Standards for the Characterization of Endurance in Resistive Switching Devices

Resistive switching (RS) devices are emerging electronic components that could have applications in multiple types of integrated circuits, including electronic memories, true random number generators, radiofrequency switches, neuromorphic vision sensors, and artificial neural networks. The main factor hindering the massive employment of RS devices in commercial circuits is related to variability and reliability issues, which are usually evaluated through switching endurance tests. However, we note that most studies that claimed high endurances >106 cycles were based on resistance versus cycle plots that contain very few data points (in many cases even <20), and which are collected in only one device. We recommend not to use such a characterization method because it is highly inaccurate and unreliable (i.e., it cannot reliably demonstrate that the device effectively switches in every cycle and it ignores cycle-to-cycle and device-to-device variability). This has created a blurry vision of the real performance of RS devices and in many cases has exaggerated their potential. This article proposes and describes a method for the correct characterization of switching endurance in RS devices; this method aims to construct endurance plots showing one data point per cycle and resistive state and combine data from multiple devices. Adopting this recommended method should result in more reliable literature in the field of RS technologies, which should accelerate their integration in commercial products

Towards Oxide Electronics:a Roadmap

At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore's law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community. Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructures and devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics

Memristor devices based on low-bandwidth manganites

This dissertation investigates the phenomenon of resistive switching (RS) in lowbandwidth mixed-valence perovskite manganite oxides. In particular, the compounds Pr0.6Ca0.4MnO3 and Gd1−xCaxMnO3 with x between 0 and 1 are studied. The steps of sample fabrication, crystalline properties and measurements to verify the quality of the devices are also reported. The thin film memristor devices were fabricated from target pellets using pulsed laser deposition on single crystal SrTiO3 substrates. The crystallinity was verified using X-ray diffraction and the elemental composition by energy dispersive X-ray spectroscopy. The fabricated thin films were used to create memristor devices by depositing patterned metal electrodes on them by either DC magnetron sputtering or e-beam physical vapor deposition. When the studied materials were combined with a reactive electrode material, the formed interface exhibited the phenomenon of resistive switching, where the resistance of the device can be modified non-volatilely by application of electric field to the terminals of the device. The noble metals Au and Ag were found to be optimal for the passive interfaces, and Al as the active interface. The RS properties of the devices made with the optimal electrode configuration were studied in detail. The devices were found to have asymmetric bipolar RS with promising characteristics. The studies encompassed varying the calcium doping of the samples, studying the endurance and timing characteristics of the RS phenomenon as well as measuring the device characteristics as a function of temperature. The RS properties were found to vary significantly over the calcium doping range. When the measurement results were used in a conduction model analysis, the switching properties were found to be correlated with the trap-energy level of the Al/GCMOinterface region. Lastly, the GCMO memristor devices were modeled successfully using a compact model compatible with circuit simulators and the biologicallyinspired spike-timing-dependent plasticity learning rule was demonstrated. In conclusion, GCMO is a promising new material for RS-based neuromorphic applications due to its stable switching properties. The unexpected differences between GCMO and PCMO show that there are still many unexplored RS properties and behaviors within the manganite family that can be explored in future research

Characterization of the doped silicon dioxide and its implications on the resistive switching phenomena in the electrochemical metallization cells

In this Master's thesis, the switching behavior of the doped and undoped SiO2-based memory cells was compared. The aim of doping was to enhance the switching behavior of the ECM memory cells. About 270 samples were sputtered using the CT1000 cluster deposition tool in the IWE2 of RWTH Aachen University. For the deposition of the thin films, the platinum, titanium nitride and Al2O3 substrates were used. The deposition was performed by using three differently doped targets. The physical characterization of the thin films was done using SEM, XRR, XRD, and EDX. Electroforming and electric characterization of the fabricated memory cells were made in the probe station with the light microscope and the Keithley electrometer. The results of the physical and electrical characterization were analyzed using the principle of Exploratory Data Analysis (EDA). The analysis of the result shows that two undoped samples on the platinum substrate and some doped samples exhibit the unexpected volatile threshold switching of metallic and semiconductive origin, respectively. Linear fitting of the measurement data in a logarithmic scale suggests that Schottky- and Frenkel- Poole conduction mechanisms are not dominant

