Schmid U., Lukes F., Christensen N.E., Alouan M., Cardona M., Kasper E., Kibbel H., Presting H.
The authors present experimental as well as theoretical data for the linear optical response of symmetrically strained Ge4Si6(001) superlattices. Ab initio calculations show that the superlattice considered has a direct gap at about 1.1 eV. The lowest direct transitions are dipole allowed, although they are rather weak. The complex dielectric function has been measured with a rotating analyser ellipsometer. Excellent agreement of the experimental second-derivative spectrum d2 in 2/d omega 2 with the theory is obtained after the calculated in 2( omega ) has been convoluted with a Lorentzian to include effects of lifetime broadening, both for the shape and position of bulk-like and for new superlattice-like transitions. A detailed analysis of the observed structures in terms of interband transitions is given.
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