Jun 23, 2011
SPM probes inhomogeneity in nanoscale resistance switching
Electric-field-induced resistance switching at the nanoscale in oxide films is an important issue for the development of resistance random access memory (RRAM) with high scalability. As the dimensions of individual device elements continue to shrink, especially to sizes comparable to the grains of the polycrystalline film, resistance switching at individual grains and grain boundaries will become more technologically important. However, conventional macroscopic measurements integrate over a fairly large area and cannot spatially resolve highly local properties.
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.
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.
About the author
Dr Da-Shan Shang received his PhD in Material Physics and Chemistry from the Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2007, and is currently an associate professor in Ji-Rong Sun's group at the Institute of Physics, Chinese Academy of Sciences, Beijing, China. Shang has been working in the field of RRAM since 2004 and his research currently focuses on nanoscale resistance switching and its physical mechanism in transition metal oxides, especially the formation and evolution of the conductive channel during resistance switching.