Defect Engineering in HfO2/TiN-based Resistive Random Access Memory (RRAM) Devices by Reactive Molecular Beam Epitaxy

Recently, there has been huge interest in emerging memory technologies, spurred by the ever increasing demand for storage capacities in various applications like Internet of Things (IoT), Big Data, etc. CMOS based flash memory, the current mainstay of the memory technology, has been able to increase its density by scaling down to a 16 nm node and further implementation of 3D architectures. However, flash memory is expected to soon run into disadvantage due to challenges in further scaling. Therefore, extensive efforts are being made towards developing new devices for the next generation of non-volatile memories with the combined advantages of flash memory like non-volatility, high density, low cost and low power consumption as well as high speed performance of DRAM. Among the many competitors, resistive random access memories (RRAM) based on resistive switching in oxides are promising due to its simple metal-insulator-metal (MIM) structure, fast switching speeds (<10 ns), excellent scalability (<10 nm) and potential for multi-level switching. RRAM devices based on the popular dielectric-metal gate combination of hafnium oxide (HfO2) and titanium nitride (TiN), which is the subject of research in this work, are particularly interesting due to its compatibility with existing CMOS technology in addition to the aforementioned advantages. Though prototype RRAM chips have already been demonstrated, key problems for commercial realization of RRAM include large variability and insufficient understanding of the complex switching physics. Resistive switching mechanism in oxides is generally understood to be mediated via the transport of oxygen ions leading to the formation of a conductive filament composed of oxygen vacancy defects. Appropriate defect engineering approaches offer potential towards tailoring the switching behavior as well as improving the performance and yield of HfO2-RRAM. In this thesis, the impact of pre-induced defects on the resistive switching behavior of HfO2-RRAM is investigated in detail and our results are presented. Defect engineered oxide thin films were deposited using reactive molecular beam epitaxy (RMBE) to fabricate metal oxide/TiN based devices. RMBE technique offers the unique possibility to precisely and reproducibly control the oxygen stoichiometry of the thin films in a wide range. Using RMBE, defects were introduced in polycrystalline HfOx thin films intrinsically by oxygen stoichiometry engineering and extrinsically via impurity doping (trivalent lanthanum and pentavalent tantalum). Both the studies were performed at at CMOS compatible deposition temperatures (< 450 °C) with an eye on practical applications. Prior to tantalum doping in HfO2, oxygen stoichiometry engineering studies were also performed in amorphous tantalum oxide (TaOx) thin films to identify the oxidation conditions of tantalum metal. The density of oxygen stoichiometry engineered thin films of HfOx and TaOx could be tuned in a wide range from that of the bulk oxide density to close to metallic density. High degree of oxygen deficiency in oxides led to the formation of defect states near the Fermi level as well as multiple oxidation states of the metal, as observed by X-ray photoelectron spectroscopy (XPS). The pure stoichiometric hafnium oxide films crystallize as expected in a stable monoclinic structure (m-HfO2) whereas, oxygen deficient HfOx thin films were found to crystallize in vacancy stabilized tetragonal like structure (t-HfO2-x). Impurity doping also led to the stabilization of higher symmetry tetragonal (t-Ta:HfOx) or cubic structures (c-La:HfOx) depending on the ionic radii of the dopant. The growth of TiN thin films was also investigated using RMBE. The devices used for electrical studies in this work mostly involved deposition of oxides by RMBE on polycrystalline TiN/Si electrodes after ex-situ transfer for further deposition. Therefore, RMBE grown TiN thin film electrodes with similar or better quality would allow in-situ uninterrupted deposition of subsequent oxide layers in future to form cleaner interfaces. Optimized conditions for growth of epitaxial TiN films on the commercially relevant (001) oriented silicon and c-cut sapphire substrates were established, with focus on achieving smooth surfaces and low resistivity. High quality epitaxial