Defect Induced Aging and Breakdown in High-k Dielectrics

abstract: High-k dielectrics have been employed in the metal-oxide semiconductor field effect transistors (MOSFETs) since 45 nm technology node. In this MOSFET industry, Moore’s law projects the feature size of MOSFET scales half within every 18 months. Such scaling down theory has not only led to the physical limit of manufacturing but also raised the reliability issues in MOSFETs. After the incorporation of HfO2 based high-k dielectrics, the stacked oxides based gate insulator is facing rather challenging reliability issues due to the vulnerable HfO2 layer, ultra-thin interfacial SiO2 layer, and even messy interface between SiO2 and HfO2. Bias temperature instabilities (BTI), hot channel electrons injections (HCI), stress-induced leakage current (SILC), and time dependent dielectric breakdown (TDDB) are the four most prominent reliability challenges impacting the lifetime of the chips under use. In order to fully understand the origins that could potentially challenge the reliability of the MOSFETs the defects induced aging and breakdown of the high-k dielectrics have been profoundly investigated here. BTI aging has been investigated to be related to charging effects from the bulk oxide traps and generations of Si-H bonds related interface traps. CVS and RVS induced dielectric breakdown studies have been performed and investigated. The breakdown process is regarded to be related to oxygen vacancies generations triggered by hot hole injections from anode. Post breakdown conduction study in the RRAM devices have shown irreversible characteristics of the dielectrics, although the resistance could be switched into high resistance state.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Multiscale modeling for application-oriented optimization of resistive random-access memory

Memristor-based neuromorphic systems have been proposed as a promising alternative to von Neumann computing architectures, which are currently challenged by the ever-increasing computational power required by modern artificial intelligence (AI) algorithms. The design and optimization of memristive devices for specific AI applications is thus of paramount importance, but still extremely complex, as many dierent physical mechanisms and their interactions have to be accounted for, which are, in many cases, not fully understood. The high complexity of the physical mechanisms involved and their partial comprehension are currently hampering the development of memristive devices and preventing their optimization. In this work, we tackle the application-oriented optimization of Resistive Random-Access Memory (RRAM) devices using a multiscale modeling platform. The considered platform includes all the involved physical mechanisms (i.e., charge transport and trapping, and ion generation, diusion, and recombination) and accounts for the 3D electric and temperature field in the device. Thanks to its multiscale nature, the modeling platform allows RRAM devices to be simulated and the microscopic physical mechanisms involved to be investigated, the device performance to be connected to the material's microscopic properties and geometries, the device electrical characteristics to be predicted, the effect of the forming conditions (i.e., temperature, compliance current, and voltage stress) on the device's performance and variability to be evaluated, the analog resistance switching to be optimized, and the device's reliability and failure causes to be investigated. The discussion of the presented simulation results provides useful insights for supporting the application-oriented optimization of RRAM technology according to specific AI applications, for the implementation of either non-volatile memories, deep neural networks, or spiking neural networks

Forming-Free Grain Boundary Engineered Hafnium Oxide Resistive Random Access Memory Devices

A model device based on an epitaxial stack combination of titanium nitride (111) and monoclinic hafnia (11 (Formula presented.)) is grown onto a c-cut Al 2O 3-substrate to target the role of grain boundaries in resistive switching. The texture transfer results in 120° in-plane rotated m-HfO 2 grains, and thus, in a defined subset of allowed grain boundary orientations of high symmetry. These engineered grain boundaries thread the whole dielectric layer, thereby providing predefined breakdown paths for electroforming-free resistive random access memory devices. Combining X-ray diffraction and scanning transmission electron microscopy (STEM)–based localized automated crystal orientation mapping (ACOM), a nanoscale picture of crystal growth and grain boundary orientation is obtained. High-resolution STEM reveals low-energy grain boundaries with facing ((Formula presented.)) and ((Formula presented.) 21) surfaces. The uniform distribution of forming voltages below 2 V—within the operation regime—and the stable switching voltages indicates reduced intra- and device-to-device variation in grain boundary engineered hafnium-oxide-based random access memory devices

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

A new opportunity for the emerging tellurium semiconductor: making resistive switching devices

Abstract: The development of the resistive switching cross-point array as the next-generation platform for high-density storage, in-memory computing and neuromorphic computing heavily relies on the improvement of the two component devices, volatile selector and nonvolatile memory, which have distinct operating current requirements. The perennial current-volatility dilemma that has been widely faced in various device implementations remains a major bottleneck. Here, we show that the device based on electrochemically active, low-thermal conductivity and low-melting temperature semiconducting tellurium filament can solve this dilemma, being able to function as either selector or memory in respective desired current ranges. Furthermore, we demonstrate one-selector-one-resistor behavior in a tandem of two identical Te-based devices, indicating the potential of Te-based device as a universal array building block. These nonconventional phenomena can be understood from a combination of unique electrical-thermal properties in Te. Preliminary device optimization efforts also indicate large and unique design space for Te-based resistive switching devices

Etude des cellules mémoires résistives RRAM à base de HfO2 par caractérisation électrique et simulations atomistiques

