About anisotropy of atomic-scale height step on (0001) sapphire surface

A vicinal (0001) alumina surface, annealed at 1500°C for 4 h in air, is studied by atomic force microscopy (AFM) at room temperature. The images show two different kinds of steps: faceted steps with a height of several nanometres, running in the [101̄0] direction and steps of 0.22 nm height (c/6 steps with reference to alumina cell) which cross terraces in between faceted steps. The orientation distribution of the c/6 steps reveals two distinct directions not parallel to any low-index crystalline direction. The main values of the angle between each c/6 steps and the [101̄0] direction, equal to 36.5° and 52.5° respectively, alternate strictly along a terrace in between faceted steps. These c/6 steps generally appear coupled. The existence of two kinds of c/6 step orientations corresponds to different step energies due to different atomic arrangements originating from the bulk lattice of alumina. The atomic packing, responsible for angular alternation of neighbouring steps, determines the third order symmetry of c/6 step properties. Statistical analysis of step orientations and further modelling yields an angular dependence of the step energy. The anisotropy of c/6 step energy is found to be weak