Growth and characterization of Ge/Si strained-layer superlattices

Ultrathin Ge/Si strained-layer superlattices (SLS's) with periodicities of a few monolayers have been successfully grown and characterized by Raman scattering spectroscopy. Structures with alternating Ge and Si layers were grown on Si substrates of different orientations. A thick 200 nm Ge _0.4Si_0.6 buffer layer was grown prior to the growth of the superlattice to make the strain distribution of the superlattice symmetrical and thus to maintain the pseudomorphic growth of the superlattices. Folded acoustic phonon peaks observed from these Ge/Si SLS samples can be used to determine the superlattice periodicity. The observed optical phonon frequencies were found to depend strongly on superlattice periodicity. A quantitative interpretation of this phenomenon was presented. Subsequent annealing of these samples reveals that the transition from pure Ge and/or Si layers to Ge _xSi_1-x alloy becomes more pronounced as the annealing time and temperature increase.