A structural study was made of Si/Ge multi-layers which had been grown, by means of molecular-beam epitaxy, onto (100)Si substrates. The layers were analyzed by means of transmission electron microscopy, high-resolution X-ray diffraction, and secondary-ion mass spectrometry. It was found that the specimens differed with regard to the number of periods, the period thickness, and the thickness ratio of the Si/Ge period. In particular, interdiffusion of Ge atoms in each superlattice period of the epilayer and in the epilayer as a whole was investigated. The interdiffusion caused broadening of the nominal thickness of the Ge layer, and produced a SiGe alloy. Moreover, the Ge content of the multilayer periods increased as a function of the growth time. That is, the superlattice periods which were close to the sample surface contained more Ge atoms when compared with periods that were close to the substrate/superlattice interface. Two steps in strain relaxation were identified. Firstly, the strain energy density in each period was partially reduced by the formation of coherent islands. Then, at a certain value of strain energy density, the shape of the islands changed and the structures partially or completely relaxed the accumulated strain energy via the nucleation of extended defects.

E.Carlino, C.Giannini, C.Gerardi, L.Tapfer, K.A.Mäder, H.Von Känel: Journal of Applied Physics, 1996, 79[3], 1441-7