An oxygen vacancy mediated Ag reduction and nucleation mechanism in SiO2 RRAM devices
Abstract Density Functional Theory (DFT) calculations were used to model the incorporation and diffusion of Ag in Ag/SiO2/Me (Me = W or Pt) resistive random-access memory (RRAM) devices. We consider an O vacancy (VO) mediated model of the initial stages of Ag clustering, where the VO is identified as the principle site for Ag+ reduction. The Ag+ interstitial is calculated to be energetically favoured inside a-SiO2 at the Fermi energies of Ag, W and Pt. The adiabatic diffusion barriers of Ag+ are found to be lower than those for Ag0 with a strong dependence on the local network structure, supporting Ag+ being the mobile species during device operation. Ag+ ions bind to VO forming the [Ag/VO]+ complex. The [Ag/VO]+ complex is then reduced by trapping an electron forming [Ag/VO]0. By sampling every VO in a 216-atom cell of a-SiO2 we demonstrate that this mechanism can occur only at 33%, 33% and 11% of O vacancies at the Ag, W and Pt electrodes, respectively. This complex can subsequently act as a nucleation site for Ag clustering with the formation of [Ag2/VO]+, which is reduced by trapping an extra electron.