TiN(111)||Al2O3(0001) and TiN(001)||Si(001) films with a low resistivity (20-200 uOhm.cm) were achieved, in spite of the large lattice mismatch. Very low surface roughness, characterized by a streaky reflection high energy electron diffraction (RHEED) pattern during TiN film growth was additionally obtained, by tuning the Ti/N flux ratios. Oxygen engineered HfOx/TiN devices were further electrically characterized to obtain I-V characteristics during quasi-static DC switching. Usually, an initial electroforming step (high voltages) is required to obtain further reproducible switching operation (at lower voltages). High device to device variability in RRAM is typically associated with the stochastic nature of electroforming process which increases at higher forming voltages. Using highly oxygen deficient HfOx and TaOx films, the forming voltages were found to be reduced to levels close to operating voltages, paving the way for forming-free devices. However, the use of high defect concentration adds to increasing the complexity of the switching mechanism. This is reflected in the rather complex and dissimilar switching behaviors observed in the myriad of similar RRAM devices reported in the rapidly growing literature. Using model Pt/HfOx/TiN-based device stacks; it is shown that a well-controlled oxygen stoichiometry governs the filament formation and the (partial) occurrence of multiple resistive switching modes (bipolar, unipolar, threshold, complementary). These findings fuel a better fundamental understanding of the underlying phenomena for future theoretical considerations. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. While a bipolar switching occurs in all the devices irrespective of defect concentration, switching modes like unipolar and threshold switching is favored only at higher oxygen stoichiometry. This suggests the suppression of thermal effects via higher heat dissipation and lowered concentration gradient of oxygen vacancies in oxygen deficient devices. A qualitative switching model based on the drift, diffusion and thermophoresis of oxygen ions is suggested to account for the partial occurrence of various switching modes depending on the oxygen stoichiometry. Further, the evolution or drift of high resistance states during endurance test of the common bipolar operation is compared for HfO2 and HfO1.5 based devices and interpreted using the quantum point contact (QPC) model. Similar observations regarding switching modes were also obtained in oxygen engineered Pt/TaOx/TiN devices, therefore allowing the findings to be generalized to other filamentary resistive switching oxides and contributing towards developing a unified switching model. Besides finding application as non-volatile memory, RRAM devices are also promising for hardware implementation of neuromorphic computing. This is motivated by the possibility of multi-level switching or gradual (analog) modulation of resistance in an RRAM device which can emulate biological synapses. Defect engineering approaches have thus been investigated in Pt/hafnium oxide/TiN devices for tuning the DC I-V switching dynamics to achieve multi-level or gradual switching electronic synapses. Higher contribution of thermal effects in pure stoichiometric HfO2 typically results in a single sharp set process and abrupt sharp current jumps during the reset process during a conventional bipolar operation. By using ~18% La-doped HfOx based device, a completely gradual reset behavior with a higher ON/OFF ratio could be achieved during the bipolar reset operation. This is likely related to filament stabilization around the dopant sites allowing a uniform rupture during reset. More interestingly, in oxygen deficient HfO1.5 based devices, intermediate conductance states corresponding to integer or half-integer multiples of quantum conductance (G0) was observed during both the set and reset operations at room temperature. These are related to the better stabilization of intermediate atomic size filament constrictions during the switching process. Occurrence of these intermediate quantum conductance states, especially during the typically abrupt set process, is likely aided by a weaker filament and better thermal dissipation in the highly oxygen deficient devices. These results suggest that a combination of doping and high oxygen vacancy concentration may lead to improved synaptic functionality with concurrent gradual set and reset behaviors

