Reconstructions of the GaN(000(1)over-bar) surface

Reconstructions of the GaN(0001) surface are studied for the first time. Using scanning tunneling microscopy and reflection high-energy electron diffraction, four primary structures are observed: 1 x 1, 3 x 3, 6 x 6, and c(6 x 12). On the basis of first-principles calculations, the 1 x 1 structure is shown to consist of a Ga monolayer bonded to a N-terminated GaN bilayer. From a combination of experiment and theory, it is argued that the 3 x 3 structure is an adatom-on-adlayer structure with one additional Ga atom per 3 x 3 unit cell.