The role of interface and exchange processes in forming and switching of ZrO2 based ReRAM devices

Redox-based resistive random access memories (ReRAM) have many promising features like high scalability, low energy consumption and fast read and write times, making it a very promising candidate not only for future non-volatile memory, but as well for new computing concepts like neuromorphic computing. To date, the understanding of these devices is not deep enough to predict the behaviour of a device based on its specifications like geometry and material properties. This is a major issue, as it slows down the development of better ReRAM devices. Therefore, the aim of this work is to find correlations between the stack parameters and the properties of ReRAM devices to help overcoming this problem. First, the influence of the device materials and geometry is investigated. For that a model picture for forming is introduced, which allows to separate the forming voltage into an oxide thickness dependent and an interface dependent part. The oxide thickness dependent part is dominated by the oxide properties (like the breakdown field), the interface dependent part dominated by the electrode properties (like the oxygen affinity). To determine those properties, the forming voltage is investigated in dependence of the oxide and the ohmic electrode thickness. The interface voltage can be reduced by using ohmic electrode materials with high oxygen affinity like Hf. From the increased resistance and decreased capacitance of devices with e.g. Hf electrodes compared to Pt electrodes, it could be deduced that several nanometres of the electrode at the oxide/electrode interface are oxidized. This oxidized electrode layer is approximately 3 nm thick on a 5 nm oxide layer and becomes thicker with increasing oxide thickness underneath. Thereafter, the influence of a substoichiometric oxide layer as electrode is investigated. An increased oxygen content increases the forming voltage but can as well cause improved resistance states of the devices due to the built-in series resistance. After the investigation of materials and structure, further aspects of forming and switching are discussed. In order to reveal the processes happening during switching, hard x-ray photo emission electron microscopy (HAXPEEM) experiments were conducted on ReRAM devices. These measurements could show, that the oxide layer is actually reduced and that an oxygen exchange at the ohmic electrode layer is happening in these devices. A closer investigation revealed, that this exchange must be due to an interaction with the surrounding. Finally, it is shown that the asymmetry in the I-V-characteristics of the devices in the high resistance state is mostly due to a volatile effect. This effect is characterized in detail and an oxygen exchange with the Pt electrode or an electronic trapping / detrapping mechanism are considered as physical origins for the effect. The question which mechanism is the relevant one is still open. With these new insights into the physical processes happening during forming and switching, a further step towards a deeper understanding of ReRAM device behaviour is taken.