Ultrathin Si_mGe_n strained layer superlattices-a step towards Si optoelectronics

Ultrathin Si_mGe_n (m monolayers (ML) Si, n ML Ge) strained layer superlattices (SLS) 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_1-ybGe_yb alloy buffer layer. The energies and the oscillator strengths of the bandgap and intersubband transitions have been studied theoretically for Si_mGe_n SLS with a variety of period lengths, particularly those of m+n=10. 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 (h nu approximately 0.8 eV) are presented and discussed as well as complementary photocapacitance measurements on a p-n doped Si₄Ge₄ SLS diode. The fabrication of test mesa diodes from Si/Ge SLS structures is described. Finally, device applications offering the possibility of monolithic integration of superlattice devices with complex silicon-based electronic circuits are outlined.