Enhanced Resistive Switching in Complex Oxide Interfacial Memristors by Device Downscaling

Resistive switching devices are integral for realising physical computing systems. A large number of switching mechanisms have been observed and depend strongly on the material system. They can loosely be distinguished by the switching location as either occurring through the material bulk between two electrodes, or interface-type where switching takes place in a region localised to the area underneath the electrodes. This first kind of is widely studied and has been identified in a wide range of materials. The scalability of memristors is important for the development of large networks. For example, in valence change memristors multilevel analogue switching is observed in larger area memristors, but when downscaled this behaviour changes, and most devices show an abrupt transition.<br/>We have demonstrated that interface-based memristors on Schottky contacts on semiconductor complex-oxide platforms have interesting properties for computing purposes, such as multilevel continuous switching, energy landscapes that can be tuned by electric fields [1], and responses that can be used in learning algorithms [2]. The scalability of memristors is important for the development of large networks, but this area is so far largely unexplored for interface memristors. By performing transmission electron microscopy measurements on biased and unbiased samples we show that interfacial defects, particularly oxygen vacancies, play a key role in governing resistive switching. This raises the important question of whether memristive action is sustained when devices are downscaled and the number of defects is reduced. We address this by investigating Nb-doped SrTiO₃ electronic devices of areas spanning several orders of magnitude. Unexpectedly, we find that smaller devices exhibit larger resistance windows, which is desirable for network implementation. We explain these surprising results by considering the kinetics of trapping and de-trapping in dielectric materials from first principles to relate the decay constants from retention measurements to the trapping density. Larger values are found for smaller junctions, suggesting an increase in the density of traps is responsible for enhancing the resistance ratio.<br/>Typically, models of the Schottky barrier and interfacial electric field consider material-specific parameters and assume spatial homogeneity. In finite devices, however, this does not describe the system completely. It is expected that field lines will accumulate near the device edges, resulting in local enhancement of the field. This is especially important to consider for Nb-doped SrTiO₃ as the dielectric constant, and consequently, the Schottky barrier profile strongly depends on electric fields. We extend existing models of the electric field to include the device edge. Our work indicates an increased contribution of the device edge compared to the inner area when downscaled. This is an important finding for network scaling as it allows for a controlled way of increasing the memory window while limiting the electric field.<br/> <br/>[1] A. S. Goossens, A. Das, and T. Banerjee. <i>Journal of Applied Physics</i> <b>124</b>, 15 (2018)<br/>[2] T. F. Tiotto, A. S. Goossens, T. Banerjee, J. P. Borst, N. A. Taatgen. <i>Frontiers in Neuroscience</i> <b>19, </b>1456 (2021)