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Memristive devices are promising candidates for synaptic memories in neuromorphic computing systems, but the insufficient reliability of the analog behavior has been a challenge. Lateral oxide scaling with bottom-up technologies such as thermal oxidation, coupled with new device architectures, can improve the reliability. However, the effect of thermal oxide growth on analog device behavior remains unclear. In this study, we fabricate a variety of resistive switching thermal oxides on tantalum electrode surfaces and develop a method for rapid full factorial electrical analysis. The analysis uses 2500 unique control parameter combinations of current limits and RESET-stop-voltages to compare device behavior across millions of modulated current–voltage sweeps. We clarify the mechanisms that shift the device behavior from abrupt towards gradual SET transitions, a desirable characteristic for emulating analog plasticity. We found that a mildly negative differential resistance and an increased internal series resistance at the metal–oxide interface contribute to the stabilization of the gradual SET transitions. These findings highlight the importance of fine-tuning the bottom-up oxide growth for improving switching performance. [ABSTRACT FROM AUTHOR]

Long-term retention is one of the major challenges concerning the reliability of redox-based resistive switching random access memories based on the valence change mechanism (VCM). The stability of the programmed state has to be ensured over several years, leaving a sufficient read window between the states, which is even more challenging at large statistics. Thus, the underlying physical mechanisms have to be understood and experimental data have to be evaluated accurately. Here, it shows that the retention behavior of the high resistive state (HRS) is more complex than that of the low resistive state and requires a different evaluation method. In this work, we experimentally investigate the retention behavior of 5M VCM devices via accelerated life testing and show the difficulties of commonly used evaluation methods in view of the HRS. Subsequently, we present a new evaluation method focusing on the standard deviation of the HRS current distribution. Hereby, an activation energy for the degradation process can be extracted, which is essential for the prediction of the devices' behavior under operating conditions. Furthermore, we reproduce the experimentally observed behavior with our 3D Kinetic Monte Carlo simulation model. We confirm the plausibility of our evaluation method and are able to connect the calculated activation energy to the migration barriers of oxygen vacancies that we implemented in the model and that we believe play a key role in the experimentally observed degradation process. [ABSTRACT FROM AUTHOR]

Resistive switching devices and other components with negative differential resistance (NDR) are emerging as possible electronic constituents of next-generation computing architectures. Due to the exhibited NDR effects, switching operations are strongly affected by the presence of resistance in series with the memory cell. Experimental measurements useful in the development of these devices use a deliberate addition of series resistance, which can be done either by integrating resistors on-chip or by connecting external components to the wafer probing system. The former approach is considered inflexible because the resistance value attached to a given device cannot be changed or removed, while the latter approach tends to create parasitic effects that impact controllability and interfere with measurements. In this work, we introduce a circuit design for flexible characterization of two-terminal nanodevices that provides a programmatically adjustable external series resistance while maintaining low parasitic capacitance. Experimental demonstrations show the impact of the series resistance on NDR and resistive switching measurements. [ABSTRACT FROM AUTHOR]

Resistive switching devices, important for emerging memory and neuromorphic applications, face significant challenges related to the control of delicate filamentary states in the oxide material. As a device switches, its rapid conductivity change is involved in a positive feedback process that would lead to runaway destruction of the cell without current, voltage, or energy limitation. Typically, cells are directly patterned on MOS transistors to limit the current, but this approach is very restrictive as the necessary integration limits the materials available as well as the fabrication cycle time. In this article, we propose an external circuit to cycle resistive memory cells, capturing the full transfer curves while driving the cells in a way that suppresses runaway transitions. Using this circuit, we demonstrate the acquisition of 105I, V loops per second without using on-wafer current limiting transistors. This setup brings voltage sweeping measurements to a relevant timescale for applications and enables many new experimental possibilities for device evaluation in a statistical context. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the unipolar resistive-switching mechanism in TiN\NiO\Ni cells, whereby the NiO layer is deposited by metal-organic chemical vapor deposition. Irrespective of the cell size down to 80 nm, controlled unipolar switching is observed only on condition the Ni electrode is used as anode. Local elemental analyzes revealed enhanced oxygen concentration at the NiO\Ni interface after reset operation. Based on this result as well as on the quantum-mechanical modeling of the conduction for different resistance states, the reset switching is analyzed in terms of electrochemically assisted oxidation process of the conductive filament close to the Ni anode. Electrical switching results on various stack configurations confirm the major impact of the electrode materials on the reset performances, bringing important insights into the mechanisms involved in the unipolar switching of this system. Literature data are discussed on the basis of this learning, suggesting possible extension of these considerations to other oxide-based switching systems. [ABSTRACT FROM AUTHOR]

