Air Force Research Laboratory, Materials and Manufacturing Directorate , Wright-Patterson Air Force Base , Dayton , Ohio 45433 , United States.
ACS Appl Mater Interfaces. 2018 Mar 21;10(11):9802-9816. doi: 10.1021/acsami.7b17645. Epub 2018 Mar 13.
Electrically switchable resistive random access memories have drawn much interest as nonvolatile memory device candidates based on metal-insulator-metal (MIM) structure concepts. However, atomic level mechanisms that lead to conductive filament (CF) formation in MIM structures are often lacking, such as for the system with NiO as the oxide layer, which was found promising for resistive random access memory (RRAM) device applications. In this work, using density functional theory with a Hubbard-type on-site Coulomb correction, which we carefully benchmarked, we analyzed the intrinsic propensity toward CF formation in NiO upon introduction of oxygen vacancies, including interfacial effects of Ag or Pt electrodes. First, for stoichiometric MIM structural models, contributions from metal-induced gap states to the electronic density of states (DOS) were identified, accommodating oxygen vacancy states and showing that the interface region is reduced more easily than the bulklike region, for example, for the Ag/NiO/Ag structure. Moreover, a tendency toward oxygen vacancy clustering was demonstrated, important for CF formation. Indeed, by introducing ordered oxygen vacancies into the oxide layer for both MIM models, several extended defect states within the forbidden gap have resulted, which lead to defect-assisted transport. These were shown to be influenced by the spatial distribution and number of oxygen vacancies in the filament, where the degree of reduction of Ni atoms changes based on the immediate surroundings. Projected electronic DOS for individual Ni atoms in regions near and away from oxygen vacancies indicated that those Ni close to oxygen vacancies contribute most to the conductivity. Interestingly, based on charge analyses, these atoms are revealed to undergo significant reduction, generating a locally conductive region in the oxide layer that consists of metallic/near-metallic Ni (Ni), formed through local reduction.
基于金属-绝缘体-金属（MIM）结构概念，电可切换电阻式随机存取存储器作为非易失性存储器件候选材料引起了广泛关注。然而，导致 MIM 结构中导电细丝（CF）形成的原子级机制通常缺乏，例如具有 NiO 作为氧化物层的系统，该系统被发现对电阻式随机存取存储器（RRAM）器件应用有很大的前景。在这项工作中，我们使用了带有 Hubbard 型局域库仑修正的密度泛函理论，经过仔细的基准测试，分析了 NiO 中由于引入氧空位而导致 CF 形成的固有倾向，包括 Ag 或 Pt 电极的界面效应。首先，对于化学计量的 MIM 结构模型，金属诱导的能隙态对电子态密度（DOS）的贡献得到了确定，容纳了氧空位态，并表明与块状区域相比，界面区域更容易被还原，例如对于 Ag/NiO/Ag 结构。此外，还证明了氧空位聚类的趋势，这对于 CF 的形成很重要。事实上，通过在两种 MIM 模型的氧化物层中引入有序的氧空位，在禁带内产生了几个扩展的缺陷态，从而导致了缺陷辅助传输。结果表明，这些缺陷态受到 CF 中氧空位的空间分布和数量的影响，其中 Ni 原子的还原程度取决于其周围环境。在靠近和远离氧空位的区域中，单个 Ni 原子的投影电子 DOS 表明，那些靠近氧空位的 Ni 原子对导电性的贡献最大。有趣的是，基于电荷分析，这些原子被揭示经历了显著的还原，在氧化物层中形成了一个由金属/近金属 Ni（Ni）组成的局部导电区域，这是通过局部还原形成的。
