Lateral tunnel junction produced by electron-beam-induced deposition

Electron-beam-induced deposition using a WF6 precursor molecule was applied to making metal/insulator/metal tunnel junctions for single-electron transport devices. Single wires 8 nm high and about 13 nm wide were produced on a SiO2 substrate with Au/Cr electrode pads and their conductance showed a rapid increase of about five orders of magnitude as electron-beam (EB) doses increased between 5 and 15 pC shot. To estimate the deposit thickness distribution, spatial thickness distribution in spot exposure was defined and obtained for specified EB doses. From this function, a single-wire resistivity at 230 and 300 K was determined to be 6×10−4 Ω cm at doses exceeding 15 pC/shot. Single-tunnel junctions, where space with a 2.5 nm increment was at the center of single wire, were produced. The electrical characteristics of these single junctions were fitted to a Fowler–Nordheim plot the absolute value of whose gradient gradually increased with increasing space width. The barrier height of this junction was estimated to be 0.17–0.2 eV, lower than that for SiO2/W junctions. This might be caused by the change from the metallic deposit to the insulator for the single wire as a function of the EB dose. This deposition technique enabled us to fabricate a transistor structure where dots were located in space and a side gate electrode was also deposited. The structure showed Coulomb oscillation even at 230 K and Monte Carlo simulation of this device showed reasonable agreement with the experiment, assuming appropriate circuit parameters of gate capacitance and tunnel resistance.