Formation and Structural Characterization of Amorphous GeSn Thin Films via Solid-State Sn Diffusion into Hydrogenated Amorphous Germanium

Integrating GeSn alloys into semiconductor manufacturing remains challenging due to the lack of scalable, low-temperature deposition methods compatible with CMOS process flows. Here, we demonstrate that solid-state diffusion of Sn into PECVD-grown hydrogenated amorphous germanium (a-Ge:H) on Si/SiO₂ substrates provides an epitaxy-free, CMOS-compatible route to amorphous GeSn thin films, bypassing the thermal and lattice-matching constraints that limit conventional growth. Sequential a-Ge:H deposition and Sn evaporation followed by low-temperature annealing (150–250 °C) drive controlled Sn interdiffusion, bond redistribution, and alloy formation within the amorphous network. Raman spectroscopy, HR-TEM, and SAED reveal a structurally complex hybrid material: a predominantly amorphous GeSn matrix containing sparsely distributed Ge-rich nanocrystalline domains. This structure emerges from the competition between diffusion-driven mixing and localized structural relaxation, leading to the coexistence of short-range order and nanoscale crystallinity. TEM-EDS confirms a heterogeneous Sn distribution of ~12 at.% across the film, while the absence of β-Sn precipitation indicates that Sn is retained within the amorphous network in a locally coordinated environment consistent with substitutional-like bonding, rather than segregating into metallic phases. These results establish solid-state alloying as a scalable, low-temperature pathway to GeSn formation, enabling control over local bonding and structural order without epitaxy. This approach opens practical routes toward silicon-compatible electronics, flexible devices, and integrated photonic platforms operating under reduced thermal budgets.