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 forma tion of structural defects of Si/Ge multilayers grown by molecular bea m epitaxy on (100)- Si substrates. The investigated specimens differ i n number of periods, period thickness, and in the Si/Ge layer thicknes s ratio. The structural analyses are performed by transmission electro n microscopy and high-resolution X-ray diffraction. We found that a Si -Ge interdiffusion induces a broadening of the nominal thickness of th e Ge layer, producing a SixGe1-x alloy as well as a higher Ge content in the last periods of the multilayer structure. Our measurements sugg est that the strain relaxation occurs in two steps: i) in each period of the multilayer the strain energy density is partially reduced by th e formation of coherent islands; ii) at a certain value of the strain energy density, the shape of the coherent islands changes and the stru ctures, partially or completely, relax the accumulated strain energy b y nucleation of dislocations. The increase of the strain energy densit y is related to the measured monotonic increase of the Ge content as a function of the growth time.