Growth and characterization of ultrathin SimGen strained-layer superlattices (Invited Paper)

Growth of ultrathin Si_mGe_n [m monolayers (ML) Si, n ML Ge] strained layer superlattices (SLS) by molecular beam epitaxy is reported. Diode structures (doping sequence p⁺-n-n⁺ on n⁺-substrate and n⁺-n-p⁺ on p⁺-substrate) were grown for optical device applications where strain symmetrization of the SLS by a thin homogeneous buffer layer was used. The concepts of bandstructure folding and strain adjustment of the SLS by a thin (approximately 50 nm) Si_1-ybGe_yb alloy buffer layer are described. The folded bandstructure with its transition matrix elements as a function of period length is calculated. Various characterization tools such as x-ray diffraction, transmission electron microscopy, Rutherford backscattering, Raman spectroscopy, and photocapacitance measurements are used to analyze the growth quality, strain distribution, periodicity, interface sharpness, and optical properties of the superlattice. Results from recent optical experiments such as photoluminescence and ellipsometry giving hints of a direct bandgap transition in a 10 ML Si₆Ge₄ SLS in the near-infrared spectral region (hw≈0.8 eV) are compared with PL data from SiGe alloys. Device applications offering the possibility of a monolithic integration of superlattice devices with complex silicon based electronic circuits are discussed.