SI-LIKE AND GE-LIKE FEATURES IN THE DIELECTRIC FUNCTION OF SI(N) GE(M) SUPERLATTICES

We have investigated the optical properties of several Si1-xGex alloys and short-period Si(n)Ge(m) superlattices (SLs) grown by molecular be am epitaxy on crystalline Si(001) substrates. The behaviour of the all oy overlayers closely resembles that of Czochralski-grown bulk materia l for which data are available in recent literature. For an Si6Ge2 SL, where every Ge atom is at an interface, the optical properties were f ound to closely resemble those of an alloy. For thicker Ge layers (Si6 Ge4 SLs) deviations from the alloy-like behaviour become apparent. Mul tilayer model results for the SLs are obtained. The optical response o f the SLs is modelled by an effective medium approximation using the d ielectric constants of bulk Si and Ge, with the appropriate volume fra ctions. The interfaces in the SLs are modelled by an Si1-xGex alloy. W e find that this approach does indeed give a good description of the S L optical response. Furthermore, we show that spectroscopic ellipsomet ry can be used to monitor the alloying in an Si12Ge8 SL on annealing. A shift from a Ge-like E1-type optical transition in the SL into an Si 1-xGex alloy-like E1-type transition is observed.