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Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen 1 Online-Ressource : Illustrationen, Diagramme (2022). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, Redox-based resistive random access memory (ReRAM) devices are due to their non-volatility, scalability and energy efficiency promising candidates for future information technology applications following the “More Moore” approach. Especially their ability to access multiple resistance states makes them attractive as artificial synapses for Beyond-von Neumann computing architectures like for example in-memory-computing concepts. Typical filamentary-type valence change mechanism (VCM) ReRAM cells consist of an insulating metal oxide layer sandwiched between an inert and a chemically active metal electrode. One of the favored metal oxides by the industry is HfO2, as it is already in use in semiconductor device manufacturing processes. Although bipolar-switching HfO2-based VCM devices are intensively studied for embedded memory and neuromorphic circuit applications, several issues are still to be addressed for a wider use. One topic arises from the need of an initial electroforming step, which creates the switching filament and influences the subsequent switching behavior. To ensure a reproducible electroforming process at a voltage compatible with the chosen complementary metal oxide semiconductor (CMOS) technology node, the effect of the ReRAM cell structure on the electroforming event needs to be well understood. Another topic arises from the high variability of certain switching parameters for standard single layer HfO2-based devices. These are in particular the rather broad cycle-to-cycle variations of the read values in high resistance state and of the threshold voltage which triggers the switching event. This limits the number of multilevel states that can be programmed into standard HfO2-based devices. In this work, these two issues are addressed by means of ReRAM cell design, nanoscale device fabrication and comprehensive characterization and interpretation of the results based on state of the art modeling. To be more precise, ReRAM devices are built from nanometer thin HfO2 single layers and HfO2/TiOx bilayers sandwiched between an electronically active platinum and a transition metal electrode which enables an oxygen exchange. The two metal oxide layers were realized by subsequent atomic layer deposition (ALD) processes without breaking the vacuum, as this technique enables pinhole free and reproducible ultrathin oxide layers on structured substrates. The results might be transferred into the realization of ultra-dense three-dimensional integrated ReRAM devices in new neuromorphic computing concepts. ReRAM devices from ALD HfO2/TiOx layers of a few nanometers are fabricated at device areas ranging from 20 μm2 to about 3,600 nm2. The large and small devices are realized in planar stack and crossbar configuration, respectively. A systematic study of the electroforming behavior of HfO2-based ReRAM cells is performed. This covers the effects of the device area, the oxide thickness, the material and thickness of the non-noble metal, and the temperature. The understanding of electrochemical interfacial redox-processes enabled the design of HfO2/TiOx bilayer stacks with a considerably reduced area dependency of the electroforming voltages. The HfO2/TiOx bilayer structures were scaled to 60 nm x 60 nm device area while keeping the electroforming voltage at a level which is suitable for CMOS-integration. Furthermore the switching variability of HfO2-based devices is addressed. In particular, it is demonstrated that the change from a HfO2 single layer to a HfO2/TiOx bilayer cell design results in a significant improvement of the switching variability. In addition, the proposed device design enables gradual-like resistance changes for both switching events. The effect of the device structure is explained by physics based simulations utilizing the recently provided “JART v1b” software of IWE-II, RWTH Aachen University. The resistive switching performance of the fabricated HfO2/TiOx-based nanostructured ReRAM devices is demonstrated by pulsed switching measurements that reveal fast switching behavior with transition times down to the 150 ps regime. To conclude, this work proposed HfO2/TiOx bilayer nanosized ReRAM devices fabricated by means of industrial relevant process technologies and demonstrates the clear potential for the use as artificial synapses in next generation neuromorphic circuits. The obtained results contribute to a deeper physical understanding of the influence of the device microstructure and the interfacial reactions on the final switching performance of the system. Furthermore they help to identify the cell parameters that can be utilized for tuning the device properties., Published by Aachen

APL materials 7(9), 091105 (2019). doi:10.1063/1.5108654, Published by AIP Publ., Melville, NY

The utilization of bipolar-type memristive devices for the realization of synaptic connectivity in neural networks strongly depends on the ability of the devices for analog conductance modulation under application of electrical stimuli in the form of identical voltage pulses. Typically, filamentary valence change mechanism (VCM)-type devices show an abrupt SET and a gradual RESET switching behavior. Thus, it is challenging to achieve an analog conductance modulation during SET and RESET. Here, we show that analog as well as binary conductance modulation can be achieved in a Pt/HfO2/TiOx/Ti VCM cell by varying the operation conditions. By analyzing the switching dynamics over many orders of magnitude and comparing to a fully dynamic switching model, the origin of the two different switching modes is revealed. SET and RESET transition show a two-step switching process: a fast conductance change succeeds a slow conductance change. While the time for the fast conductance change, the transition time, turns out to be state-independent for a specific voltage, the time for the slow conductance change, the delay time, is highly state-dependent. Analog switching can be achieved if the pulse time is a fraction of the transition time. If the pulse time is larger than the transition time, the switching becomes probabilistic and binary. Considering the effect of the device state on the delay time in addition, a procedure is proposed to find the ideal operation conditions for analog switching