Strain relaxation of pseudomorphic Si1-xGex/Si(100) heterostructures after hydrogen or helium ion implantation for virtual substrate fabrication

Strain relaxed Si1-xGex layers on Si (100) are used as virtual substrates for the growth of e.g. Si/Si1-xGex quantum well structures. We investigated the effects of H+ and He+ ion implantation and subsequent annealing on pseudomorphic Si1-xGex/Si(100) heterostructures grown by molecular beam epitaxy (MBE). A narrow defect band is generated by ion implantation slightly underneath the interface inducing the formation of strain-relieving misfit dislocations (MDs) during subsequent thermal annealing. Using H+ ion implantation, nearly complete strain relaxation of Si1-xGex layers with Ge fractions up to 22 at.% was obtained at temperatures as low as 800 degreesC and the samples appeared free of threading dislocations (TDs) within the SiGe layer to the limit of transmission electron microscopy (TEM) analysis. Efficient strain relaxation was demonstrated even for Si1-xGex layers with Ge fractions up to 30 at.% using He+ ion implantation. We have thus developed a method for producing high-quality, thin, relaxed Si1-xGex films on Si(100) with TD densities well below 10(7) cm(-2) by standard techniques as MBE and ion implantation. The heterostructures were analyzed using X-ray diffraction (XRD), Rutherford backscattering/channeling spectrometry and TEM, We propose a model of strain relaxation in which dislocations generated in conjunction with the formation of H or He filled overpressurized cavities glide to the interface where they form strain-relieving misfit segments. On the basis of this assumption, the conditions for efficient strain relaxation are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.