Non-standard switching phenomena and interface effects in resistive switching devices (2018)

The rapid progress of electronics over the past decades has altered many aspects of our lives fundamentally. As the conventional concepts for non-volatile memory technologies are approaching their physical scaling limits, new memory concepts are demanded. One of the most promising approaches to push the scalability limit further is the redox-based resistive switching random access memory (ReRAM). The resistive switching in these ReRAM memory devices is controlled by applying appropriate voltage pulses, whereby the information is stored as the resistance level of the device. Its simple, two-terminal structure in combination with a strong local confinement of the device region responsible for resistive switching allow for a remarkable scaling potential of ReRAM technology. Together with a high potential of energy efficiency, fast access and low cost it is considered as a potential competitor for both Flash and DRAM. Resistive switching is observed in various material systems, however, many questions regarding interplay between different materials and its impact on switching properties remain unclear. In this thesis, metal-oxide-based, VCM-type ReRAM cells are analyzed, with Ta2O5 material as the metal-oxide. While the basic bipolar resistive switching process can be understood from the presence and redistribution of oxygen vacancies inside the insulating metal-oxide layer, this cannot explain the occurrence (and structural control) of non-standard switching processes (i.e. complementary switching and eight-wise switching “sub-loops”). In this thesis it is shown that these phenomena can be explained by the physical processes taking place at the interfaces between the metal-oxide and the electrodes. The research work in this thesis is divided into three parts: In the first part, resistive switching in ReRAM cells is studied with regard to the material of the electrode at the switching interface, i.e. the active electrode. It is postulated that the oxygen exchange between the oxide and the active electrode can cause an additional competing resistive switching mode, the so-called eight-wise switching “sub-loops”. Introduction of an oxygen blocking interlayer at this interface allows for suppression of this oxygen exchange reaction and therefore of this additional switching mode, which is also important to achieve a higher OFF resistance state during RESET of the cell. In the second part, effects of the oxygen exchange at the opposite interface between the metal-oxide and a reactive metallic electrode are studied. This reactive metal electrode is also called oxygen exchange layer (OEL), due to its ability to extract oxygen out of the adjacent metal-oxide layer. It has been observed that the OEL thickness has a significant impact on the electrical properties of pristine ReRAM cells. Furthermore, a change of the preferential switching mode can be induced by tuning the OEL thickness. In this work it is postulated that oxygen incorporation into the metallic OEL electrode can increase its work function affecting the interfacial electronic barrier and by that induce occurrence of complementary switching instead of the standard bipolar switching. In the third and final part, two applications based on the oxygen exchange at the OEL electrode are experimentally analyzed and discussed, providing useful tools for direct tuning of the ReRAM device properties.