GaN homoepitaxial growth by molecular-beam epitaxy under both excess gallium (Ga) and excess nitrogen (N) conditions is investigated. Based on two-dimensional island shape and surface step structures, we suggest the growth is kinetic-limited under the excess-Ga condition but diffusion-limited in the excess-N regime. The triangular GaN islands and double step bunching seen on surfaces prepared under excess-Ga are attributed to a difference in adatom attachment and/or site exchange rates between $A$ and $B$ steps, which is induced by surfactant Ga adlayers on GaN(0001).

• Received 31 October 2005

DOI:https://doi.org/10.1103/PhysRevB.74.085314