Proximal probe characterization of nanoscale charge transport properties in Co/SiO2 multilayer structures

We have used scanning force microscopy to study localized charge injection and subsequent charge transport in discontinuous Co/SiO₂ multilayer structures. Charge was injected by applying a bias voltage pulse between a conductive proximal probe tip and the sample. Electrostatic force microscopy was used to image charged areas, to determine quantitatively the amount of stored charge, and to characterize charge transport. Charge was deposited controllably and reproducibly within areas ∼20–50 nm in radius and an exponential decay in the peak charge was observed. The decay times were observed to be dependent on the nominal Co film thickness and on the sign of the deposited charge, with longer decay times for positive charge than for negative charge. These results are interpreted as a consequence of Coulomb-blockade effects, considering charge transport both within the Co layer as well as from the Co layer into the Si substrate.