First-principles study on metal-TaO$_x$-metal heterostructures: response to applied bias voltages*

Metal-TaO$_x$-metal heterostuructures are promising as a novel nonvolatile memory device [1]. The formation of conduction paths in the TaO$_x$ layer is responsible for the low resistance state and the switching mechanism is understood as the electrochemical redox reaction involved with the bias-voltage application. However, microscopic details of the conduction path and switching mechanism have not been clarified yet. We examine electronic states, and electron and ion transport in Cu-TaO$_x$ (x$\sim$2.5)-Pt(or Cu) heterostructures from first principles, focusing on the response to applied bias voltages. We show that Cu interstitials in crystalline Ta$_2$O$_5$ enhances electronic conduction considerably [2], while does not in amorphous one. We also show that the potential change due to the bias application is sensitive to the structure of TaO$_x$ layer and/or metal-TaO$_x$ interface: in some case, the potential change may be very small near the Cu/TaO$_x$ interface in the TaO$_x$ layer so that the bias application hardly change the mobility of Cu ions in this region. These results will be discussed in terms of electronic states. [1] T. Sakamoto et al., APL 91 (2007) 092110; [2] T. K. Gu et al., ACS Nano 4 (2010) 6477.