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We report on the temperature stability of pseudomorphic GeSn films grown bymolecular beam epitaxy on Ge(001) substrates. Both the growthtemperature-dependence and the influence of post-growth annealing steps wereinvestigated. In either case we observe that decomposition of metastableepilayers with Sn concentrations around 10% sets in above 230{\deg}C, theeutectic temperature of the Ge/Sn system. Time-resolved annealing experimentsin a scanning electron microscope reveal the crucial role of liquid Sn dropletsin this phase separation process. Driven by a gradient of the chemicalpotential, the Sn droplets move on the surface along preferentialcrystallographic directions, thereby taking up Sn and Ge from the strained GeSnlayer at their leading edge. While Sn-uptake increases the volume of the melt,dissolved Ge becomes re-deposited by a liquid-phase epitaxial process at thetrailing edge of the droplet. Secondary droplets are launched from the rims ofthe single-crystalline Ge trails into intact regions of the GeSn film, leadingto an avalanche-like transformation front between the GeSn film andre-deposited Ge. This process makes phase separation of metastable GeSn layersparticularly efficient at rather low temperatures.

Free-running Sn precipitates: an efficient phase separation mechanism for metastable GeSn epilayers