Electronic band structure and free carrier properties of strained tin-germanium alloys on InSb from FTIR ellipsometry*

From Fourier-transform infrared ellipsometry, we provide spectroscopic evidence about the valence band structure of α-tin. The mid-infrared dielectric function of α-tin grown pseudomorphically by molecular beam epitaxy shows a strong peak near 0.41 eV. The peak can be attributed to the allowed intra-valence band transitions from the Γ₇^- (electron-like) to the Γ₈^+υ heavy hole valence band and/or interband transitions from the Γ₇^- band to the Γ₈^+c light “hole” conduction band. Possible sources for the strength of the peak and its temperature dependence will be discussed. Our results are significant, because intra-valence band transitions have not previously been reported as a peak in ellipsometry spectra. This peak cannot be described as a van Hove singularity with a critical point lineshape. It should be universal, i.e., common to all zero-gap semiconductors. We will also show the dependence of this peak on composition and strain in Ge_1-xSn_x alloys with up to 6% Ge. At photon energies below 0.4 eV, FTIR ellipsometry spectra are dominated by the Drude response of free carriers and a discussion on the species of the carriers will be provided.