X-RAY REFLECTOMETRY STUDY OF INTERDIFFUSION IN SI/GE HETEROSTRUCTURES

Interdiffusion in molecular-beam-epitaxy-grown Si-Ge heterostructures has been investigated by grazing incidence x-ray reflectometry. Measurements on Si1-xGex/Si superlattices reveal that the thickness of the Si1-xGex layers increases when intermixing is induced by high-temperature annealing. This results from a strong composition dependence of the Ge diffusion coefficient in Si1-xGex. Experiments on coherently strained superlattices show that the diffusion rate is faster in the early stage of annealing, before the strain is relieved. This suggests that strain enhances interdiffusion at the interfaces. Similar experiments performed on (Si(m)Ge(n))p atomic layer superlattices show that considerable intermixing occurs in these heterostructures even after moderate annealing treatments (i.e., 20 s at 700-degrees-C). Due to the very low diffusion rate of Ge in Si-rich Si1-xGex alloys, interdiffusion is more pronounced in structures of smaller periodicity or in which the inter-faces are initially intermixed.