Probing the resistance switching mechanisms in SiOₓ/Ag RRAM devices

Resistive random access memory (RRAM) devices represent promising candidates for emerging non-volatile data storage applications and neuromorphic computing. In those devices, the resistance of a dielectric -often a binary oxide- is switched between a low resistance state (LRS) and one or more high resistance states (HRS) by the application of an appropriate external electrical bias. This resistance switching could be filamentary, i.e., involves the formation of a conductive filament. This filament can be thought of as chains of conductive oxygen vacancies (intrinsic resistance switching) or metallic atoms from an active device electrode (extrinsic resistance switching). In this thesis, the relationship between device electrode material and its resistance switching mechanism in SiOx (x∼1.9)-based RRAM devices was studied. Although it’s widely reported that RRAM devices with electrochemically active top electrodes, such as Ag, switch extrinsically, I show that both mechanisms and their associated conductive filaments can be triggered during device switching in ambient conditions. Resistance vs temperature measurements and conduction mechanism analysis were used to probe the nature of the formed filaments within device oxide layer. Results show that the two mechanisms can coexist within the device during switching. The type of filament generated by the initial electroforming of the device, however, depends on the polarity of the applied voltage during the electroforming step. This finding could help in optimising those RRAM devices for the different storage applications. Although the two mechanisms were observed under ambient conditions, SiOx/Ag devices showed extrinsic switching behaviour only under vacuum. In such an oxygen-poor environment, the contribution of intrinsic resistance switching mechanism appears to be reduced or probably eliminated. In extrinsically electroformed RRAM devices with Ag top electrodes, a metallic filament is likely to form within the switching layer. Using conductance tomography technique, the metallic filament of those SiOx/Ag devices was partially imaged using a conductive AFM (CAFM) tip

Local Characterization of Resistance Switching Phenomena in Transition Metal Oxides

The development of neuromorphic computing systems that emulate the analog charge states and plasticity of the brain’s neuron-synapse architecture has been a major driver of resistance switching materials exploration. Materials that demonstrate changes in conductance with tunable ratios and volatility of resistance states within a single layer are highly desirable. Although excellent resistance switching device performance has been demonstrated in a range of transition metal oxides, a lack of understanding of the fundamental microscale evolution of a material during resistance switching presents a key limitation to controlling switching parameters. Here, we examine the role of materials defects on local resistance switching structures in two representative transition metal oxide materials: HfOv2 thin films and hydrothermally synthesized VOv2 single crystals. In each material, we seek to clarify the structure of resistance switching domains and the kinetics of domain formation resulting from intentional defect introduction. This thesis is therefore divided into two main parts concerning (1) the introduction of planar defects in HfOv2 filamentary resistance switching devices, and (2) the impact of introduction of point defects on the metal-insulator transition in VOv2 single crystals. Part I (Sections 2 – 3) details investigation of Cu ion migration rates in Cu/HfOv2/p+Si and Cu/HfOv2/TiN devices in which oxide microstructure varies between amorphous, polycrystalline, and oriented polycrystalline. Ion migration across the oxide layer is shown to be rate limiting and faster in polycrystalline layers than in amorphous HfO2 layers at equivalent electric field. Moreover, the 3D shape of conductive filaments is investigated by a scribing atomic force microscopy experiment in Cu/HfOv2/p+Si devices and reveals a broad range of filament shapes under identical electrical stress conditions. Thermal dissipation is interpreted as the principal determinant of filament area, while oxide microstructure is shown to direct the location of filaments within the device. In part II (Sections 4 – 5), the hysteresis of the metal-insulator transition (switching volatility) in VOv2 is shown to intrinsically derive from nucleation limited transformations in individual particles. Here, hysteresis is a strong function of particle size, but may be increased or decreased by synthesis techniques that affect the concentration and potency of intrinsic point defects. Upon chemical doping with boron at interstitial lattice sites, a unique kinetic effect on the hysteresis of the current driven metal-insulator transition in two terminal BxVOv2 devices is observed. Dependence of the critical switching current on thermal relaxation time and temperature is characterized and recommendations for further kinetic testing are made. Finally, a few experimental extensions of the work presented in this thesis are made in Section 6

