Morphology and surface reconstructions of m-plane GaN

M-plane GaN(1 -1 0 0) is grown by plasma assisted molecular beam epitaxy on ZnO(1 -1 0 0) substrates. A low-temperature GaN buffer layer is found to be necessary to obtain good structural quality of the films. Well oriented (1 -1 0 0) GaN films are obtained, with a slate like surface morphology. On the GaN(1 -1 0 0) surfaces, reconstructions with symmetry of c(2x2) and approximate "4x5" and are found under N- and Ga-rich conditions, respectively. We propose a model for Ga-rich conditions with the "4x5" structure consisting of monolayers of Ga terminating the GaN surface.