Enhanced thermal stability of SiGeSn by suppressing surface-mediated degradation
$\text{SiGeSn}$ alloys are promising silicon-compatible semiconductors for monolithic infrared photonics. However, their metastable nature limits the thermal budgets available for post-growth device processing, and the mechanisms governing their thermal degradation remain unresolved. Here, we investigate the thermal stability of $\text{Si}_{0.08}\text{Ge}_{0.83}\text{Sn}_{0.04}$ alloys using in situ spectroscopic ellipsometry (SE) during isothermal annealing at 550 °C. We show that adding an ultrathin oxide cap kinetically suppresses Sn exchange with the free surface while leaving bulk diffusion pathways largely unaffected. Uncapped films undergo phase separation after 50 min, accompanied by void formation, a 60% thickness reduction, and a 400 meV blueshift of the $E_{2}$ critical point (CP) transition, consistent with substitutional Sn depletion from the probed volume through surface segregation. In contrast, oxide-capped films exhibit a small compositional change (<1 at.% Sn) and optical shift (<20 meV) over the same period, with suppressed void formation, strain relaxation, and alloy decomposition. This surface-kinetic control additionally yields a 25-fold reduction in contact resistivity relative to annealed uncapped alloys. These results identify surface Sn transport as the dominant degradation pathway in SiGeSn and demonstrate that an ultrathin oxide cap extends the thermal stability of metastable group-IV alloys, providing a practical route toward their integration into advanced silicon photonic and electronic platforms.