  Hydrogen resist lithography using the tip of a scanning tunneling microscope
(STM) is employed for patterning p-type nanostructures in silicon. For this,
the carrier density and mobility of boron $\delta$-layers, fabricated by
gas-phase doping, are characterized with low-temperature transport experiments.
Sheet resistivities as low as $300\thinspace\Omega$ are found. Adsorption,
incorporation and surface diffusion of the dopants are investigated by STM
imaging and result in an upper bound of 2\,nm for the lithographic resolution
which is also corroborated by fabricating a 7.5\,nm wide p-type nanowire and
measuring its electrical properties. Finally, to demonstrate the feasibility of
bipolar dopant device fabrication with this technique, we prepared a 100\,nm
wide pn junction and show that its electrical behavior is similar to that of an
Esaki diode.
