Nonequilibrium diffusion of reactive solid islands

We report on the observation of the random walk of solid-state silicon islands on SiO2 substrates during annealing at high temperatures. The mean square displacement (MSD) of the islands exhibits three regimes. At short times, the islands undergo equilibrium diffusion and begin to etch the surface thereby creating ringlike trenches. Then, an unusual size independent diffusionlike behavior is observed with a linear increase of the MSD. This behavior is attributed to a pinning instability of the triple line. Finally, as etching proceeds pits are formed in the substrate, and the MSD saturates as the islands are self-trapped in their own pits. Kinetic Monte Carlo simulations reproduce the main features of the three regimes, and provide a consistent picture of the microscopic reaction mechanisms at play in the experiments. © 2014 American Physical Society.