Redox-based resistive switching random access memory (ReRAM) are currently of high interest for scaled non-volatile memory in embedded, storage class, and mass memory applications. The broad field of application demands for optimization of device characteristics in different directions, that is, for example, low voltage or low current operation, electroforming free behavior, large memory window, low variability of the switching parameters, high nonlinearity in the current-voltage characteristics, high switching endurance or long data retention. From this rather long list it already becomes clear that there will not be a single ReRAM cell with all criteria fulfilled at the same time. Therefore, it is necessary to understand the correlation between material properties and cell design on the switching behavior of the devices.

A single valence-change-mechanism (VCM)-type switching cell is typically built from a transition metal oxide layer sandwiched between metal electrodes of different oxidation enthalpy and it is accepted in the scientific community that oxygen exchange and drift/diffusion processes play a major role in the resistive switching process. Thus, changing the oxidation/reduction enthalpies by means of a variation of the layer stack or introduction of additional barrier layers is of utmost importance.

Aiming towards future three-dimensional (3D) integrated arrays of ReRAM cells atomic layer deposition (ALD) will become the method of choice to grow ultra-thin, conformal, homogeneous and dense oxide films at a thermal budget below 450 °C.

This presentation will focus on VCM-type bipolar switching devices built from nano-crossbar structures sandwiching different stacks of metal oxide layers. We will show that the variability of the switching parameters of HfO₂ based ReRAM cells can be significantly improved for HfO₂/TiO₂ bilayer based devices where the resistive TiO₂ layer acts as an internal current limiter. The effect of thickness variation on the switching characteristics of Al₂O₃/TiO₂ bilayer cells will be covered as well as possibilities to achieve electroforming-free devices for two different materials, which are TiO₂ and HfO₂. Furthermore, the appearance of two types of switching polarity in TiO₂ cells will be demonstrated and will be discussed based on a competition of different redox chemical reactions. Finally, we will demonstrate the functionality of memristive plus threshold-type selector functionality obtained after electroforming of nano-crossbar cells based on a 10 nm thin amorphous niobium oxide layer.

All devices were smaller than 0.01 µm² in size and the oxide layer thickness was less than 10 nm indicating the important need for understanding the effect of interfacial reactions on the restive switching behavior of oxide based ReRAM cells. At the same time it opens the possibility for device tailoring by means of proper materials design.