LAUSR

HfO$$_2$$2-based memristive switching devices are currently under intensive investigation due to their high performance and mature fabrication techniques. However, several critical issues have to be addressed to bring them from laboratory to market. We have recently looked into two important issues with the use of density functional theory methods. One is the wide distribution of device resistance in off-states. We have modeled the switching process of a Pt–HfO$$_2$$2–Pt structure for which quantized conductance was observed. Oxygen atoms moving inside a conductive oxygen vacancy filament divide the filament into several quantum wells. Device conductance changes exponentially when one oxygen atom moves away from interface into filament. We propose that the high sensitivity of device conductance to the position of oxygen atoms results in the large variation of device off-state resistance. Another issue that we have recently addressed is the poor switching performance of devices based on a TiN–HfO$$_2$$2–TiN structure. While recent experiments have shown that by inserting an “oxygen scavenger” metal between positive electrode and oxide significantly improves device performance, the fundamental understanding of the improvement is lacking. We provide detailed understanding how scavenger layers improve device performance. First, we show that Ta insertion facilitates the formation of on-states by reducing the formation energy. Second, the inserted Ta layer reduces the Schottky barrier height in the off-states by changing interface electric dipole at the oxide/electrode interface. Nevertheless, the device maintains a high on/off-resistance ratio. Finally, with Ta insertion the on-state conductance becomes much less sensitive to the specific location from which the oxygen was removed from the oxide. Our studies provide fundamental understanding needed for enabling realization of a nonvolatile memory technology with reduced energy consumption.