Evolution of step morphology on vicinal sapphire (1 −1 0 2) surfaces accompanied with self-assembly of comb-shaped chemical domains

Morphology of vicinal sapphire (1 −1 0 2) surfaces was observed in air by atomic force microscopy (AFM) after annealing at temperatures between 1073 K and 1273 K. Surfaces of one type of the samples exhibited high densities of islands and voids with single-atomic height near the step edges at the early stage of annealing, and the crystallographic anisotropy appears as their elongated shapes. During the subsequent annealing, their densities were decreased through the Ostwald ripening process, and finally they were incorporated into the upper and lower terraces. On surfaces of another type of the samples, a comb-shaped pattern consisting of parallel-striped domains running along the [1 −1 0 −1] direction appeared upon the annealing at 1273 K for 3 h. Difference in hydrophilicity was observed between the striped domains and the other areas using frictional force microscopy, which detects the amount of adsorbed water on the surface through meniscus force. The striped pattern that is one of the features of the surface anisotropy is considered to be formed because the surface energy can be decreased when two striped stress domains are alternately arranged.