title: Switching in Emerging Memories and their Response to Heavy Ion Irradiation
Creators: name: Vogel, Tobias
Publisher: name: UNSPECIFIED
Year: 2023

description: Memory technologies are ubiquitous in our everyday life and the number of electronic devices relying on them keeps increasing rapidly. In established memories such as static random-access memory (SRAM), dynamic random-access memory (DRAM) and flash memory, further downscaling of transistors has slowed down leading to deviations from Moore’s law. However, this was so far considered a fundamental step towards ever increasing data storage, reduced energy consumption and lowered fabrication cost for decades. Furthermore, new applications such as image recognition or autonomous driving, which implement artificial neural networks, represent a challenge for the classic computer architecture. These challenges are the driving force for the development of new technologies such as emerging memories, including phase-change-based resistive, redox-based resistive and ferroelectric memories. Those memory types store information using different physical concepts than the established, mainly directly charge-based technologies. One important concept can be summarized under the term “memristor” or “memristive system”. It is based on a resistance change of a memory cell triggered by electric stimuli, which is dependent on the history of the memory material. The resistance in phase-change memories relies on the reversible transition of a phase-change material such as Ge2Sb2Te5 from an amorphous to a crystalline state. In contrast, the resistance in filamentary redox-based systems dependents on ion-migration and the formation/control of a conductive filament in a thin dielectric layer such as HfO2. This allows the achievement of at least two distinguishable memory states, which are repeatably accessible by applying voltage/current stimuli in a switching process. In ferroelectric memories, the data storage is based on the control of the electric polarization of a ferroelectric material such as doped HfO2. Due to their good scalability, non-volatility and fast as well as energy-efficient operation, these memories are currently discussed as promising candidates to replace or complement established memory concepts. Additionally, they have been demonstrated to show e.g., interesting synaptic-like properties usable for neuromorphic applications and prospective artificial intelligence. Furthermore, since the information storage principle of these emerging memory types is not directly based on charges, they are promising candidates for utilization in radiation-harsh environments, including e.g., aerospace applications. In general, all types of ionizing radiation of natural or artificial origin are a threat for microelectronics, as they can affect the functionality of electronic components. An extreme case is represented by high-energy heavy ions due to their large ionization potential and high penetration depth. This type of radiation can induce defects and phase transitions in materials relevant for memory technologies such as HfO2. This could adversely affect the functionality of emerging memories. Currently, all emerging memories are still in their infancy and not yet fully competitive with established technologies, especially in relation to a detailed understanding of the working principles of the memories, the fabrication of integrated circuits in complementary metal-oxide-semiconductor (CMOS)-compatible processes and production costs. In order to unveil the full potential of the emerging memories and to enable their use for different applications, a deeper understanding of the material properties and the basic physical switching processes is required. This also includes a better understanding of radiation-induced effects such as phase transitions in the materials and related switching properties of emerging memories. In this regard, research needs to be further advanced. In this context, this work addresses relevant influences on the switching characteristics of three selected emerging memory types, in particular HfO2-based resistive and ferroelectric memories as well as Germanium-Antimony-Tellurium (Ge-Sb-Te)-based phase-change memories and their response to heavy ion irradiation. For studies on non-irradiated HfO2-based memristive devices, specific model systems consisting of single metal-insulator-metal (MIM) capacitor stacks were prepared. This includes doping of HfO2 films with Zr to potentially induce ferroelectric properties in Hf0.5Zr0.5O2-based memory, which represent one of the most-promising candidates for ferroelectric memory applications. Additional studies were designed to investigate the influence of Zr-doping, the choice of the electrode materials and the oxide layer thickness on the resistive switching behavior of HfO2-based devices relevant for resistive memory applications (Pt/Hf0.5Zr0.5O2/Pt, Pt/HfO2/Pt and Cu/HfO2/Pt). For the preparation of Zr-doped HfO2 films, a growth routine was successfully established. It includes a co-evaporation process