OPTICAL-TRANSITIONS IN STRAINED GE SI SUPERLATTICES/

Spectroscopic ellipsometry has been used to determine the dielectric f unctions of ultrathin Ge/Si superlattices with varying strain states a nd periodicity at room temperature. The E1-like transitions could be r esolved 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 al loys, as predicted theoretically. The E2 transitions are shifted towar ds lower energies and split into a doublet. Both of its components sho w a shift due to the hydrostatic component of the internal strain, whi ch is approximately half of what one would expect from the correspondi ng deformation potentials of the constituent bulk materials. While bot h transitions decrease in energy with increasing period, only the lowe r peak shows a variation of its amplitude and broadening with period, yielding evidence for confinement effects.