  First-principles full-potential linearized augmented plane wave (FLAPW)
calculations are performed to clarify the role of the interface geometry on
piezoelectric fields and on potential line-ups at the [0001]-wurtzite and
[111]-zincblende GaN/Al junctions. The electric fields (polarity and magnitude)
are found to be strongly affected by atomic relaxations in the interface
region. A procedure is tested to evaluate the Schottky barrier in the presence
of electric fields and used to show that their effect is quite small (a few
tenths of an eV). These calculations assess the rectifying behaviour of the
GaN/Al contact, giving very good agreement with experimental values for the
barrier. Stimulated by the complexity of the problem, we disentangle chemical
and structural effects on the relevant properties (such as the potential
discontinuity and electric fields) by studying auxiliary unrelaxed
nitride/metal systems. Focusing on simple electronegativity arguments, we
outline the leading mechanisms that result in the final values of the electric
fields and Schottky barriers in these ideal interfaces. Finally, the
transitivity rule in the presence of two inequivalent junctions is proved to
give reliable results.
