The role of the local chemical environment of Ag on the resistive switching mechanism of conductive bridging random access memories

Abstract

Conductive bridging random access memories (CBRAMs) are one of the most promising emerging technologies for the next generation of non-volatile memory. However, the lack of understanding of the switching mechanism at the nanoscale level prevents successful transfer to industry. In this paper, Ag/GeSx/W CBRAM devices are analyzed using depth selective X-ray Absorption Spectroscopy before and after switching. The study of the local environment around Ag atoms in such devices reveals that Ag is in two very distinct environments with short Ag-S bonds due to Ag dissolved in the GeSx matrix, and longer Ag-Ag bonds related to an Ag metallic phase. These experiments allow the conclusion that the switching process involves the formation of metallic Ag nano-filaments initiated at the Ag electrode. All these experimental features are well supported by ab initio molecular dynamics simulations showing that Ag favorably bonds to S atoms, and permit the proposal of a model at the microscopic level that can explain the instability of the conductive state in these Ag-GeSx CBRAM devices. Finally, the principle of the nondestructive method described here can be extended to other types of resistive memory concepts.

Role of Sb dopant in Ag:GeSx‐based conducting bridge random access memories

“…Forming the entire device (sample C1) or repeated cycles of resistance switching (sample C2) do not either alter the Sb site so demonstrating its considerable stability. This differs from the behaviour of Ag that was reported to increase its metallic phase fraction upon cycling [11]. This also confirms the vision that Sb enters in the matrix and binds to available S ions and remains a spectator in the switching process.…”

“…Figure 5 shows the FT spectra of two identical samples with and without Sb and prior to any switching process. The two peaks correspond to Ag-S and Ag-Ag bonds, respectively, relative to the previously proposed structural model consisting in Ag ions dispersed in the matrix plus a fraction forming a metallic fcc phase (or small nuclei of it) [11]. The quantitative analysis, based on the apparent coordination number of the first shell of the Ag metal, shows that a metal fraction f m noSb ¼ 2 AE 2% is present in absence of Sb (as in sample C3) whereas f m Sb ¼ 13 AE 3% in presence of Sb (sample C1).…”

“…By opposite, a bond between Ag and S is evident, as also extensively discussed in Ref. [11] that was instead ruled out in Ref. [12].…”

“…This study was conducted via X‐ray absorption spectroscopy (XAS) at the Sb‐K and Ag‐K absorption edges making use of the grazing incidence data collection mode to enhance the signal from the surface layer. This experimental technique (grazing incidence X‐ray absorption spectroscopy) has already been used in the case of Ag:GeS x layers providing a direct description of the chemical state of Ag in different conditions (switched, unswitched) and space regions (entire chalcogenide layer, surface under the Ag electrode, ..) of the cell. Previous studies on CBRAMs based on XAS have been already presented in the past but were only focused on the major component elements of the materials (Ge, S) whereas in the present case the relevant information is derived directly on the ‘dopants’ (Sb and Ag).…”

Preferred location for conducting filament formation in thin-film nano-ionic electrolyte: study of microstructure by atom-probe tomography

“…Using X-ray absorption spectroscopy, Souchier et al [39] detected different local environments of Ag atoms in Ge-S doped by electroformation, a process which may result in locally distributed Ag-doped regions [3], therefore being sensitive to a host-matrix microstructure. The fraction of metallic-like bonding of Ag increased from <15 to 30-40% after an external voltage was applied to switch the sample into the ON state.…”