Effect of Oxygen Exchange between Two Oxide Layers of a Memristive Bilayer Valence-Change Memory Cell on the Switching Polarity

Abstract

Valence-change memory (VCM) cells are promising candidates for future nonvolatile memory devices. A special setup of VCM devices consists of bilayer cells where two thin oxide layers are placed in between two metal electrodes. One oxide layer serves as a tunnel barrier, whereas the second oxide layer is a highly doped conductive semiconductor. Experiments show that an exchange of oxygen between the two layers changes the resistance of the cell. However, the exchange process and how it influences the resistance is not well understood yet. With a drift-diffusion model for electrons and oxygen vacancies, we investigate the movement and exchange of oxygen vacancies and their influence on the band structure as well as on the shape of the tunnel barrier. The simulation results show that a high oxygen-vacancy concentration lowers the height of the tunnel barrier; thus it increases the conductivity of the bilayer cell. The effect of the band lowering is stronger in materials with low permittivity. Hence, two different resistance states evolve if there is an exchange of oxygen between the two oxide layers with different permittivities. Thereby, the switching polarity depends on the relation of the permittivities of the two oxide layers. Furthermore, it is revealed that resistance switching can be induced by the movement of vacancies only inside the conductive oxide, without any oxygen exchange between the layers.