Sapphire evolution of the vicinal (0001) sapphire surface upon annealing in air

The evolution of a terrace-step nanostructure (TSN) on the sapphire (0001) surface misoriented by an angle of 0.1° with respect to the ( 101̅0 ) plane was observed by atomic force microscopy (AFM) at temperatures from 1273 to 1673 K. It was established that, with an increase in the annealing temperature to 1373 K, the step height attains 0.44 nm at a distance of 220 nm between steps; i.e., heating by 100 K doubles these parameters. In this case, the relief periodicity is retained. Rapid cooling of the substrate to 973 K leads to partial freezing of the surface structure, which makes it possible to observe the transition from one TSN to another. It was established that two steps coalescence upon annealing to 1373 K toward the ( 101̅0 ) plane, which has the lowest rigidity and, consequently, the lowest atomic density. The coalescence of two steps at a specified temperature is completed at a sufficiently large distance between the steps, at which their interaction energy is negligible. Upon further annealing of the samples above 1373 K, the steps overgrow to 1 nm; however, their periodicity is broken in this case.