Properties of pseudomorphic and relaxed germanium1−xtinx alloys (x < 0.185) grown by MBE

Epitaxial layers of Ge1−xSnx with Sn compositions up to 18.5% were grown on Ge (100) substrates via solid-source molecular beam epitaxy. Crystallographic information was determined by high resolution x-ray diffraction, and composition was verified by Rutherford backscattering spectrometry. The surface roughness, measured via atomic force microscopy and variable angle spectroscopic ellipsometry, was found to scale with the layer thickness and the Sn concentration, but not to the extent of strain relaxation. In addition, x-ray rocking curve peak broadening was found not to trend with strain relaxation. The optical response of the Ge1−xSnx alloys was measured by spectroscopic ellipsometry. With increasing Sn content, the E1 and E1 + Δ1 critical points shifted to lower energies, and closely matched the deformation potential theory calculations for both pseudomorphic and relaxed Ge1−xSnx layers. The dielectric functions of the high Sn and strain relaxed material were similar to bulk germanium, but with slightl...