Surface states and Fermi-level pinning at clean and Al covered GaN surfaces

Ab initio electronic structure calculations have been performed for the clean GaN surface (both N- and Ga-terminated) and for the GaN/Al 1-ML system, obtained from the clean surface by addition of an Al monolayer. In the most stable configuration, Al is found to occupy the bridge site at the clean unreconstructed N-terminated surface. All the systems studied show an appreciable surface charge density due to midgap states strongly localized in the surface region; this localization is particularly enhanced for the N-terminated case, showing the high instability of the unreconstructed clean surface. On the other hand, the Ga-terminated surface is found to be more stable and to behave very similarly to the Al-covered system. The Fermi level $E_F$ is shifted toward the conduction band with respect to the clean N-terminated surface. Additional coverage of Al layers does not significantly affect the initial position of $E_F$ within the GaN band gap, showing that the Schottky barrier height is well established after the first Al layer.