Nickel oxide thin films grown by chemical deposition techniques: Potential and challenges in next‐generation rigid and flexible device applications

Funder: Aziz FoundationFunder: Downing College, CambridgeFunder: Isaac Newton Trust; Id: http://dx.doi.org/10.13039/501100004815Abstract: Nickel oxide (NiO x ), a p‐type oxide semiconductor, has gained significant attention due to its versatile and tunable properties. It has become one of the critical materials in wide range of electronics applications, including resistive switching random access memory devices and highly sensitive and selective sensor applications. In addition, the wide band gap and high work function, coupled with the low electron affinity, have made NiO x widely used in emerging optoelectronics and p‐n heterojunctions. The properties of NiO x thin films depend strongly on the deposition method and conditions. Efficient implementation of NiO x in next‐generation devices will require controllable growth and processing methods that can tailor the morphological and electronic properties of the material, but which are also compatible with flexible substrates. In this review, we link together the fundamental properties of NiO x with the chemical processing methods that have been developed to grow the material as thin films, and with its application in electronic devices. We focus solely on thin films, rather than NiO x incorporated with one‐dimensional or two‐dimensional materials. This review starts by discussing how the p‐type nature of NiO x arises and how its stoichiometry affects its electronic and magnetic properties. We discuss the chemical deposition techniques for growing NiO x thin films, including chemical vapor deposition, atomic layer deposition, and a selection of solution processing approaches, and present examples of recent progress made in the implementation of NiO x thin films in devices, both on rigid and flexible substrates. Furthermore, we discuss the remaining challenges and limitations in the deposition of device‐quality NiO x thin films with chemical growth methods. imag

In situ studies of catalytic processes by Near Ambient X-ray Photoelectron Spectroscopy

