Atomic Transport in Au-Ge Droplets: Brownian and Electromigration Dynamics

Presolidification in Eutectic Droplets

Evidence of presolidification, the counterpart to premelting, is reported. Near the eutectic temperature, T_{C}, the propagation direction of thermal gradient driven motion of eutectic Ge-Pt droplets on Ge(110) is determined by presolidification. Well above T_{C}, the micron-sized droplets move towards the hottest location at the substrate, irrespective of crystalline direction. At 90 K above T_{C}, a strong, unanticipated preference for propagation along the substrate [001] azimuth suddenly emerges, which is attributed to presolidification at the liquid-solid interface. The propagation along [001] is accompanied by a distinct change in shape from compact to elongated along [001].

Microscopic Study of the Spinodal Decomposition of Supported Eutectic Droplets During Cooling: PtGe/Ge{110}

We embarked on an in situ low-energy electron microscopy, photo-electron emission microscopy, and selected area low-energy electron diffraction study during the cooling of huge eutectic droplets through the critical stages of the eutectic transition. On this journey through uncharted waters, we revealed an expected initial shrinking of the exposed area of the droplet, followed by an unanticipated expansion. We attribute this behavior to an initial fast amorphization of the interface between the droplet and surface, followed by the recrystallization of Ge expelled from the droplet at the interface. As a major surprise, we discovered the emergence of extensive "spaghetti"-like patterns, which are rationalized in terms of parallel Ge ripples oriented along, mainly, [-554] and [-55-4] directions. They emerge during spinodal decomposition when passing the eutectic temperature of the system. Their sides are defined by Ge{111} and Ge{11-1} vicinals covered with Pt-modified (√3 × √3) superstructures. The distance between adjacent ripples is about 18 nm.

Growth, phase transition, and island motion of Au on Ge(111)

Using low energy electron microscopy, Au on Ge(111) is determined to follow a Stranski-Krastanov growth mode consisting of a single layer up to one monolayer (ML), followed by three-dimensional Au-Ge alloy droplets. Near 600 °C, we report the first observation of a reversible first-order phase transition that occurs from the (3 × 3)R30° phase to a (1 × 1) phase, which has a coverage of 0.367 ML. The transition gradually occurs through a coexistence region with a temperature range of about 2 °C and weakly depends on coverage, varying from 640 °C at 1 ML down to 580 °C at 0.8 ML. The phase transition is accompanied by phase fluctuations of small domains or the fluctuations of phase boundaries of large domains. At coverage >1 ML and above 250 °C, the 3D droplets move with stick-slip hopping behavior that has previously been explained by dissolution of Ge at step edges into the alloy droplet, which then comes to concentration and thermal equilibrium via the island motion.