Strain/composition interplay in thin SiGe layers on insulator processed by Ge condensation

We study the interplay between strain and composition during the elementary process steps which allow the fabrication of strained ultra-thin SiGe layers on insulators from a Silicon-On-Insulator (SOI) substrate by the Ge condensation technique. Strain maps with subnanometer resolution and high precision are obtained using the dark-field electron holography technique. We confirm that two basic mechanisms drive the final composition of the top layer, namely Ge injection during oxidation and Si/Ge interdiffusion, both thermally activated. We show that during this process the observed strain results from the out-of-plane relaxation of the stress generated by the substitution of Si by Ge atoms in the Si lattice, rigidly bounded to the underlying buried oxide.