The resistive switching mechanism of CBRAM clarified by using synchrotron characterizations

In  recent  years,  resistive  random  access  memories  have  received  extensive  interest  for applications  as  non-volatile  memories  or  neuromorphic  computing.  Conductive  Bridging Random Access Memories (CBRAM) based on a glassy Ag-GeS_x  layer sandwiched between an  Ag  anode  and  an  inert  W  cathode  are  considered  to  be  one  of  the  most  promising technologies. Under the influence of an electric field Ag ions are produced at the anode and migrate in the electrolyte reaching the cathode and forming a conducting wire. This process is reversible by applying a bias with opposite polarity. However, the lack of understanding of the  switching  mechanisms  at  a  nanoscale  level  prevents  the  successful  transfer  of  this technology to the industry. CBRAM devices were characterized in their different resistive states using depth-selective X-Ray Absorption Spectroscopy (XAS) at the GILDA-CRG beamline at the ESRF in Grenoble.

Figure (a) presents the XAS spectra depending on the switching state of the device and on the depth.  As-deposited  CBRAMs  shows  a  lower  silver  metal  fraction  with  respect  to  the switched ones. Moreover, XAS surface data show a higher metal fraction below the anode with respect to the entire layer. Our experiments highlight that the switching process involves the  formation  of  metallic  Ag  cone-shaped  nanofilaments  with  their  base  on  the  active electrode.  Moreover,  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 due to Ag dissolved in the GeS_x  matrix and longer Ag-Ag bonds related to an Ag metallic phase. Ab initio molecular dynamics (MD)  simulations  confirm that Ag  favorably binds  to  S atoms. This  provides  an explanation  on  the  reported  instability  of  the  ON  state  that  can  be attributed  to  Ag sulphidation and hence a rupture of the Ag–Ag metallic conduction path in the stack.

Figure  a)  EXAFS  spectra  of  3  prototypical  samples:  1=switched  CBRAM  at  the  surface, below the anode 2= switched CBRAM in the whole layer, 3=pristine CBRAM at the surface. Data (points) and best fit curves (continuous lines). b) Schematic of a silver nano-filament in Ag–GeS_x  and the silver filament dissolution as obtained by MD.

 

 

Principal publication and authors

E. Souchier, F. d’Acapito, P. Noé, P. Blaise, M. Bernard and V. Jousseaume, Phys. Chem.

Chem. Phys. 17, 23931 (2015). DOI: 10.1039/c5cp03601a