We have produced single, synthetic nanometer-diameter pores by using a tightly focused, high-energy electron beam to sputter atoms in 10-nm-thick silicon nitride membranes. Subsequently, we measured the ionic conductance as a function of time, bath concentration, and pore diameter to infer the conductivity and ionic mobility through the pores. The pore conductivity is found to be much larger than the bulk conductivity for dilute bath concentrations, where the Debye length is larger than the pore radius, whereas it is comparable with or less than the bulk for high bath concentrations. We interpret these observations by using multiscale simulations of the ion transport through the pores. Molecular dynamics is used to estimate the ion mobility, and ion transport in the pore is described by the coupled Poisson-Nernst-Planck and the Stokes equations that are solved self-consistently for the ion concentration and velocity and electrical potential. We find that the measurements are consistent with the presence of fixed negative charge in the pore wall and a reduction of the ion mobility because of the fixed charge and the ion proximity to the pore wall.ion conduction ͉ nanopore ͉ nanostructured materials N anometer-diameter pores formed by proteins are prevalent in biology where they are used to regulate the flow of ions and molecules through the otherwise impermeable cell membrane. Even though the structure is known with atomic precision in some cases, there does not seem to be a simple relationship between the pore geometry and the conductivity (1). This difficulty may be due to the nonuniform, high concentration of charge in the pore (2).As a first step toward understanding the conductivity, we produced synthetic pores ranging in diameter from 1 to 3.2 nm in ultra-thin silicon nitride (Si 3 N 4 ) membranes and measured the ionic conductance as a function of time and electrolyte concentration to infer the conductivity and ionic mobility. This work measures the conductance through pores with radii comparable with the van der Waals radius of an ion (3-5). Similar claims by Siwy and Fuliński (6) that rely on the conductance through a single pore to estimate the pore diameter are unreliable, because conductance does not scale with the diameter. Like previous work in nanofiltration membranes that contain ensembles of pores with varying diameter (7), we find that the pore conductivity associated with a single pore is found to be much larger than the bulk conductivity for dilute electrolyte concentrations, where the Debye length is larger than the pore radius, whereas it is comparable with or less than the bulk for high concentrations. These observations are also consistent with a recent report by Stein et al. (8) of ion transport in silica channels Ͼ70 nm wide that is governed by surface charge. To interpret our observations, we use multiscale simulations of the ion transport through the pores. Molecular dynamics (MD) is used to estimate the ion mobility, and ion transport in the pore is described by the coupled Poisson-N...

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Effect of gate oxidation method on electrical properties of metal-oxide-semiconductor field-effect transistors fabricated on 4H-SiC C (0001) face Appl.The key stages of the dry oxidation of the SiC͑0001͒ surface are analyzed based on first-principles calculations. It is found that an abrupt SiC/ SiO 2 interface model results in a large activation barrier of oxygen penetration to the silicon carbide, and thus the penetration is probably the rate-limiting step for the entire dry-oxidation process. The subsequent reactions of SiC oxidation after oxygen penetration are investigated, and it is found that CO release is competing with carbon dimer formation. These dimers probably are responsible for near-interface traps in the silica layer generated during SiC oxidation. The possible passivation reactions of a carbon dimer defect by active species, such as O 2 , NO, and H 2 are investigated. It is found that an oxygen molecule can break a Si-C bond via dissociation in the triplet state and finally can produce two CO molecules from the carbon dimer defect. The NO molecule can easily break a Si-C bond of a carbon dimer defect and form cyano groups -CN, which can finally recombine to form a C 2 N 2 molecule. This molecule can hardly diffuse in silica matrix, and it is suggested that it is further oxidized by an NO molecule to CO and N 2 molecules. It is suggested that the process of passivation by O 2 and NO molecules is restricted by the incorporation of these molecules in small voids near the carbon defect. Based on the calculated results, a simple kinetic mechanism of dry SiC oxidation is proposed and kinetic modeling of the oxidation process is performed. It is found that in the framework of this mechanism, the carbon defect density should weakly depend on temperature.

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