Fabrication of supersaturated Ge1-xSnx alloys via Pulsed Laser Melting

Over the last two decades, Ge1-xSnx alloys have received significant attention since Sn alloying with x > 8 at. % converts Ge into a direct bandgap semiconductor. This paves the way for the development of mid-infrared lasers and photodetectors sensitive to wavelengths ranging from the short-wave infrared (SWIR) (1.4–3 μm) to the long-wave infrared (LWIR) (8–14 μm). The fabrication of Sn-rich Ge1-xSnx alloys on Ge presents different challenges, the first being the 14 % relaxed lattice mismatch between Ge and Sn. In the pseudomorphic growth regime, Ge1-xSnx layers grown on Ge experience compressive strain, which shifts the indirect-to-direct bandgap crossover to higher Sn concentrations. Another issue concerns the the poor solubility of Sn in Ge (< 1 at. % at room temperature), causing Sn to segregate especially at high temperatures. This work comprises two parts. In the first, we demonstrate the fabrication of supersaturated Ge1-xSnx alloys (up to 20 at. % Sn) on Ge and Ge-on-Si substrates by employing a novel ex-situ approach articulated in two stages. First, a thin film of metallic Sn (thickness < 70 nm) was deposited on the substrate surface by DC magnetron sputtering. Subsequently, we performed nanosecond pulsed laser melting (PLM) to melt the topmost region of the substrate and allow Sn atoms to in-diffuse and be ultimately incorporated in the re-grown host lattice at concentrations well exceeding the equilibrium solubility of Sn in Ge. Ge1-xSnx alloys were characterized through a set of techniques including secondary ions mass spectrometry (SIMS), high resolution x-ray diffraction (HRXRD), Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM). In-depth Sn concentration profiles acquired by SIMS showed the formation of box-like (i.e., plateau) regions located near the sample surface, proving that our ex-situ approach leads to Ge1-xSnx alloys with uniform composition on Ge. By varying Sn sputtering/PLM process parameters, we verified that it is possible to finely tune the length of the plateau region, the Sn concentration within it and the broadening of the diffusion profile (i.e., the maximum melt depth). Reciprocal space maps acquired by HRXRD revealed that the difference in thermal expansion coefficient between Ge1-xSnx and Ge leads to the formation of a tensile strained (~ 0.30 %) alloy. By combining SIMS and HRXRD analyses and performing heat flow simulations, we demonstrated that increasing the regrowth velocity beyond 3.5 m/s results in the Sn substitutional fraction χ_Sn improving from ~ 60 % to ~ 100 %. Drawing on this result, we tested the use of an additional laser pulse at low energy density for the recovery of the non-substitutional Sn fraction χ_Sn in Ge1-xSnx samples fabricated by PLM, showing that it can be increased up to ~ 95 %. This same method was applied to Ge1-xSnx samples fabricated on Ge-on-Si substrate, obtaining a slightly lower increase in χ_Sn compared to Ge. In the second part of this work, we demonstrate that ammonia peroxide mixture (APM) allows the selective wet etching of Ge1-xSnx alloys on Ge-on-Si substrates. We also show that maskless lithography is a feasible approach to achieve high selectivity between Ge and Ge1-xSnx while ensuring surface integrity required for applications. In summary, this thesis managed to prove the efficacy of PLM in the fabrication of thick, supersaturated Ge1-xSnx alloys on Ge and Ge-on-Si substrates, having uniform composition near the surface and χ_Sn up to 95 %. The possibility to selectively etch thick Ge layers while preserving Ge1-xSnx offered by maskless lithography sets the stage for the integration of Ge1-xSnx alloys in devices such as mid-infrared photodetectors and mid-infrared lasers.