Infrared dielectric response, index of refraction, and absorption of germanium-tin alloys with tin contents up to 27% deposited by molecular beam epitaxy

摘要

The dielectric spectral response of Ge1-xSnx thin film alloys with relatively high Sn contents (0.15 = x = 0.27) and thickness from 42 to 132 nm was characterized by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 mu m. The Ge1-xSnx thin films were deposited on Ge substrates by molecular beam epitaxy using an electron-beam source for Ge to achieve a substrate temperature below 150 degrees C to prevent the surface segregation of Sn. From the measured dielectric function, the complex refractive index was calculated indicating an increase in the real index with the Sn content at mid-infrared wavelengths. The ellipsometry revealed that the band structure critical point energies red-shifted with the increasing Sn content. The optical absorption coefficient was calculated from the imaginary index and showed a strong absorption into, and beyond, the mid-infrared with the increasing Sn content. Published by AIP Publishing.