Structural characterization of Si_mGe_n strained layer superlattices

Si_mGe_n strained layer superlattice (SLS) structures were grown by molecular beam epitaxy on Ge_xSi_1-x buffer layers on 〈100〉 Si substrates to determine the effects of buffer layer composition, SLS thickness ratio, and superlattice periodicity, on the overall quality of these structures. X-ray diffraction methods were used to determine how closely actual periodicities and compositions met targeted values, and to evaluate the quality of these samples. In most instances the as-grown structures matched the targeted values to within 10%, though in some instances deviations of 20-25% in either the period or composition were observed. The quality of the SLS structures was greatly dependent on the composition of the buffer layer on which it was grown. Si_mGe_n SLS structures grown on Si- and Ge-rich buffer layers were of much higher quality than Si_mGe _m SLSs grown on Ge_0.50Si_0.50 layers, but the x-ray rocking curves of the Si_mGe_n samples indicated that they were far from perfect and contained moderate levels of defects. These results were confirmed by cross sectional transmission electron microscopy, which showed that the Si_mGe_m structures contained significant numbers of dislocations and that the layers were nonuniform in thickness and wavy in appearance. Si_mGe_n structures, however, displayed fewer defects but some dislocations and nonparallelism of layers were still observed.