Entropy-controlled fully reversible nanostructure formation of Ge on miscut vicinal Si(001) surfaces

Abstract

Entropy effects substantially modify the growth of self-assembled Ge nanostructures on vicinal Si (001) surfaces. As shown by variable temperature scanning tunneling microscopy, this leads to new types of one dimensional nanostructures that are not only tunable in size and shape but can be fully reversible erased and reformed without changes in final sizes and shapes. This unique behavior is caused by the free surface energy renormalization caused by the large step entropy of vicinal surfaces. In thermodynamic equilibrium, this favors the formation of a planar 2D surface at higher temperatures, where-as the nanostructured surface is the preferred low-temperature configuration. Taking the step entropy into account, the critical phase transition temperature is derived by free energy calculations and is shown to scale nearly linearly with the Ge coverage, in excellent agreement with the experiments. Due to self-limitation, the nanowire size is solely controlled by the Ge coverage and vicinal angle, completely independent of the growth or an-nealing conditions. Thus, highly reproducible nanostructures with tunable geometries are obtained. This opens new avenues for controlled nanostructure formation for practical de-vice applications.