  Ab-initio calculations have been used to study the influence of the interface
morphology and, notably, of the exchange reaction on the electronic properties
of Al/GaN (100) interfaces. In particular, the effects of interface structure
(i.e. interfacial bond lengths, semiconductor surface polarity, and reacted
intralayers) on the SBH at the Al/GaN (001) junction are specifically
addressed. The electronic structure of the following atomic configurations have
been investigated theoretically: (i) an abrupt, relaxed GaN/Al interface; (ii)
an interface which has undergone one monolayer of exchange reaction; and
interfaces with a monolayer-thick interlayer of (iii) AlN and (iv)
Ga$_{0.5}$Al$_{0.5}$N. Intermixed interfaces are found to pin the interface
Fermi level at a position not significantly different from that of an abrupt
interface. Our calculations also show that the interface band line--up is not
strongly dependent on the interface morphology changes studied. The p-type SBH
is reduced by less than 0.1 eV if the GaN surface is Ga-terminated compared to
the N-terminated one. Moreover, we show that both an ultrathin
Ga$_x$Al$_{1-x}$N ($x$ = 0, 0.5) intralayer and a Ga$\leftrightarrow$Al atomic
swap at the interface does not significantly affect the Schottky barrier
height.
