STRAIN RELAXATION OF SI GE MULTILAYERS - COHERENT ISLANDS FORMATION AND THEIR EVOLUTION AS A FUNCTION OF THE STRAIN/

We report on a structural study of Si/Ge multilayers grown by molecula r-beam epitaxy on (100)-Si substrates. The analyses have been performe d by using transmission electron microscopy, high-resolution x-ray dif fraction, and secondary-ion-mass spectrometry. The investigated specim ens differ in number of periods, period thickness, and in the Si/Ge pe riods thickness ratio. In particular, we investigate the interdiffusio n of the Ge atoms in each superlattice period of the epilayer and in t he epilayer as whole. The interdiffusion causes a broadening of the no minal thickness of the Ge layer producing a SixGe1-x alloy. Furthermor e, the Ge content in the multilayer periods increases as a function of the growth time, i.e., the superlattice periods close to the sample s urface contain more Ge atoms if compared to the periods close to the s ubstrate/superlattice interface. We find two steps in the strain relax ation: (i) In each period the strain energy density is partially reduc ed by the formation of coherent islands; (ii) at a certain value of th e strain energy density the shape of the islands changes and the struc tures relax partially or completely the accumulated strain energy by n ucleation of extended defects. (C) 1996 American Institute of Physics.