Thermomigration of Au-Ge microdroplets on Ge(111): Atomic mechanisms and effects on surface morphology

The motion of microdroplets under thermal gradients, also known as thermomigration (TM), plays a critical role in a wide range of surface phenomena and technological applications. In this work, we study the TM behavior of Au-Ge micro-droplets on Ge(111) surfaces and show that the droplet motion leads to the formation of atomically flat terraces, which are essential for thin film epitaxy. Experimental observations are combined with a microscopic model to investigate droplet motion and the formation of flat atomic terraces. Our results show that, at low temperatures, droplet velocity increases with size, but this size dependence diminishes as the temperature rises. The microscopic model we develop is based on thermally activated detachment, diffusion, and attachment of solute (Ge) atoms across the droplet, driven by a temperature-dependent concentration difference between the leading edge and the trailing edge. The model reproduces the experimental trends. Our results provide insights into the fundamental mechanisms of droplet TM and suggest ways to control surface morphology.