Step bunching of vicinal GaN(0001) surfaces during molecular beam epitaxy

Step bunching of vicinal GaN(0001) surface during epitaxial growth is obser ved by scanning tunneling microscopy. Large step stiffness and repulsive st ep-step interaction are suggested based on step morphology observations. Th e size of the bunch changes with time, depending on the direction in which the substrate is heated by a direct current. This observation provides evid ence for the electromigration effect causing the step bunching, and from th e field dependence we infer that adatoms, which are likely N, have effectiv e positive charges.