Mater. Res. Soc. Symp. Proc. Vol. 1729 © 2015 Materials Research Society DOI: 10.1557/opl.201 . Nanosecond Fast Switching Processes Observed in Gapless-Type, Ta 2 O 5 –Based Atomic Switches Tohru Tsuruoka 1,2 , Tsuyoshi Hasegawa 1,2 and Masakazu Aono 1 1 International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-044, Japan 2 Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 7 Gobancho, Chiyoda-ku, Tokyo 102-0075, Japan ABSTRACT The switching speed of a Cu/Ta 2 O 5 /Pt atomic switch between a high-resistance (OFF) state and a low-resistance (ON) state was evaluated by transient current measurements under the application of a short voltage pulse. It was found that the SET time from the OFF state to the ON state decreased as low as 1 ns, and the RESET time from the ON state to the OFF state reached a few ns using moderate pulse amplitudes. The switching time depends strongly on the pulse amplitude and the cell resistance before applying a voltage pulse. This observation indicates that oxide-based atomic switches hold potential for fast-switching memory applications. It was also found that Cu nucleation on the Pt electrode is likely to the rate-limiting process determining the SET time and the REST time appears to be preferentially determined by thermochemical reaction. INTRODUCTION In recent years, resistive-switching random access memory (RRAM) has been recognized as one of the most promising next-generation memory technologies, because of its superior characteristics such as excellent scalability and ease of operation. Of various types of RRAM available, a simple metal/ionic conductor/metal (MIM) structure whose resistance is changed based on the formation and dissolution of a metal filament is very attractive for practical applications. Since metal oxides are highly compatible with the fabrication process of complementary metal-oxide-semiconductor devices, a thin oxide film is a good candidate as the ionic conductor material. In general, this type of cells is composed of a thin oxide film sandwiched between an electrochemically active metal electrode (usually Ag or Cu) and an inert metal electrode (such as Pt or Au). From the similarity of the operation mechanism to an atomic switch, in which resistance of a nanometer gap between a mixed conductor electrode and an inert metal electrode is changed by the formation and annihilation of a metal bridge [1], the MIM- structured cell can be referred to as a gapless-type atomic switch [2]. Among the various properties of a RRAM operation, switching speed is one of the most important switching characteristics. Although many research groups have reported the switching time of RRAM cells [3-5], it is still a challenging task to measure on or below the order to nanoseconds due to the broadening and ringing of voltage pulses arising from an impedance mismatch in the measurement system with RRAM cells. Torrezan et al. succeeded to measure a switching time of ~0.1 ns of a Pt/Ta 2 O 5 /Pt cell, which was integrated into a coplanar waveguide (CPW) structure [6]. In this case, the resistive switching originates from the migration of oxygen vacancies in the Ta 2 O 5 film, and the ON/OFF resistance ratio is ranging from 10 to 10 2 . In contrast, the atomic switch generally has higher ON/OFFF ratios than 10 4 , leading to a larger 5 93