linearly with layer thickness. Using strongly reduced oxygen deficient hafnium oxide thin films grown on polycrystalline TiN/Si(001) substrates, the thickness dependence of the forming voltage is strongly suppressed. Instead, an almost constant forming voltage of about 3 V is observed up to 200 nm layer thickness. This effect suggests that filament formation and switching occurs for all samples in an oxidized HfO2 surface layer of a few nanometer thickness while the highly oxygen deficient thin film itself merely serves as a oxygen vacancy reservoir. (C) 2014 AIP Publishing LLC.

In this work a SET/RESET investigation in cycling on ReRAM arrays has been performed, in order to find the most reliable SET/RESET operation conditions. The analysis will compare DC and pulsed SET/RESET operations featuring different durations and voltages on previously DC formed 1T-lR4kbits memory arrays. A thorough analysis of the ReRAM reliability joining the cell-to-cell variability analysis to that of cycling evaluations in complete arrays is addressed. A comparison between DC and Pulse SET/RESET in terms of switching yield, read window, device-to-device uniformity and bit error rate is reported. Finally, the impact of a temperature bake at 1250C on a cycled array is shown to study the temperature impact on the array variability.

The Ti/HfO2 interface plays a major role for resistance switching performances. However, clear interface engineering strategies to achieve reliable and reproducible switching have been poorly investigated. For this purpose, we present a comprehensive study of the Ti/HfO2 interface by a combined experimental-theoretical approach. Based on the use of oxygen-isotope marked Hf*O2, the oxygen scavenging capability of the Ti layer is clearly proven. More importantly, in line with ab initio theory, the combined HAXPES-Tof-SIMS study of the thin films deposited by MBE clearly establishes a strong impact of the HfO2 thin film morphology on the Ti/HfO2 interface reactivity. Low-temperature deposition is thus seen as a RRAM processing compatible way to establish the critical amount of oxygen vacancies to achieve reproducible and reliable resistance switching performances.

We have investigated the resistive switching behavior in stoichiometric HfO2 and oxygen-deficient HfO2−x thin films grown on TiN electrodes using reactive molecular beam epitaxy. Oxygen defect states were controlled by the flow of oxygen radicals during thin film growth. Hard X-ray photoelectron spectroscopy confirmed the presence of sub-stoichiometric hafnium oxide and defect states near the Fermi level. The oxygen deficient HfO2−x thin films show bipolar switching with an electroforming occurring at low voltages and low operating currents, paving the way for almost forming-free devices for low-power applications.

Resistive switching in Ti/HfO2/TiN was investigated in-operando by hard x-ray photoelectron spectroscopy. In comparison with the virgin-state, ON- and OFF-states show enhanced Ti/TiOx/HfO2 interface oxidation, resulting from an oxygen-gettering activity of Ti. The formed TiOx layer acts in the resistive switching process as an oxygen reservoir in exchange with the non-stoichiometric HfO2−δ. A Ti1+/Ti3+ valence change redox reaction occurs between OFF- and ON-states. The peak shifts are attributed to space charge potentials created by the varying oxygen vacancy concentration at the interface. A push-pull model of oxygen vacancies as a function of voltage polarity is proposed to describe the mechanism.

In this paper, we describe a new non-volatile memory, based on metal-insulator-metal that provides performance benefits compared to standard Flash memory. In addition and more importantly, it comes with some advantages with respect to side channel attacks, i.e., its structure prevents by default optical analysis.

Back-end-of-line integrated 1 x 1 mu m(2) TiN/HfO2/Ti/TiN MIM memory devices in a 0.25-mu m complementary metal-oxide-semiconductor technology were built to investigate the conduction mechanism and the resistive switching behavior as a function of temperature. The temperature-dependent I-V characteristics in fresh devices are attributed to the Poole-Frenkel mechanism with an extracted trap energy level at phi approximate to 0.2 eV below the HfO2 conduction band. The trap level is associated with positively charged oxygen vacancies. The electroformed memory cells show a stable bipolar switching behavior in the temperature range from 213-413 K. The OFF-state current increases with temperature, whereas the ON-state current can be described by a weak metallic behavior. Furthermore, the results suggest that the I-V cycling not only induces significant changes in the electrical properties of the MIM memory devices, i.e., the increase in the OFF-state current, but also stronger temperature dependence. The temperature effect on the ON-state and OFF-state characteristics is modeled within the framework of the quantum point-contact model for dielectric breakdown using an effective temperature-dependent confinement potential.

Process compatible high-k dielectric thin films are one of the key solutions to develop high performance metal–insulator–metal (MIM) structures for future microelectronic devices. Engineered cerium–aluminate (CexAl2–xO3) thin films were deposited on titanium nitride metal electrodes by electron-beam co-evaporation of ceria and alumina in a molecular beam deposition chamber. X-ray photoelectron spectroscopy clearly reveals that Ce cations can be stabilized in the 3+ valence state in CexAl2–xO3 up to x=0.7 by accommodation in the alumina host matrix. Higher Ce content was observed to result in cerium dioxide segregation in cerium aluminate matrix, probably due to the chemical tendency of Ce cations to exist rather in the 4+ than in the 3+ state. Electrical characterization of the X-ray amorphous Ce0.7Al1.3O3 films reveals a dielectric constant value of about 11 and leakage current lower than 10−4A/cm2. No parasitic low-k interface formation between the high-k Ce0.7Al1.3O3 film and the TiN metal electrode is detected.

A physics-based analytical model for the current-voltage (I-V) characteristics corresponding to the low and high resistive states in electroformed metal-insulator-metal structures with HfO2 layers is proposed. The model relies on the Landauer theory for the electron transport in mesoscopic systems. The switching phenomenon is ascribed to the modulation of the constriction's bottleneck cross-sectio...