Resistive switching in electrochemical metallization memory cells (2009)

New memory technologies must be easily integrable into standard semiconductor processes and allow for a high memory density. The aim is the fabrication of non-volatile memory devices with a low power consumption. The devices should be read out non-destructively, and have a high endurance and high scalability. In this context, two terminal memory cells become interesting which consist of only three layers and whose resistive state is defined by the growth and dissolution of a metallic filament. In this study, resistive switching due to filament formation in Ag-Ge-Se- and Cu-SiO2-based memory devices was investigated. Sputter processes for the thin film deposition of Ge0.3Se0.7 and SiO2 were developed and optimized. The memory devices consisted of an active layer, Ge0.3Se0.7 or SiO2, sandwiched between an inert electrode, Pt or Ir, and an easily oxidizable electrode, Ag or Cu. Latter is oxidized under the application of an appropriate positive bias voltage, and metal ions migrate along the electric eld towards the counter-electrode where they are reduced. Starting at the inert electrode, a metallic filament grows switching the initially highly resistive cell to a low resistance state when the contact to the anode is established. Under reverse bias the filament is dissolved and the high resistance state is restored. Due to the switching mechanism, these cells are referred to as electrochemical metallization memory cells. Ge0.3Se0.7 is known to be a good metal ion conductor. In this work, the same switching mechanism as observed in Ag-Ge-Se was also found in SiO2-based memory devices, whose oxide layer thickness could be reduced to 5 nm. The layer stack was physically and electrically investigated. The surface roughness was investigated by atomic force microscopy, and X-ray techniques were used for non-destructive thickness measurements and surface characterization. A depth profile of the memory cells was obtained by time of flight secondary ion mass spectroscopy to visualize diffusion barriers. In combination with Rutherford backscattering analysis the metal content in fresh memory cells was determined. The switching behavior of the memory cells was electrically characterized. Due to the metal content in the active layer, an electrical forming procedure was necessary before repetitive switching could be observed. With respect to this, the two material systems, Ag-Ge-Se and Cu-SiO2, were compared. Furthermore, the capability of multi-bit data storage by limiting the maximum write current was disclosed. For the first time, ultra low power consumption was demonstrated by write currents in the lower nano- to picoampère range and switching voltages of a few hundred millivolts. Several thousand switching cycles were achieved and first pulse measurements showed that electrochemical memory devices are switchable within a few nanoseconds. The filament formation was locally investigated by conductive atomic force microscopy. Single metallic filaments could be grown and dissolved. Measurements without current limitation allowed for the identication of the mass transport through the active layer due to strong topographical changes. The high scalability of electrochemical memory cells was demonstrated by measurements with current limitation: metallic filaments with a diameter of only a few nanometers were repetitively switchable.