of Hf and Zr in a reactive molecular beam epitaxy setup allowing a precise control of the composition. The resistive switching studies have revealed functioning devices with no significant influence of the Zr-doping. Furthermore, a strong dependence of the choice of the electrode materials and the switching types on the dominating switching mechanism was found in Pt/HfO2/Pt and Cu/HfO2/Pt devices. While the switching in Pt/HfO2/Pt devices is based on a thermochemical mechanism, the obtained switching in Cu/HfO2/Pt devices yielding the best switching performance is based on an electrochemical mechanism. As a reduction of the layer thicknesses of electronic components is of huge interest, studying the related influences on the switching characteristics is essential. In a specifically designed study, the switching mechanism in Cu/HfO2/Pt devices under bipolar resistive switching was found to be dependent on the oxide layer thickness. A qualitative model based on a strongly Joule heating-assisted electrochemical mechanism was developed. Furthermore, to investigate the impact of ionizing radiation in the electronic energy loss regime on different memory materials, HfO2- and Ge-Sb-Te-based films containing different compositions and phases/crystallinity, respectively, were exposed to high-energy Au ions. These studies were conducted in close collaboration with project partners from CEA-Leti and LTM CNRS (Grenoble, France) as well as from Fraunhofer IPMS CNT (Dresden, Germany). Irradiation experiments were performed at the Helmholtzzentrum für Schwerionenforschung (Darmstadt, Germany) in close collaboration with the materials research group of Prof. Dr. C. Trautmann. By combining different characterization methods such as X-ray diffraction, X-ray photoelectron spectroscopy and different scanning transmission electron microscopy methodologies, an improved understanding of the basic mechanisms of the occurring phase transitions was achieved. This was supported by a direct comparison of irradiated with as grown HfO2-x films of different stoichiometry and crystal structure. Those films were prepared using a reactive molecular beam epitaxy setup, which allows a precise control of the oxygen content. In monoclinic HfO2 films, crystalline-to-crystalline phase transition based on induced oxygen vacancies to a rhombohedral phase of hafnium oxide was observed. This phase transition was found to be accompanied by a significant grain fragmentation at high fluences. In this context, a pattern matching routine was developed in cooperation with the electron microscopy research group of Prof. Dr. L. Molina-Luna for the analysis of a recorded, complex scanning precession electron diffraction dataset. This allowed the investigation of the phase transition at high spacial resolution. The irradiation of Zr- and Si-doped ferroelectric HfO2 as well as in Ge2Sb2Te5 (GST) and Ge-rich GST (GGST) phase-change layers showed similar irradiation-induced phase transitions. In ferroelectric HfO2-based layers, the transition from a polar to a non-polar phase occurred, whereas the phase transitions found in Ge-Sb-Te-based layers were related to beam-induced breaking and formation of chemical bonds. Structural characterization revealed that for all tested memory materials the irradiation effects strongly depend on the initial crystallinity and crystal structure as well as on the composition and microstructure of the layers. Moreover, the occurring phase transitions could be directly correlated to switching properties obtained from electrical measurements performed on memory devices. For oxide-based and phase-change random access memories (OxRAM and PCRAM), detrimental beam-induced effects on the access transistors of the one transistor-one memory cell (1T1R) arrays are found to be dominating the failure mechanism at ion fluences exceeding 5×1010 ions/cm², while it is indicated that the memory cells are radiation-hard as long as no phase transitions occur in the layers. In ferroelectric capacitors, the induced phase transition from the polar orthorhombic to a non-polar phase could even be reversed by electric field cycling after irradiation. Hence, all tested memory types were found to be extremely radiation-resilient. It was shown that a combination of different characterization techniques is needed to achieve a full understanding on occurring structural and compositional changes and related electrical properties. This can also be recommended as general guidelines for the characterization and interpretation of complex irradiation-induced effects in emerging memories. Overall, this work provides an improved understanding of resistive switching phenomena and of basic mechanisms of irradiation effects in HfO2- and Ge-Sb-Te-based memories. This is important since these emerging memories are promising candidates for future memory applications replacing or supplementing existing memory technologies. In this context, this work provides useful knowledge for the development of new strategies for the fabrication and characterization of emerging memories.