  A mechanism of controlling the degree of strain relaxation in GeSn epilayers,
grown by molecular beam epitaxy on Ge/Si(001) substrates, is reported in this
work. It is demonstrated that by suitably controlling the thickness and the
growth recipe of the underlying Ge buffer layer, both fully-strained and
highly-relaxed GeSn epilayers can be obtained, without significant Sn
segregation. The strain relaxation of the GeSn epilayer is mediated by
threading dislocations of the Ge buffer layer, propagating across the Ge-GeSn
interface. Systematic estimation of the threading dislocation density in both
the alloy epilayer and the Ge buffer layer, by the approach developed by
Benediktovich et al. [A. Benediktovitch, A. Zhylik, T. Ulyanenkova, M. Myronov,
A. Ulyanenkov (2015), J. Appl. Cryst. 48, 655-665] supports this observation,
and also reveals that no additional dislocations are generated at the Ge-GeSn
interface. Together with recently reported techniques to arrest dislocation
propagation in GeSn epilayers, these results bode extremely well for
realization of highly-relaxed GeSn epilayers, much coveted for development of
GeSn-based emitters.
