Growth and Characterization of Ultrathin Si_mGe_n Strained Layer Superlattices

Summary

Growth of Ultrrathin Si_mGe_n (m monolayers (ML) Si, n ML Ge) strained layer superlattices (SLS) by molecular beam epitaxy (MBE) 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 with strain symmetrization of the SLS by a thin homogeneous buffer layer serving as a virtual substrate. The concept of zone folding is described and the transition matrix elements of the folded bandstructure as a function of period length are calculated. The concept of a virtual substrate consisting of the actual Si-substrate and a thin buffer layer is explained. The strain adjustment of the SLS by choice of the buffer layer composition and the strain symmmetrization is outlined. Different structural (TEM-and X-ray analysis, Rutherford Back Scattering) and optical (Raman, Photoluminescence, modulation spectroscopy) characterization methods of the SLS are discussed. A Raman spectrum of a Si₆Ge₄ SLS is shown together with an Si_0.6Ge_0.4 alloy sample, the occuring peaks are discussed.

The performance of future microelectronic circuits will be strongly enhanced by the monilithic integration of superlattice devices with conventional integrated circuits on top of a silicon substrate. With this material concept the mismatch between the superlattice materials and the silicon substrate has to be accomodated. The SinGem SLS is a model system for the study of mismatch effects because of similar chemistry and well pronounced strain effects.