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

Probing the resistance switching mechanism in silicon suboxide memory devices

Redox-based resistive random access memory has the scope to greatly improve upon current electronic data storage, though the mechanism by which devices operate is not understood completely. In particular, the connection between oxygen migration, the formation of conductive filaments and device longevity is still disputed. Here, I used atomic force microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy to characterise the growth of filaments and the movement of oxygen in silicon-rich silicon oxide memory devices. As such, I was able to establish some of the chemical and structural differences between states of different resistance, which would correspond to binary data storage states. The oxide active layer is reduced simultaneously to the appearance of surface distortion and volumes of high conductivity in an otherwise-insulating material. These results support the established model of a resistance switching mechanism that relies on the migration of oxygen ions under an electrical bias, forming conductive pathways in the switching material. Notably, I demonstrate a reduction in the active layer stoichiometry as a result of electrical stress and show for the first time the presence of multiple filamentary growths in three dimensions in an intrinsic switching material. In addition, I have proven the efficacy of an extension to the method of profiling conductivity variations in insulators in three dimensions using conductive atomic force microscopy. However, in this case my findings conflict with the status quo of this methodology. In particular, I demonstrate that the measurement process significantly affects the scanning probe, leading to the likelihood of data inaccuracy. This highlights the needed for further development of the technique and careful analysis of the data obtained

