Influence of the exchange reaction on the electronic structure of GaN/Al junctions

Ab initio full-potential linearized augmented plane-wave (FLAPW) 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. Although the detailed mechanism is not understood, the exchange reaction has been purported to influence the Schottky barrier height (SBH) as a result of the formation of an interfacial ${\mathrm{Ga}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ layer. 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. Thus, the electronic structure of the following atomic configurations have been investigated theoretically: (i) an abrupt, relaxed GaN/Al interface; (ii) an interface that has undergone one monolayer of exchange reaction; and interfaces with a monolayer-thick interlayer of (iii) AlN and (iv) ${\mathrm{Ga}}_{0.5}{\mathrm{Al}}_{0.5}\mathrm{N}.$ The exchange reaction is found to be exothermic with an enthalpy of 0.1 eV/atom. We find that the first few layers of semiconductor are metallic due to the tailing of metal-induced gap states; therefore, the presence of a monolayer-thick interfacial alloy layer does not result in an enhanced band gap near the interface. 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 lineup 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 ${\mathrm{Ga}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ ($x=0,$ 0.5) intralayer and a $\mathrm{Ga}\leftrightarrow \mathrm{Al}$ atomic swap at the interface do not significantly affect the Schottky barrier height.