Optical transitions in strained Ge/Si superlattices

Spectroscopic ellipsometry has been used to determine the dielectric functions of ultrathin Ge/Si superlattices with varying strain states and periodicity at room temperature. The ${\mathit{E}}_1$-like transitions could be resolved with the multiple-angle-of-incidence technique and in a thick, Ge-rich sample; they split up into various contributions and start to absorb the light at lower energies than compositionally equivalent alloys, as predicted theoretically. The ${\mathit{E}}_2$ transitions are shifted towards lower energies and split into a doublet. Both of its components show a shift due to the hydrostatic component of the internal strain, which is approximately half of what one would expect from the corresponding deformation potentials of the constituent bulk materials. While both transitions decrease in energy with increasing period, only the lower peak shows a variation of its amplitude and broadening with period, yielding evidence for confinement effects.