  Self-assembled Ge wires with a height of only 3 unit cells and a length of up
to 2 micrometers were grown on Si(001) by means of a catalyst-free method based
on molecular beam epitaxy. The wires grow horizontally along either the [100]
or the [010] direction. On atomically flat surfaces, they exhibit a highly
uniform, triangular cross section. A simple thermodynamic model accounts for
the existence of a preferential base width for longitudinal expansion, in
quantitative agreement with the experimental findings. Despite the absence of
intentional doping, first transistor-type devices made from single wires show
low-resistive electrical contacts and single hole transport at sub-Kelvin
temperatures. In view of their exceptionally small and self-defined cross
section, these Ge wires hold promise for the realization of hole systems with
exotic properties and provide a new development route for silicon-based
nanoelectronics.
