  Controlling the growth kinetics from the vapor phase has been a powerful
paradigm enabling a variety of metastable epitaxial semiconductors such as
Sn-containing group IV semiconductors (Si)GeSn. In addition to its importance
for emerging photonic and optoelectronic applications, this class of materials
is also a rich platform to highlight the interplay between kinetics and
thermodynamic driving forces during growth of strained, nonequilibrium alloys.
Indeed, these alloys are inherently strained and supersaturated in Sn and thus
can suffer instabilities that are still to be fully elucidated. In this vein,
in this work the atomic-scale microstructure of Ge0.82Sn0.18 is investigated at
the onset of phase separation as the epitaxial growth aborts. In addition to
the expected accumulation of Sn on the surface leading to Sn-rich droplets and
sub-surface regions with the anticipated equilibrium Sn composition of 1.0at.%,
the diffusion of Sn atoms also yields conspicuous Sn-decorated filaments with
nonuniform Sn content in the range of ~1 to 11at.% . The latter are attributed
to the formation and propagation of dislocations, facilitating the Sn transport
toward the surface through pipe diffusion. Furthermore, the interface between
the Sn droplet and GeSn shows a distinct, defective layer with Sn content of
~22at.%. This layer is likely formed by the expelled excess equilibrium Ge as
the Sn solidifies, and its content seems to be a consequence of strain
minimization between tetragonal Sn-rich and cubic Ge-rich equilibrium phases.
The elucidation of these phenomena is crucial to understand the stability of
GeSn semiconductors and control their epitaxial growth at a uniform
composition.
