  Germanium-Tin (GeSn) alloys have emerged as a promising material for future
optoelectronics, energy harvesting and nanoelectronics owing to their direct
bandgap and compatibility with existing Si-based electronics. Yet, their
metastability poses significant challenges calling for in-depth investigations
of their thermal behavior. With this perspective, this work addresses the
interdiffusion processes throughout thermal annealing of pseudomorphic GeSn
binary and SiGeSn ternary alloys. In both systems, the initially pseudomorphic
layers are relaxed upon annealing exclusively via thermally induced diffusional
mass transfer of Sn. Systematic post-growth annealing experiments reveal
enhanced Sn and Si diffusion regimes that manifest at temperatures below
600{\deg}C. The amplified low-temperature diffusion and the observation of only
subtle differences between binary and ternary hint at the unique metastability
of the Si-Ge-Sn material system as the most important driving force for phase
separation.
