  We report a theoretical study of the structural, electronic and optical
properties of hBN-AlN superlattice heterostructures (SL) using a
first-principles approach based on standard and hybrid Density Functional
Theory. We consider short-period ($L<10$ nm) SL and find that their properties
depend strongly on the AlN layer thickness $L_{AlN}$. For $L_{AlN}\lesssim1$
nm, AlN stabilizes into the hexagonal phase and SL display insulating behavior
with type II interface band alignment and optical gaps as small as $5.2$ eV.
The wurtzite phase forms for thicker AlN layers. In these cases built-in
electric fields lead to formation of polarization compensating charges as well
as two-dimensional conductive behavior for electronic transport along
interfaces. We also find defect-like states localized at interfaces which are
optically active in the visible range.
