Strain relaxation of Si/Ge multilayers investigated by transmission electron microscopy and high-resolution X-ray diffractometry

In this work we investigate the strain-relief mechanisms and the formation of structural defects of Si/Ge multilayers grown by molecular beam epitaxy on (100)- Si substrates. The investigated specimens differ in number of periods, period thickness, and in the Si/Ge layer thickness ratio. The structural analyses are performed by transmission electron microscopy and high-resolution X-ray diffraction. We found that a Si-Ge interdiffusion induces a broadening of the nominal thickness of the Ge layer, producing a SixGe1-x alloy as well as a higher Ge content in the last periods of the multilayer structure. Our measurements suggest that the strain relaxation occurs in two steps: i) in each period of the multilayer the strain energy density is partially reduced by the formation of coherent islands; ii) at a certain value of the strain energy density, the shape of the coherent islands changes and the structures, partially or completely, relax the accumulated strain energy by nucleation of dislocations. The increase of the strain energy density is related to the measured monotonic increase of the Ge content as a function of the growth time.