  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.