Among non-volatile memory technologies, NAND Flash represents a significant portion in the IC market and has benefitted from the traditional scaling of semiconductor industry allowing its high density integration. However, this scaling seems to be problematic beyond the 22 nm node. In an effort to go beyond this scaling limitation, alternative memory solutions are proposed among which Resistive RAM (RRAM) stands out as a serious candidate for NAND Flash replacement. Hence, in this PhD thesis we try to respond to many open questions about RRAM devices based on hafnium oxide (HfO2), in particular, by addressing the lack of detailed physical comprehension about their operation and reliability. The impact of scaling, the role of electrodes, the process of defects formation and diffusion are investigated. The impact of alloying/doping HfO2 with other materials for improved RRAM performance is also studied. Finally, our study attempts to provide some answers on the conductive filament formation, its stability and possible composition.La mémoire NAND Flash représente une part importante dans le marché des circuits intégrés et a bénéficié de la traditionnelle miniaturisation de l’industrie des sémiconducteurs lui permettant un niveau d’intégration élevé. Toutefois, cette miniaturisation semble poser des sérieux problèmes au-delà du noeud 22 nm. Dans un souci de dépasser cette limite, des solutions mémoires alternatives sont proposées parmi lesquelles la mémoire résistive (RRAM) se pose comme un sérieux candidat pour le remplacement de NAND Flash. Ainsi, dans cette thèse nous essayons de répondre à des nombreuses questions ouvertes sur les dispositifs RRAM à base d’oxyde d’hafnium (HfO2) en particulier en adressant le manque de compréhension physique détaillée sur leur fonctionnement et leur fiabilité. L’impact de la réduction de taille des RRAM, le rôle des électrodes et le processus de formation et de diffusion des défauts sont étudiés. L’impact de l’alliage/dopage de HfO2 avec d’autres matériaux pour l’optimisation des RRAM est aussi abordé. Enfin, notre étude tente de donner quelques réponses sur la formation du filament conducteur, sa stabilité et sa possible composition

IMPROVING THE HAFNIA-BASED RESISTIVE RANDOM-ACCESS MEMORY THROUGH MATERAL ENGINEERING

Ph.DDOCTOR OF PHILOSOPH

OXIDE-BASED MEMRISTIVE DEVICES BY BLOCK COPOLYMER SELF-ASSEMBLY

Oxide-based memristive systems represent today an emerging class of devices with a significant potential in memory, logic, and neuromorphic circuit applications. These devices have a simple capacitor structure and promise superior scalability together with favorable memory performances. This thesis presents a study of resistive switching phenomena in HfOx-based nanoscale memristive devices, with focus on material properties and development of bottom-up approaches for the fabrication of structures with dimension down to the nanoscale. One of the main issues for practical applications regarding device variability is first assessed by doping hafnium oxide films with different concentrations of aluminum atoms. Testing devices are analyzed by physico-chemical and electrical techniques in order to define the effect of oxide doping on the device properties. In the following part of the thesis, the scalability limit is explored in very high density arrays of nanodevices produced exploiting a lithographic approach based on the bottom-up self-assembly of block copolymer templates. This technique allows a tight control over the size and density of the defined features, and the possibilities offered by block copolymer patterning are here discussed. Electrical measurements of the nanodevices are performed through conductive atomic force microscopy. The device variability is examined and related to the inherent oxide non-homogeneity at the nanoscale, while a non-volatile switching of the resistance of the nanodevices is demonstrated. Further, this analysis draws the attention to a crosstalk phenomenon occurring at the nanoscale in a continuous thin film geometry. This result suggests to select different system configurations. A promising technique based on selective reactions with one copolymer block is finally discussed which allows the direct production of oxide patterns from block copolymer templates avoiding a pattern transfer process. In conclusion, the results reported in this thesis highlight the high scalability potential of oxide-based memristive devices, providing a missing piece of information for the understanding and practical development of very high density arrays

Pyroelectric and electrocaloric effects in hafnium oxide thin films

The material class of hafnium oxide-based ferroelectrics adds an unexpected and huge momentum to the long-known phenomenon of pyroelectricity. In this thesis, a comprehensive study of pyroelectric and electrocaloric properties of this novel ferroelectric material class is conducted. hafnium oxide is a lead-free, non-toxic transition metal oxide, and abundant in the manufacturing of semiconductor devices. The compatibility to existing fabrication processes spawns the possibility of on-chip infrared sensing, energy harvesting, and refrigeration solutions, for which this dissertation aims to lay a foundation. A screening of the material system with respect to several dopants reveals an enhanced pyroelectric response at the morphotropic phase boundary between the polar orthorhombic and the non-polar tetragonal phase. Further, a strong pyroelectric effect is observed when applying an electric field to antiferroelectric-like films, which is attributed to a field-induced transition between the tetragonal and orthorhombic phases. Primary and secondary pyroelectric effects are separated using high-frequency temperature cycles, where the effect of frequency-dependent substrate clamping is exploited. The piezoelectric response is determined by comparing primary and secondary pyroelectric coefficients, which reproduces the expected wake-up behavior in hafnium oxide films. Further, the potential of hafnium oxide for thermal-electric energy conversion is explored. The electrocaloric temperature change of only 20 nm thick films is observed directly by using a specialized test structure. By comparing the magnitude of the effect to the pyroelectric response, it is concluded that defect charges have an important impact on the electrocaloric effect in hafnium oxide-based ferroelectrics. Energy harvesting with a conformal hafnium oxide film on a porous, nano-patterned substrate is performed, which enhances the power output. Further, the integration of a pyroelectric energy harvesting device in a microchip for waste heat recovery and more energy-efficient electronic devices is demonstrated. High dielectric breakdown fields of up to 4 MV/cm in combination with a sizable pyroelectric response and a comparably low dielectric permittivity illustrate the prospect of hafnium oxide-based devices for future energy conversion applications