Analysis of electron states in nanometer-thin NiO layers grown by metallo-organic chemical vapor deposition on SiO2 or Al2O3 by means of spectroscopic ellipsometry and internal photoemission measurements points to the presence of a metal-like electron density, in correlation with the enhanced electrical conductivity of the films. These metallic states are suggested to be related to Ni clusters formed by partial reduction of NiO by hydrogen-containing by-products of the deposition reaction. [ABSTRACT FROM AUTHOR]

In this paper, we show the coexistence of the bipolar and unipolar resistive-switching modes in NiO cells realized using an optimized oxidation process of a Ni blanket layer used as the bottom electrode. The two switching modes can be activated independent of the cell switching history provided the appropriate programming conditions are applied. The bipolar and unipolar switching modes are discussed as driven by electrochemical- and thermal-based mechanisms, respectively. The switching versatility between these two modes is demonstrated both for large oxidized Ni films and for Ni films oxidized at the bottom of small dimension contact holes. The perspective of selecting the desired switching mode in a scaled device made in a small diameter single hole is highly attractive because the specific advantages of the two modes broaden the application scope of the cell and enable larger flexibility in terms of memory architecture. [ABSTRACT FROM AUTHOR]

The oxidation characteristics of TiAlN films integrated in the bottom electrode (BE) stack of three-dimensional SrBi2Ta2O9-based (SBT) ferroelectric capacitors are investigated in the range of 650–800 °C. The patterned TiAlN\Ir\IrO2\Pt BE is encapsulated by a thin ferroelectric SBT film deposited by metal organic chemical vapor deposition and then crystallized ex situ at temperatures higher than 650 °C in oxygen. During this annealing step the TiAlN film oxidizes from the lateral side of the patterned BE mesas. Compared to the vertical oxidation in blanket TiAlN layers, the lateral oxidation rate in our capped patterned films is much larger for similar oxidation conditions. This lateral oxidation of the TiAlN is strongly correlated with the in-film SBT stress that depends upon the deposition temperature and the thickness of the SBT film: the higher the tensile stress in the SBT, the larger the TiAlN oxidation length induced. From a kinetic study, the lateral oxygen diffusion was found to be a self-limited process with activation energy of about 2.2 eV and a preexponential factor D0 of 7×10-3 cm2/s. This factor, one to two orders of magnitude higher than the one obtained in blanket TiAlN layers, is indicative of interfacial diffusion. The mechanism of the increased lateral oxidation can be understood by the high tensile stress of the SBT film at the sidewall of the ferroelectric capacitors that tends to tear apart the Ir/TiAlN interface and causes enlarged interfacial oxygen diffusion. The limiting step of the oxidation mechanism is identified as oxygen diffusion in the Al-depleted/Ti-rich TiAlN layer that lies beyond the Al-rich film found at the very edge of the capacitors, whereas on blanket films the oxygen diffusion is limited in the Al-rich surface layer region. [ABSTRACT FROM AUTHOR]

In this work, we characterize the imprint properties of three-dimensional Sr0.8Bi2.2Ta2O9 (SBT) ferroelectric capacitors integrated in 0.18 μm technology using metal-organic chemical vapor deposition of SBT. We investigated the dependence of hysteresis loops on the storage temperature and duration after poling the capacitors. Also, the influence of unipolar and bipolar voltage cyclings, respectively, during and after the imprint stress was studied. We show that the imprint-induced shift of the hysteresis is limited for all the investigated stress conditions, and we associate this low shift mainly to a pronounced polarization decrease with the temperature, deviating from the Landau-Devonshire law. Our results also suggest that emission within the SBT layer is dominant over injection from the electrode in the imprint mechanism at 0 V bias. On the other hand, dynamic imprint stress results suggest an increased contribution of carrier injection. Regarding the switchable polarization, it was observed to be higher after an imprint stress due to a wake-up of ferroelectric domains, which may be thermally driven but was evidenced to be prominently field driven. The wake-up and the imprint are discussed in terms of trapping/detrapping mechanisms. [ABSTRACT FROM AUTHOR]

