Steady-state motion of silicon islands driven by a DC current

The understanding and control of structures at the surface of crystals is a fashionable topic nowadays. The role of an electric heating current in shaping the morphology of vicinal (111) and (001) silicon surfaces during sublimation in ultra-high vacuum is well known. Less known is the behaviour of surface features when the crystal is in contact with its own saturated vapour, and thus at equilibrium. We report here the observation by reflection electron microscopy of two-dimensional, micrometre-sized silicon islands on a resistively heated Si(001) substrate held at equilibrium (vanishing super- and undersaturation) at temperatures between 1000 and 1100°C. Surprisingly, the islands are seen to perform a gliding motion at a constant velocity in or against the current direction, depending on the island reconstruction. The value of the velocity is, on the contrary, independent of island reconstruction and size. A simple model based on adatom electromigration is discussed, that allows us to account for all observed features.