Coherent islands as preferential sites for sticking of Ge atoms in Si/Ge multilayers: formation of "conical shaped defects"

In this work we investigate the role of coherent islands in the strain relaxation mechanism of Si/SixGe1?x multilayers by using conventional transmission electron microscopy, high-resolution transmission electron microscopy, and transmission electron spectroscopic imaging. The samples investigated were grown by molecular beam epitaxy and they differ in the number of periods, period thickness, and in the Si/Ge layer thickness ratio. The formation of defect free coherent islands in strained SixGe1?x layers is at the origin of the peculiar morphology of the interfaces ?waviness?. We show that coherent islands act as preferential sticking sites for Ge adatoms, producing regions of higher Ge concentration. The higher concentration of Ge corresponding to the greater lattice spacing on the top of the coherent islands depletes the relevant strained layers. The corresponding accumulation of strain energy produces the formation of ''conical-shaped defects.'' Inside these conical-shaped defects the elastic energy is relieved by nucleation of dislocations.