Optical and x-ray characterization of Ge-Sn alloys on GaAs*

In this poster, we describe the optical and x-ray characterization of a thick Ge_1-ySn_y alloy grown on GaAs by chemical vapor deposition. From (224) x-ray reciprocal space maps we find that the alloy layer is grown pseudomorphically on the GaAs substrate. Thus, we can use (004) rocking curves and reciprocal space maps to determine the alloy composition based on Vegard’s Law. We find y=0.011.

We acquired the ellipsometric angles ψ and Δ from 0.5 to 6.5 eV photon energy and 60 to 75° incidence angles using a vertical variable angle of incidence ellipsometer (VASE) equipped with a Berek wave plate compensator. Due to low tin content, the ellipsometric angles and the resulting pseudo-dielectric function is described very well, resulting in an epilayer thickness of 1600 nm.

After fixing the thickness, we obtained the dielectric function ε of the Ge_1-ySn_y layer from a point-by-point fit, giving similar results to bulk Ge. The second derivative was fitted with analytical line shapes to determine the critical point parameters. The energy was compared with predictions from continuum elasticity theory based on established deformation potentials for Ge.