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.