First-principles investigation of the conductive filament configuration in rutile TiO 2-x ReRAM
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First-principles investigation of the conductive filament configuration in rutile TiO2-x ReRAM Liang Zhao1, Seong-Geon Park2, Blanka Magyari-Köpe1, Yoshio Nishi1 1 Department of Electrical Engineering, Stanford University, 420 Via Palou Mall, Stanford, CA 94305, U.S.A. 2 Department of Material Science and Engineering, Stanford University, 420 Via Palou Mall, Stanford, CA 94305, U.S.A. ABSTRACT The interactions and ordering of oxygen vacancies in rutile TiO2 were thoroughly investigated by density functional calculations to search for atomic configurations of the conductive filament. As random isolated vacancies could not support the low-resistance state conduction in TiO2 ReRAM, vacancy ordering was introduced in [110] and [001] directions of the lattice to study the electronic structures. The calculation results revealed that a di-vacancy chain in [001] direction makes the electrons delocalized in that direction, which is identified as a possible configuration of the conductive filament. This low-resistance state can be effectively disrupted by moving oxygen vacancies out of the filament to reach high-resistance states. INTRODUCTION In recent years, resistance change random access memory (ReRAM) is attracting intensive research and development efforts in the semiconductor industry as a promising candidate for the next-generation non-volatile memory devices [1]. State-of-the-art ReRAM devices should possess the advantages of high density, fast switching speed, low operating power and long endurance. Among many resistance change materials, transitional metal oxide ReRAM based on oxygen vacancy diffusion and migration is by far the most widely studied category [2,3]. The fabrication processes of these binary oxides (e.g. TiO2, NiO and HfO2) are usually less complicated and more reproducible than other switching materials, and they are fully compatible with CMOS processes in addition to the advantages mentioned above [4]. Despite the experimental success, the physical mechanisms of TiO2 and other metal oxide ReRAM are still under investigations. In-depth understanding of the resistance switching mechanism has become a burning issue for designing the device structure and reliability for practical applications. Although the filamentary switching mechanism is now widely recognized, the atomistic-level details of the localized conductive path are mostly unavailable and very difficult to study experimentally. In order to connect the atomic movements with resistance change, we need to develop a good understanding of both high-resistance (HRS) and lowresistance states (LRS) of ReRAM. In this work, TiO2 is selected as a prototypical material for the theoretical investigations of ReRAM [5]. We assume the conductive filament is formed by oxygen vacancies in rutile TiO2 lattice, and apply detailed density functional theory (DFT) calculations to investigate the electronic structures and conduction properties of various vacancy configurations. It is discovered that the oxygen di-vacancy chain in [001] direction of rutile TiO2 can f
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