Structure of GaN(0001): The laterally contracted Ga bilayer model

We discuss the energetics and structure of a laterally contracted Ga bilayer model for the Ga-rich pseudo-$1\times 1$ phase of the GaN(0001) surface. First-principles total energy calculations reveal that a laterally contracted overlayer of Ga atoms bonded to a $1\times 1$ Ga adlayer is energetically favorable in the Ga-rich limit. The calculations also show that the energy of this surface structure is very insensitive to the lateral position of the contracted layer with respect to the underlying Ga layer. The flatness of the energy surface suggests the presence of rapidly moving domain boundaries separating regions of the surface having different registries. Such motion may lead to the $1\times 1$ corrugation pattern seen in scanning tunneling microscopy images.