Step bunching on the vicinal GaN(0001) surface

Nominally 2^∘ vicinal GaN(0001) surfaces exhibit monolayer-height steps at 990^∘C in the metal-organic chemical vapor deposition environment. Real-time x-ray scattering observations at 715--990^∘C indicate that there is a tendency for step bunching during growth. Below 850^∘C, step bunches nucleated during growth remain and coarsen after growth, while above 850^∘C, the surface reverts to monolayer-height steps after growth. Surfaces vicinal toward the 1100 and the 1120 planes exhibit similar behavior. We suggest a simple equilibrium surface orientational phase diagram for vicinal GaN(0001) that is consistent with these observations.