Optical vibrational and structural properties of Ge _1-xSn _x alloys by UHV-CVD

UHV-CVD growth based on a deuterium stabilized Sn hydride and digermane produces Ge-Sn alloys with tunable bandgaps. The Ge _1-xSn _x (x=2-20 at.%) alloys are deposited on Si (100) and exhibit superior crystallinity and thermal stability compared with MBE grown films. Composition, crystal and electronic structure, and optical and vibrational properties are characterized by RBS, low energy SIMS, high resolution electron microscopy TEM, x-ray diffraction, as well as Raman and IR spectroscopies. TEM studies reveal epitaxial films with lattice constants between those of Ge and Sn. X-ray diffraction shows well-defined (004) peaks and rocking curves indicate a tightly aligned spread of the crystal mosaics. Resonance Raman indicate a E ₁ bandgap reduction relative to Ge, consistent with a decrease of the E ₂ critical point observed in spectroscopic ellipsometry. IR transmission spectra indicate an increase in absorption with increasing Sn content consistent with a decrease of the direct bandgap.