Alumina is a technologically important oxide crystal because of its use as a catalyst and as a substrate for microelectronic applications. Knowledge of its surface morphology is a prerequisite for understanding and controlling the physical processes involved in many of its applications. In this work we have studied the smoothening of monoatomic step edges of (1-102) vicinal surfaces of sapphire after isochronal (1h) annealing at temperatures from 973K up to 1573K by atomic force microscopy in contact mode at room temperature. The initial relief of the
step edges results on the final mechanico-chemical polishing of sapphire samples.
AFM images of surfaces annealed in air at increasing temperatures show two distinct regimes. The first one corresponds to annealing temperatures from 973K up to 1273K. At 973K, the steps present a very rough edge with sharp protrusions together with islands of atoms and vacancies on the terraces. After annealing at higher temperature, the step edges become smoother while the density of islands decreases and their typical size increases. The second regime appears after annealing at temperatures higher than 1273K where all the islands have disappeared and the step edges gently meander on flat terraces. After annealing at temperature higher than 1573K, the steps become straight and the terrace width is typically 300nm.
To quantify the evolution of step edges within the first regime, we have measured both the decrease of the step width and the average size of islands. Plotting these measurements in an Arrhenius diagram, we found the common activation energy of 1.3 eV. For all temperatures above 1273K, we found that the spatial correlation functions C(y,T) along step edges follows the very same power law: C(y,T) = α(T)*y0.74, where y is the distance taken along the average step direction. The activation energy associated to the prefactor α(T) is of 0.78 eV. Thus, the activation energy we measure for matter diffusion in the island coarsening regime (low T) is well higher than the activation energy for step smoothing (high T). Statistical analysis of the AFM images will be discussed together with the dynamics of the system.
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