We report on the temperature stability of pseudomorphic GeSn films grown by molecular beam epitaxy on Ge(001) substrates. Both the growth temperature-dependence and the influence of post-growth annealing steps were investigated. In either case we observe that decomposition of metastable epilayers with Sn concentrations around 10% sets in above 230{deg}C, the eutectic temperature of the Ge/Sn system. Time-resolved annealing experiments in a scanning electron microscope reveal the crucial role of liquid Sn droplets in this phase separation process. Driven by a gradient of the chemical potential, the Sn droplets move on the surface along preferential crystallographic directions, thereby taking up Sn and Ge from the strained GeSn layer 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 the trailing edge of the droplet. Secondary droplets are launched from the rims of the single-crystalline Ge trails into intact regions of the GeSn film, leading to an avalanche-like transformation front between the GeSn film and re-deposited Ge. This process makes phase separation of metastable GeSn layers particularly efficient at rather low temperatures.
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