  We study the electronic properties of GaAs nanowires composed of both the
zincblende and wurtzite modifications using a ten-band k.p model. In the
wurtzite phase, two energetically close conduction bands are of importance for
the confinement and the energy levels of the electron ground state. These bands
form two intersecting potential landscapes for electrons in zincblende/wurtzite
nanostructures. The energy difference between the two bands depends sensitively
on strain, such that even small strains can reverse the energy ordering of the
two bands. This reversal may already be induced by the non-negligible lattice
mismatch between the two crystal phases in polytype GaAs nanostructures, a fact
that was ignored in previous studies of these structures. We present a
systematic study of the influence of intrinsic and extrinsic strain on the
electron ground state for both purely zincblende and wurtzite nanowires as well
as for polytype superlattices. The coexistence of the two conduction bands and
their opposite strain dependence results in complex electronic and optical
properties of GaAs polytype nanostructures. In particular, both the energy and
the polarization of the lowest intersubband transition depends on the relative
fraction of the two crystal phases in the nanowire.
