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, on which one-dimensional nanowire-like structures are formed. As shown by variable temperature scanning tunneling microscopy, these nanostructures are not only tunable in size and shape, but can be fully reversibly erased and reformed without changes in sizes and composition.
This unprecedented behavior is caused by the strong free surface energy renormalization due to the large step entropy of vicinal surfaces that strongly increases with increasing temperature. This favors a planar two-dimensional surface at higher temperatures in thermodynamic equilibrium, whereas the nanostructured surface is the preferred low-temperature configuration.
Taking the step entropy into account, the critical transition temperature between these surface states derived by free-energy minimization is shown to scale nearly linearly with the Ge coverage-in excellent agreement with the experiments. Most importantly, the nanowire sizes are found to be deterministically controlled by the Ge thickness and vicinal angle, independently of the growth or annealing conditions.
Thus, highly reproducible structures with tunable nanogeometries and -dimensions are obtained, which opens promising avenues for device applications.