Ferroelectric random access memory (FeRAM) is an attractive candidate technology for embedded nonvolatile memory, especially in applications where low power and high program speed are important. Market introduction of high-density FeRAM is, however, lagging behind standard complementary metal-oxide semiconductor (CMOS) because of the difficult integration technology. This paper discusses the major integration issues for high-density FeRAM, based on SrBi2Ta2O9 (strontium bismuth tantalate or SBT), in relation to the fabrication of our stacked cell structure. We have worked in the previous years on the development of SBT-FeRAM integration technology, based on a so-called pseudo-three-dimensional (3D) cell, with a capacitor that can be scaled from quasi two-dimensional towards a true three-dimensional capacitor where the sidewalls will importantly contribute to the signal. In the first phase of our integration development, we integrated our FeRAM cell in a 0.35 μm CMOS technology. In a second phase, then, possibility of scaling of our cell is demonstrated in 0.18 μm technology. The excellent electrical and reliability properties of the small integrated ferroelectric capacitors prove the feasibility of the technology, while the verification of the potential 3D effect confirms the basic scaling potential of our concept beyond that of the single-mask capacitor. The paper outlines the different material and technological challenges, and working solutions are demonstrated. While some issues are specific to our own cell, many are applicable to different stacked FeRAM cell concepts, or will become more general concerns when more developments are moving into 3D structures. [ABSTRACT FROM AUTHOR]

The difficult scaling of ferroelectric random access memories with the complementary metal-oxide semiconductor technology roadmap requires integration of three-dimensional (3D) ferroelectric capacitors (FeCAP’s). In this work the unusual electrical behavior of 3D FeCAP sidewalls was studied by comparing the electrical properties of two-dimensional and 3D integrated FeCAP structures. We evidenced composition variations of the SrBi2Ta2O9 (SBT) film in the sidewalls with marked bismuth segregation during metal-organic chemical-vapor deposition (MOCVD) of the SBT film. The segregation was reduced after decreasing the deposition temperature from 440 °C, whereby the Bi-rich phase in the sidewalls does not contribute to polarization, down to 405 °C, whereby sidewall SBT contributes to polarization. After further optimization of the MOCVD conditions at 405 °C, the segregation is minimized and the ferroelectric contribution of the sidewall SBT is almost the same as the contribution of the planar SBT. As a result, 3D FeCAP’s integrated up to metal interconnection exhibit a remnant polarization Pr∼7.5 μC/cm2. [ABSTRACT FROM AUTHOR]

Elementary Pt/SrBi2Ta2O9/Pt ferroelectric capacitors have been structurally characterized by x-ray diffraction using highly brilliant synchrotron radiation. A microstructural analysis of the stacked layers was performed from the collection of high-quality one-dimensional and two-dimensional diffraction patterns. During x-ray diffraction experiments, peculiar electrical behaviors under irradiation were evidenced. Indeed, depending upon their initial state (poled or nonpoled), the capacitors have exhibited drastic changes in their electrical characteristics after or under irradiation, both “fatiguelike” (polarization reduction) and/or “imprintlike” (voltage shift) phenomena being observed. Using a sample environment specially designed to measure in situ the evolutions of ferroelectric characteristics, the kinetics of both degradation and restoration of ferroelectric properties of the SrBi2Ta2O9-based capacitors under x-ray radiation have been analyzed. Reduction and recovery of switchable polarization have been explained in terms of interaction between ferroelectric domain configuration and photoinduced charge carriers. Mechanisms of charge trapping proposed in the literature are discussed to explain aging and rejuvenation. [ABSTRACT FROM AUTHOR]

The emerging nonvolatile memory technology of redox-based resistive switching (RS) devices is not only a promising candidate for future high density memories but also for computational and neuromorphic applications. In neuromorphic as well as in memory applications, RS devices are configured in nanocrossbar arrays, which are controlled by CMOS circuits. With those hybrid systems, brain-inspired artificial neural networks can be built up and trained by using a learning algorithm. First works on hardware implementation using relatively large and high current level RS devices are already published. In this work, the influence of small and low current level devices showing noncontinuous resistance levels on neuromorphic networks is studied. To this end, a well-established physical-based Verilog A model is modified to offer continuous and discrete conduction. With this model, a simple one-layer neuromorphic network is simulated to get a first insight and understanding of this problem using a backpropagation algorithm based on the steepest descent method. [ABSTRACT FROM AUTHOR]

Presents information on a study that investigated the explosive crystallization of silicon-nitrogen layers deposited on silicon samples during laser melting of the silicon. Evidence for explosive crystallization; Quantitative analysis of the crystallization threshold dependencies; Use of transmission electron microscopy.

