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 EF 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 EF within the GaN band gap, showing that the Schottky barrier height is well established after the first Al layer.