A a portada logotip del Sincrotò "Alba"(English) Labels misplacement in the bottles are currently causing problems in wine cellar's labelling lines. According to data provided by Codorníu and Torres cellars, 1 % of the bottles present this issue. Once the wine or cava have been bottled, the bottles pass through the labeling station and the scanner separates the correctly labeled bottles from those with imperfections. When a considerable number of bottles are involved, the wine is emptied and rebottled, while the labeled bottle is discarded. However, if few bottles are mislabeled, a worker manually removes the labels with a scraper and retums the bottles to the labeling line. Of course, this is a problem. In this thesis, a system has been developed consisting of a pressure-sensitive adhesive anda detaching station that allows the adhesive to be deactivated for easy removal of misplaced labels. The adhesive had to meet the current market requirements and to be easily peeled off easily in the detaching station to leave the bottle completely clean to it can be relabelled without being taken out of the labelling lines. As bottles are full when they pass through the detaching station, a technology that could quickly remove the labels from the bottles at 25ºC was required to avoid affecting the quality of the product. lt was designed to include a cleaning solution bath, a thermostatic, and a mechanical agitator. Several chemical components with different weight ratios were tested for the cleaning solution formulation to determine the most effective solution. The design of the adhesive was sub-divided into 5 parts. First, the influence of the soft monomers: n-butyl acrylate and 2- ethylhexyl acrylate influence was studied via semi-batch emulsion polymerization taking as a starting point a formulation recipe composed of n-butyl acrylate and acrylic acid. In the second part, the influence of including a hard monomer, such as acrylonitrile, was studied. In both studies it was observed that the adhesives showed poor water resistance. For it, in the third part, different polymerizable surfactants were studied as stabilizer of the emulsion and were compared with the conventional surfactant used in the base formulation. The use a polymerizable surfactant clear1y increased the water resistance of the adhesives. Although the incorporation of 2-ethylhexyl acrylate showed better adhesive performance, the acrylonitrile showed better results in the detaching bath. For it, it was decided to continue the experiments introducing in the formulation the weight ratio of acrylonitrile that showed the best balance between the adhesive properties and its performance in the detaching bath and the polymerizable surfactant that showed the highest water resistance. In the fourth part, the optimization of the adhesive properties balance was carried out by the study of the influence of acrylic acid as functional monomer and the tert-dodecyl mercaptan as chain transfer agent. Clearly, the acrylic acid improved all adhesive properties. However, the maximum amount of acrylic acid testad only improved the shear resistance since with the gel content increase, the peel resistance and tack properties deceased. On the other hand, the chain transfer agent only improved the peel resistance and tack. Therefore, it was not possible to achieve a good performance among the three adhesive properties. Finally, the influence of adhesiva preparation process was investigated. In this case an improvement in the balance of adhesive properties was observed. Several of the pressure-sensitive adhesives carne close to current market requirements, as well as showing excellent results in the detaching station.(Català) Els nanomaterials han rebut malta atracció com a catalitzadors no només perla seva elevada relació superfície-volum, sinó també perla capacitat de sintetitzar-los en diferents dimensions i formes, les quals poden influir en els processos catalítics tot exposant plans cristal·logràfics específics. Degut al caràcter superficial de la catàlisi, l'ús de tècniques sensibles a la superfície és clau, però la investigació de sistemes catalítics s'ha de fer en les condicions més realistes possibles (en condicions de reacció) per poder identificar i comprendre el comportament de les espècies actives en condicions de treball. Aquest tipus d’estudis són possibles mitjançant l’espectroscòpia fotoelectrònica de raigs-X en pressions properes a l’ambient (NAP-XPS). En aquest context, les principals línies d'investigació d'aquesta tesi, dividida entres capítols, s'han centrat en el disseny, síntesi i caracterització fisicoquímica de catalitzadors basats en cèria i deis seus centres actius, especialment mitjançant espectroscòpia fotoelectrònica de raigs-X en condicions “operando”, en dues reaccions especifiques: la oxidació del sutge i la oxidació del CO. La primera part esta dedicada a la síntesi de múltiples catalitzadors diòxid de ceri amb diferent nanoforma (cubs, bastonets, octaedres i poliedres, així com cubs i bastonets microestructurats amb plans {111}) per estudiar la influència deis plans exposats en la seva activitat catalítica per la reacció d’oxidació del sutge. S'ha avaluat la seva activitat catalítica per l'oxidació del sutge, i els nanocubs i bastonets de cèria (amb domini de cares {100} i {110}) mostren la millar activitat específica d'oxidació del sutge. Les mesures NAP-XPS han demostrat que les cares {100} i {11O} presenten la quantitat més elevada de Celll i de vacants d'oxigen durant el seu escalfament en atmosfera d'argó, i que la presència de sutge ha promogut altament la formació de Celll, confirmat també mitjançant ex-situ i “operando” Raman. La segona partes focalitza en l'estudi de la reacció d'oxidació de CO sobre un catalitzador policristal·lí de Pd/CeO2 per investigar la interacció entre el pal·ladi i el suport de céria i determinar la seva morfologia i la composició química i estructura electrònica deis diferents elements en la superfície de la mostra. S'ha preparat i caracteritzat fisicoquímicament un catalitzador convencional en pols (Pd/CeO2) abans d'estudiar la seva activitat per l'oxidació de CO amb NAP-XPS. Les mesures “operando” han permès la identificació de dues especies de Pd diferents en condicions d'oxidació de CO, essent PdxCe1-xO2--0 la fase activa. Aquesta part també inclou una anàlisi no destructiu del perfil de profunditat tot utilitzant energies d'excitació de sincrotró variables, dut a terme per esclarir la ubicació i el paper d'aquestes espècies de pal·ladi en la reacció d'estudi. La tercera i última partes basa en el disseny i preparació de catalitzadors inversos cèria/Pd mitjançant la deposició de capa atòmica (ALD) per investigar la reacció d'oxidació de CO en condicions “operando”. S'ha preparat i caracteritzat morfològicament un sistema model Pd/CeO2/Si i diversos sistemes inversos CeO2/Pd per investigar el rol de les diferents interfícies formades en l'activitat de la reacció esmentada. La optimització del disseny del sistema invers cèria/Pd ha comportat un estudi de la influència de la temperatura i del nombre de cicles de deposició sobre l'estructura i l'activitat d'aquests sistemes, i s'ha establert una relació entre la rugositat superficial deis sistemes i el nombre de cicles de deposició. També s'ha fet un seguiment de l'efecte de la temperatura sobre la uniformitat i continuïtat de les capes fines deis sistemes. Les mesures de NAP-XPS han permès concloure que 100 cicles d'ALD són el valor òptim que permet estudiar les fases de Ce i Pd deis sistemes, i que la presència de la fase activa PdxCe1-xOPostprint (published version