Charge Transport Properties of Multilayer Nanostructures
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The demand to increase device speeds and to obtain higher data storage densities leads to decreased device sizes and new device design proposals, of which some were based on multilayer or granular metal/insulator structures, which consist of metallic nanoclusters embedded within an insulating matrix. The design and optimization of devices incorporating these materials require a detailed understanding of the relevant nanoscale electrical transport properties, which can be investigated by scanning probe techniques. This article reviews recent studies of the nanoscale charge transport properties of multilayer or granular metal/insulator structures, and the application of the results of these studies in a novel magnetic field sensor design based on the combination of charge storage and tunnel-magnetoresistance in magnetic discontinuous magnetic multilayer structures is discussed briefly. In particular, charge deposition into Co nanoclusters embedded in a SiO2 matrix and the decay of the charge as a function of time are discussed. Local charge deposition into and removal from Co nanoclusters was achieved by applying a voltage pulse to a conductive probe tip in a scanning probe microscope. Electrostatic force microscopy (EFM) was used to image charged areas, to determine quantitatively the amount of stored charge, and to characterize charge transport within the Co layer and into the Si substrate. Measurements of decay times for positive and negative charge as a function of nominal Co layer film thickness are presented, and the dynamics of the charge decay for positively and negatively charged nanoclusters is analyzed as a consequence of Coulomb-blockade effects at room temperature considering a detailed model for charge transport within the Co layer as well as from charge Co nanoclusters into the Si substrate.