Oxygen-polar ZnO films are grown in step flow mode by molecular beam epitaxy. Driving by the step flow anisortropy, the growth leads to the occurrence of specific hexagonal pits in the surface. The specific pits are formed by interlacing steps of the ?101?4?? facets, thus quenches the macroscopic dipole moment along the c-axis and satisfies the stabilization principles. Nitrogen (N) doping trials are then performed on the basis of the stable surface. In the doping, growth remains in step flow mode but the step flow anisotropy vanishes, resulting in an obvious change of the surface morphology. Besides, a distinct acceptor state appears by in-situ scanning tunneling spectroscopy analysis. First-principles calculations reveal that N is readily substituting for step-edge Zn and acts as a NO2 adsorbed at the step edge. Desorption of the NO2 facilitates the formation of NO-VZn shallow acceptor complexes, which contribute to the appearance of acceptor state. According to the peculiarities of N dopants on O-polar surface, vicinal O-polar substrates (e.g., ?101?4?? substrate) are promising in ZnO:N due to the easily achieved step flow growth and high density of step edges for N’s incorpration.
