Ultrathin Si , Ge, (m monolayers (ML) si, n ML Ge) strained layer superlattices (SE) have been grown by molecular beam epitaxy. The optical properties of these structures depend on the concept of band-structure engineering by Brillouin zone folding and Strain adjustment of the SLS by a Si, _, , Ge, , alloy buffer layer. The energies and the oscillator strengths of the bandgap and Intersubband transitions have been studied theoretically for Si , Ge, SLS with a variety of period lengths, particularly those of m + n = I O . Various characterization tools such as x-ray diffraction, transmission electron microscopy Raman spectroscopy, photoluminescence (PL) and photocapacitance measurements have been used to analyse growth quality, interface sharpness morphology. strain distribution and optical properties of the superlattice experimentally. The PL data indicative of the quasidirect energy gap of the 10 ML strain-symmetrized SLS in the near-infrared spectral regime (hv -0.8eV) are presented and discussed as well as complementary photocapacitance measurements on a p-n doped Si ,Ge, SLS diode. The fabrication of test mesa diodes from Si/Ge SLS structures is described. Finally. device applications olfering the possibility of monolithic integration of Superlattice devices with complex silicon-based electronic circuits are outlined. Material K (W m K -' ) Si 145 GaAs 46 Diamond 2000 Copper 384

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We report on band-gap luminescence in strain-symmetrized, (Si) /(Ge)"superlattices grown on a step-graded, alloy buffer with a reduced dislocation density, using Sb as a surfactant. The luminescence efficiency for a (Si)9/(Ge)6 and (Si)6/(Ge)& superlattice is strongly enhanced compared with a corresponding Slo 6Geo 4 alloy reference sample. The luminescence signals can be attributed to interband transitions