Examines laser recrystallization using continuous silicon nitride or oxynitride capping layers or a structured antireflective silicon nitride capping layer. Material structures and used recrystallization conditions; Overview of the measured electrical characteristics of metal-oxide-semiconductor field-effect transistors made in different materials; Discussion on the incorporation of nitrogen from the capping layer in the laser recrystallized silicon-on-insulator structures.

Deals with a study which reported the seeded and unseeded zone melting recrystallization of ten *mum low pressure chemical vapor deposited polycrystalline silicon layer using a scanned mercury-arc lamp heater. Experimental details; Results and discussion; Conclusion.

The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum. [ABSTRACT FROM AUTHOR]

In this work, we studied the leakage mechanism at different temperatures in Pt/Sr–Ta–O/Pt and Pt/Bi–Ta–O/Pt thin film capacitors. The dielectrics were deposited by metal-organic chemical vapor deposition at 360 °C as amorphous layers, and the complete process flow of the capacitors was kept below 500 °C. Conduction is governed by Poole-Frenkel emission down to ∼15 and ∼20 nm for Sr–Ta–O and Bi–Ta–O, respectively. From the Poole-Frenkel plots, we extracted an index of refraction of ∼2.1 for Sr–Ta–O and ∼2.5 for Bi–Ta–O, and an energy of ∼0.8 eV from involved trap centers to the conduction band for both dielectrics. [ABSTRACT FROM AUTHOR]

While standard bipolar switching RRAM memory devices can be programmed into different resistance states, the complementary switching mechanism allows for two distinct switching locations that each can be programmed to these resistance states. In this letter, we present a technique to discriminate these switching locations and report on a novel scheme allowing for sub- \mu \texts pulse write and read of eight different logic states in Pt/Ta2O5/Ta/Pt devices by using only four different resistive states. Thus, in addition to the multilevel capability of bipolar switching devices, double the information can be stored and read in a single complementary switching device. [ABSTRACT FROM AUTHOR]

The influence of two different ohmic electrodes (W and Ta) on the resistive switching characteristics of TaOX-based resistive random access memory (ReRAM) devices has been studied. Consistently, higher resistance OFF states have been observed with the W-ohmic electrode under the same operational conditions for all reset stop voltages ( V\text {reset-stop}) during both the dc and ac measurements. The deeper reset for samples with W-electrode is attributed to the less negative Gibbs-free energy of W-oxide compared with Ta-oxide, resulting in easier re-oxidation of the filament through oxygen exchange with the W-electrode. The higher R\mathrm{\scriptscriptstyle OFF}/R\mathrm{\scriptscriptstyle ON}(> 10^{3}) with the W-electrode enables 3-bit multilevel cell operation in the Pt/W/TaOX/Pt ReRAM device. An excellent retention for these eight states is demonstrated at 125 °C for 10^4 s. Furthermore, the TaOX ReRAM device with both the electrodes shows high endurance up to 10^6 cycles based on two states. [ABSTRACT FROM PUBLISHER]

The origin of the ferroelectric polarization switching in orthorhombic HfO2 has been investigated by first principles calculations. The phenomenon can be regarded as being the coordinated displacement of four O ions in the orthorhombic unit cell, which can lead to a saturated polarization as high as 53 μC/cm². We show the correlation between the computed polarization reversal barrier and the experimental coercive fields. [ABSTRACT FROM AUTHOR]

In this letter, CMOS-compatible Ni/HfO2/TiN resistive random access memory stacks demonstrated attractive unipolar switching properties, showing >103 endurance and long retention at 150 °C. The Ni bottom electrode (BE) improved the switching yield over the NiSiPt BE. To better understand the unipolar forming mechanism, ab initio simulation and time of flight-secondary ion mass spectroscopy were utilized. Compared to the NiSiPt BE, Ni BE gives larger Ni diffusion in the HfO2 and lower formation enthalpy of Ni2+ species during electrical forming. Both the electrical and physical results supported a Ni-injection mechanism for the filament formation. [ABSTRACT FROM AUTHOR]

