Effect of metallic buffer at electrode-oxide interface on current-voltage characteristics of resistive random access memories (ReRAMs): A first-principles study

We present the electric current ($I$)-voltage ($V$) characteristics (-1.0 eV $<$ $V$ $<$ +1.0 eV) for a model of ReRAM devices with metal-oxide-metal structures, based on first principles nonequilibrium Green's function (NEGF) theory [1]. We choose TiN and hafnia (HfO$_2$) for the electrode and oxide materials, respectively, because this combination has been widely known in literature. We investigate the $I$-$V$ characteristics for two different compositions of the TiN/HfO$_2$ interface, (a) with and (b) without the Ta buffer layer between TiN and HfO$_2$. We assume cubic HfO$_2$ layers for simplicity. For case (a), a clear distinction between the ``ON" and ``OFF" states appears depending on the occurrence and absence of the oxygen vacancies (V$_{\rm O}$s), respectively. For case (b), however, little electric current flows even when the V$_{\rm O}$s exist in hafnia. In the latter, the O atoms abstracted from hafnia are strongly bound to N, leading to substantial separation of TiN from HfO$_2$. In contrast, in the former, the Ta buffer not only absorbs the O atoms but also bridges TiN and HfO$_2$ to secure the occurrence of the ``ON" state. [1] H. Nakamura et al., J. Phys. Chem. C \underline{115}, 19931 (2011).