  Metal-semiconductor contacts play a key role in electronics. Here we show
that for quasi-one dimensional (Q1D) structures such as nanotubes and
nanowires, side contact with the metal only leads to weak band realignment, in
contrast with bulk metal-semiconductor contacts. Schottky barriers are much
reduced compared with the bulk limit, and should facilitate the formation of
good contacts. However, the conventional strategy of heavily doping the
semiconductor to obtain Ohmic contacts breaks down as the nanowire diameter is
reduced. The issue of Fermi level pinning is also discussed, and it is
demonstrated that the unique density of states of Q1D structures makes them
less sensitive to this effect. Our results agree with recent experimental work,
and should apply to a broad range of Q1D materials.
