First-Principles Modeling for Current-Voltage Characteristics of Resistive Random Access Memories
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First-Principles Modeling for Current-Voltage Characteristics of Resistive Random Access Memories Takehide Miyazaki1, Hisao Nakamura1, Kengo Nishio1, Hisashi Shima2, Hiroyuki Akinaga2, Yoshihiro Asai1 1 Nanosystem Research Institute, National Institute for Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 2 Nanoelectronics Research Institute, National Institute for Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ABSTRACT We present results of first-principles non-equilibrium Green’s function calculations for current-voltage (IV) characteristics of the electrode/HfO2/electrode model systems. In order to investigate the effect of the electrode materials on the IV characteristics, we considered two transition metals for electrode, Ta and W, which are both body-centered-cubic elemental metals but have different valence numbers. We simulated the ON state by placing oxygen vacancies in the HfO2 layer while the OFF state was modeled with HfO2 without oxygen vacancies. At the OFF state, no electric current flowed for -1 V up to +1 V, as expected. At the ON state, however, we found that the absolute current for the Ta electrode was twice as large as that for the W electrode. The analysis of the IV characteristics shows that the electronic coupling between Ta and HfO2 is substantially stronger than that between W and HfO2. Our study demonstrates the importance of the matching between electrode and insulator materials to achieve a high ON- to OFF-current ratio in ReRAMs at a low bias. INTRODUCTION Resistive random access memories (ReRAMs) are expected to be a promising candidate for next-generation memories to meet both low electric power consumption and high switching speed [1]. A ReRAM cell often adopts a structure with a transition-metal oxide layer sandwiched by two metal electrodes [2,3], where formation and destruction of conductive regions in the oxide layer correspond to the “ON” and “OFF” states, respectively, in response to the polarity of bias voltage applied to the metal electrodes. Among theoretical modeling studies of the switching mechanisms, first principles calculations have revealed that the “ON” states originate from the defects in the oxides, including oxygen vacancies [4-7], dual defects of oxygen and metal vacancies [8] and metal interstitials [9], depending on the metal elements in the oxides and combinations of the electrodes and oxides. In this work, we present the electric current (I)-voltage (V) characteristics (-1 V < V < +1 V) of ReRAM devices with metal-oxide-metal structures, based on first principles nonequilibrium Green’s function (NEGF-DFT) theory. We choose hafnia (HfO2) as a representative example of the oxides used for ReRAMs. We investigate the IV characteristics for two kinds of metal electrodes, Ta and W. For Ta, a clear distinction between the “ON” and “OFF” states appears for hafnia with and without oxygen vacancies, respectively. For W, however, the ON/OFF di
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