To address this issue, researchers at the Institute of Physics, Chinese Academy of Science, China, are using atomic force microscopy apparatus with a conductive tip (Rh-coated silicon tip with a tip diameter of 25 nm) to investigate the local electrical properties of oxygen-annealed polycrystalline tungsten oxide film, a material that exhibits non-volatile memory behaviour. The conductive tip occupies a smaller area than the grain size and can serve as one of the electrodes for the electrical measurement.

Looking at the AFM images, the current distribution correlates well with the topographic structure of the film. Also, the grain surface shows more conductive features than the grain boundary regions.

Current mapping

The inhomogeneity in conductivity at the film surface results in inhomogeneous resistance switching behaviour. Both local I-V measurements and current mapping reveal that reversible resistance switching occurred only at the grain surface region.

These results are meaningful for the fabrication of nanoscale RRAM devices, although the mechanism that describes the inhomogenous conductivity and resistance switching requires further investigation.

The researchers presented their work in the journal Nanotechnology.