Resistive switching devices with improved control of oxygen vacancies dynamics

L'abstract è presente nell'allegato / the abstract is in the attachmen

Substoichiometric Phases of Hafnium Oxide with Semiconducting Properties

Since the dawn of the information age, all developments that provided a significant improvement in information processing and data transmission have been considered as key technologies. The impact of ever new data processing innovations on the economy and almost all areas of our daily lives is unprecedented and a departure from this trend is unimaginable in the near future. Even though the end of Moore's Law has been predicted all too often, the steady exponential growth of computing capacity remains unaffected to this day, due to tremendous commercial pressure. While the minimum physical size of the transistor architecture is a serious constraint, the steady evolution of computing effectiveness is not limited in the predictable future. However, the focus of development will have to expand more strongly to other technological aspects of information processing. For example, the development of new computer paradigms which mark a departure from the digitally dominated van Neumann architecture will play an increasingly significant role. The category of so-called next-generation non-volatile memory technologies, based on various physical principles such as phase transformation, magnetic or ferroelectric properties or ion diffusion, could play a central role here. These memory technologies promise in part strongly pronounced multi-bit properties up to quasi-analog switching behavior. These attributes are of fundamental importance especially for new promising concepts of information processing like in-memory computing and neuromorphic processing. In addition, many next-generation non-volatile memory technologies already show advantages over conventional media such as Flash memory. For example, their application promises significantly reduced energy consumption and their write and especially read speeds are in some cases far superior to conventional technology and could therefore already contribute significant technological improvements to the existing memory hierarchy. However, these alternative concepts are currently still limited in terms of their statistical reliability, among other things. Even though phase change memory in the form of the 3D XPoint, for example, has already been commercialized, the developments have not yet been able to compete due to the enormous commercial pressure in Flash memory research. Nevertheless, the further development of alternative concepts for the next and beyond memory generations is essential and the in-depth research on next-generation non-volatile memory technologies is therefore a hot and extremely important scientific topic. This work focuses on hafnium oxide, a key material in next-generation non-volatile memory research. Hafnium oxide is very well known in the semiconductor industry, as it generated a lot of attention in the course of high-k research due to its excellent dielectric properties and established CMOS compatibility. However, since the growing interest in so-called memristive memory, research efforts have primarily focused on the value of hafnium oxide in the form of resistive random-access memory (RRAM) and, with the discovery of ferroelectricity in HfO₂, ferroelectric resistive random-access memory (FeRAM). RRAM is a next-generation non-volatile memory technology that features a simple metal-insulator-metal (MIM) structure, excellent scalability, and potential 3D integration. In particular, the aforementioned gradual to quasi-continuous switching behavior has been demonstrated on a variety of RRAM systems. A significant change of the switching properties is achievable, for example, by the choice of top and bottom electrodes, the introduction of doping elements, or by designated oxygen deficiency. In particular, the last point is based on the basic physical principle of the hafnium oxide-based RRAM mechanism, in which local oxygen ions are stimulated to diffuse by applying an electrical potential, and a so-called conducting filament is formed by the remaining vacancies, which electrically connects the two electrode sides. The process is characterized by the reversibility of the conducting filament which can be dissolved by a suitable I-V programming (e.g., reversal of the voltage direction). In the literature there are some predictions of sub-stoichiometric hafnium oxide phases, such as Hf₂O₃, HfO or Hf₆O, which could be considered as conducting filament phases, but there is a lack of conclusive experimental results. While there are studies that assign supposed structures in oxygen-deficient hafnium oxide thin films, these assignments are mostly based on references from various stoichiometric hafnium oxide high-temperature phases such as tetragonal t-HfO₂ (P4₂/nmc) or cubic c-HfO₂ (Fm-3m), or high-pressure phases such as orthorhombic o-HfO₂ (Pbca). Furthermore, the structural identification of such thin films proves to be difficult, as they are susceptible to arbitrary texturing and reflection broadening in X-ray diffraction. In addition, such thin films are usually synthesized as phase mixtures with monoclinic hafnium oxide. A further challenge in property determination is given by their usual arrangement in MIM configuration, which is determined by the quality of top and bottom electrodes and their interfaces to the active material. It is therefore a non-trivial task to draw conclusions on individual material properties such as electrical conductivity in such (e.g., oxygen-deficient) RRAM devices. To answer these open questions, this work is primarily devoted to material properties of oxygen-deficient hafnium oxide phases. Therefore, in the first comprehensive study of this work, Molecular-Beam Epitaxy (MBE) was used to synthesize hafnium oxide phases over a wide oxidation range from monoclinic to hexagonal hafnium oxide. The hafnium oxide films were deposited on c-cut sapphire to achieve effective phase selection and identification by epitaxial growth, taking into account the position of relative lattice planes. In addition, the choice of a substrate with a high band gap and optical transparency enabled the direct investigation of both optical and electrical properties by means of UV/Vis transmission spectroscopy and Hall effect measurements. With additional measurements via X-ray diffraction (XRD), X-ray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), the oxygen content-dependent changes in crystal as well as band structure could be correlated with electrical properties. Based on these results, a comprehensive band structure model over the entire oxidation range from insulating HfO₂ to metallic Hf was established, highlighting the discovered intermediate key structures of rhombohedral r-HfO₁.₇ and hexagonal hcp-HfO₀.₇. In the second topic of this work, the phase transition from stoichiometric monoclinic to oxygen-deficient rhombohedral hafnium oxide was complemented by DFT calculations in collaboration with the theory group of Prof. Valentí (Frankfurt am Main). A detailed comparison between experimental results and DFT calculations confirms previously assumed mechanisms for phase stabilization. In addition, the comparison shows a remarkable agreement between experimental and theoretical results on the crystal- and band stucture. The calculations allowed to predict the positions of oxygen ions in oxygen-deficient hafnium oxide as well as the associated space group. Also, the investigations provide information on the thermodynamic stability of the corresponding phases. Finally, the orbital-resolved hybridization of valence states influenced by oxygen vacancies is discussed. Another experimental study deals with the reproduction and investigation, of the aforementioned substoichiometric hafnium oxide phases in MIM configuration which is typical for RRAM devices. Special attention was given to the influence of surface oxidation effects. Here, it was found that the oxygen-deficient phases r-HfO₁.₇ and hcp-HfO₀.₇ exhibit high ohmic conductivity as expected, but stable bipolar switching behavior as a result of oxidation in air. Here, the mechanism of this behavior was discussed and the role of the r-HfO₁.₇ and hcp-HfO₀.₇ phases as novel electrode materials in hafnium oxide-based RRAM in particular. In collaboration with the electron microscopy group of Prof. Molina Luna, the studied phases, which have been characterized by rather macroscopic techniques so far, have been analyzed by wide-ranging TEM methodology. The strong oxygen deficiency in combination with the verified electrical conductivity of r-HfO₁.₇ and hcp-HfO₀.₇ shows the importance of the identification of these phases on the nanoscale. Such abilities are essential for the planned characterization of the "conducting-filament" mechanism. Here, the ability to distinguish m-HfO₂, r-HfO₁.₇, and hcp-HfO₀.₇ using high-resolution transmission electron microscopy (HRTEM), Automated Crystal Orientation and Phase Mapping (ACOM), and Electron Energy Loss Spectroscopy (EELS), is demonstrated and the necessity of combined measurements for reliable phase identification was discussed. Finally, a series of monoclinic to rhombohedral hafnium oxide was investigated in a cooperative study with FZ Jülich using scanning probe microscopy. Since recent studies in particular highlight the significance of the microstructure in stoichiometric hafnium oxide-based RRAM, the topological microstructure in the region of the phase transition to strongly oxygen deficient rhombohedral hafnium oxide was investigated. Special attention was given to the correlation of microstructure and conductivity. In particular, the influences of grain boundaries on electrical properties were discussed. In summary, this work provides comprehensive insights into the nature and properties of sub-stoichiometric hafnium oxide phases and their implications on the research of hafnium oxide-based RRAM technology. Taking into account a wide range of scientific perspectives, both, the validity of obtained results and the wide range of their application is demonstrated. Thus, this dissertation provides a detailed scientific base to the understanding of hafnium oxide-based electronics

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

Conductive filaments multiplicity as a variability factor in CBRAM

In this work we investigate the origin of the resistance variability for the low resistive state in conductive bridging memory devices (CBRAM). We use C-AFM tomography to enable the three-dimensional observation of the filaments and correlate the presence of double-branched conductive filaments to the variability in the device performance