Synthesis of supersaturated GeSn alloys on Ge and Ge-on-Si by sputter deposition and nanosecond pulsed laser melting.

Over the last two decades, GeSn alloys have attracted considerable attention owing to their potential to achieve a direct bandgap at Sn concentrations above ~8 at.%, enabling the development of Si-compatible mid-infrared photonic devices. However, the low solid solubility of Sn in Ge remains a major challenge for the fabrication of high-quality layers with high Sn content. Here, we report the fabrication of supersaturated GeSn alloys with Sn concentrations up to 20 at.% on Ge and Ge-on-Si substrates using a novel ex-situ two-step approach. First, a thin Sn film (<50 nm) is deposited on Ge(100) and Ge-on-Si substrates by DC magnetron sputtering. Nanosecond pulsed laser melting (PLM) then induces melting of the near-surface region, enabling rapid Sn diffusion and incorporation into the regrown Ge lattice. Owing to the strongly out-of-equilibrium nature of PLM, Sn concentrations well above the equilibrium solid solubility are achieved [1,2]. Secondary ion mass spectrometry (SIMS) depth profiles reveal box-like Sn distributions near the surface, confirming the formation of uniform GeSn regions. By adjusting the initial Sn thickness and the PLM parameters (laser energy density and number of pulses), both the thickness and the Sn content of the alloyed region can be finely tuned. Transmission electron microscopy (TEM) highlights Sn segregation along threading dislocations originating from misfit dislocations at the GeSn/Ge interface. The results indicate that the substitutional Sn fraction correlates with regrowth velocity: lower laser energy densities promote substitutional incorporation, whereas higher energies reduce it. An additional low-energy pulse (300–400 mJ/cm²) effectively restores substitutional Sn in samples initially treated at higher energy density (800 mJ/cm²). Reciprocal space maps reveal tensile strain arising from thermal expansion mismatch, which lowers the Sn threshold for the indirect-to-direct bandgap transition. This approach provides a viable route to thick, uniform, supersaturated GeSn layers for advanced electronic and mid-infrared photonic devices. [1] E. Di Russo et al., Synthesis of relaxed Ge0. 9Sn0. 1/Ge by nanosecond pulsed laser melting. Applied Surface Science 612, 155817 (2023). [2] D. Fontana et al., Ex-situ n-type heavy doping of Ge1-xSnx epilayers by surface Sb deposition and pulsed laser melting. Applied Surface Science 600, 154112 (2022).