  We model the quantum confined Stark effect in AlN/GaN/AlN heterostructures
grown on top of [0001]-oriented GaN nanowires. The pyro- and piezoelectric
field are computed in a self-consistent approach, making no assumption about
the pinning of the Fermi level, but including an explicit distribution of
surface states which can act as a source or trap of carriers. We show that the
pyro- and piezoelectric field bends the conduction and valence bands of GaN and
AlN and transfers charges from the top surface of the nanowire to an electron
gas below the heterostructure. As a consequence, the Fermi level is likely
pinned near the valence band of AlN at the top surface. The electron gas and
surface charges screen the electric field, thereby reducing the Stark effect.
The efficient strain relaxation further weakens the piezoelectric polarization.
We compute the electronic properties of the heterostructures with a sp3d5s*
tight-binding model, and compare the theoretical predictions with the available
experimental data.