Future development of ferroelectric random access memory requires the integration of three-dimensional ferroelectric capacitors (FeCAP) in a microelectronic architecture. In this paper, pin-shaped two-dimensional and three-dimensional Sr0.8Bi2.2Ta2O9-based FeCAP arrays were characterized using an original approach combining micro-focused synchrotron X-ray fluorescence and diffraction. From a modelling approach, dimensional, chemical and microstructural details were extracted over a series of capacitor arrays. Hence, these techniques proved insightful for a non-destructive benchmarking approach. In addition, for all investigated capacitor geometries, the micro-fluorescence experiments evidenced a well controlled chemical composition within the FeCAP arrays, attesting the stability of the elaboration and integration steps. Nevertheless, micro-diffraction experiments have indicated a lower control of the ferroelectric film crystallographic orientation from one capacitor to the next, despite a well defined chemical composition. This feature was attributed to the presence of patterned bottom electrodes, bringing to light the role of the slanted capacitor sidewalls in the three-dimensional geometry and partially explaining the non-optimized electrical polarization usually measured in three-dimensional capacitors with respect to planar geometry. [ABSTRACT FROM AUTHOR]

This paper reports on the integration of MOCVD SBT based stacked ferroelectric capacitors in a 0.35 μm CMOS technology for embedded FeRAM, using a ferroelectric capacitor (FeCap) that is scalable to a 3D configuration. The electrical parameters of the CMOS devices with and without FeCap integration are presented. No significant shifts of the CMOS characteristics are noticed after FeCap integration. Also the effects of the CMOS processing on the FeCaps are discussed. There is no degradation of the hysteresis loops of the FeCaps throughout the CMOS back-end-of-line processing, which proves that the integrated encapsulation layer effectively prevents any hydrogen induced damage. Another key element in the successful integration of stacked ferroelectric devices is the conductive oxygen barrier, which protects the plugs from oxidation. A low contact resistance and a high yield are obtained on plug contacts covered by the oxygen barrier. Also damage free etching is a critical issue in FeRAM integration. Excellent results are presented for the etch process to form the contacts to the electrodes. [ABSTRACT FROM AUTHOR]

A new, environmentally friendly, aqueous chemical solution deposition (CSD) route for the fabrication of BLT thin films is presented. Well crystallized, preferentially c-axis oriented films were obtained at 650°C. The grain size and remanent polarization were increased by the addition of an excess of Bi3+ to the precursor solution. At 10% Bi excess a Pr of 6.2 μC/cm2 and Ec of 105 kV/cm were obtained. [ABSTRACT FROM AUTHOR]

Sr0.9Bi2.2Ta2O9 in films were prepared on a Pt-substrate by aqueous chemical solution deposition. The applied heat treatment seems to have a significant influence on the ferroelectric response, crystallinity and surface morphology. For two-layer thin films annealed at 700°C in oxygen, the Pr-value increased from 3.6 until 5.5 μC/cm2, c-axis preferred orientation was considerably lowered and rod-like instead of spherical grains were grown when the second pyrolysis step at 520°C was omitted. [ABSTRACT FROM AUTHOR]

This paper presents a preliminary study of the degradation and restoration of ferroelectric properties under X-ray irradiation of Pt/SBT/Pt ferroelectric capacitors (FeCAP) and the degradation in Pt/PZT/Pt FeCAPs. The results seem to point out that aging of irradiated capacitors originates from the pinning of domain walls due to trapping of photo-generated carriers at domain boundaries. For SBT FeCAPs, the recovery of a significant switchable polarisation after electrical cycling under X-ray irradiation indicates that the domain walls are probably weakly pinned. In PZT FeCAPs, the fatigue behaviour during X-ray exposure can be explained as a result of X-ray induced acceleration of the bulk screening process due to increasing of the concentration of bulk carriers, which leads to both static and kinetic imprint effects. [ABSTRACT FROM AUTHOR]

Provides information on a study that examined the relationship between sol-gel processing conditions and orientation selection in lead(Zr[sub1-x]Ti[subx]O[sub3] films on platinum electrodes. Methodology of the study; Results and discussion on the study.

The thermal and mechanical stability of Ir and Ir\Pt metals spacers deposited on top of Ti(Al)N\Ir\IrO 2 patterned structures has been investigated in pseudo 3D stacked SrBi 2 Ta 2 O 9 (SBT) capacitors. Their stability was compared to standard TEOS spacers. The high compressive stress at the edge of patterned electrodes, as a consequence of the high thermal expansion mismatch between the metals used in the electrode and TEOS, make the system mechanically unstable at the SBT crystallization conditions (700°C for 1 hour). The mechanical problems could be overcome if the same noble metals used in the electrode are incorporated as spacers. However, thermal stability during the SBT crystallization conditions is still an issue. For the case of Ir, surface oxidation decreases the SBT polarization values. In the case of Ir\Pt, Ir diffuses through Pt and oxidizes, leading to unstable patterned structures and to the oxidation of the Ti(Al)N layer. [ABSTRACT FROM AUTHOR]