Mechanical tuning of tunnel gaps for the assembly of single-electron transistors

We have fabricated gold single-electron transistors (SETs), operating up to 25 K, with tunnel gaps that could be individually tuned during fabrication. A combination of atomic-force-microscopy manipulation of nanodiscs and in situ electrical measurements was used to form statically stable tunnel gaps between the discs and lithographically defined electrodes. The gap resistances could be tuned to predetermined values over three orders of magnitude between similar to 1 M Omega and similar to 2 G Omega, corresponding to gap widths in the range of 3-10 Angstrom. We report on SETs with symmetrically and asymmetrically coupled islands, i.e., with equal or different tunnel resistances. In the asymmetric SET a distinct Coulomb staircase was observed. (C) 1999 American Institute of Physics. [S0003-6951(99)01036-0].