  Understanding the nature and behavior of vacancy-like defects in epitaxial
GeSn metastable alloys is crucial to elucidate the structural and
optoelectronic properties of these emerging semiconductors. The formation of
vacancies and their complexes is expected to be promoted by the relatively low
substrate temperature required for the epitaxial growth of GeSn layers with Sn
contents significantly above the equilibrium solubility of 1 at.%. These
defects can impact both the microstructure and charge carrier lifetime. Herein,
to identify the vacancy-related complexes and probe their evolution as a
function of Sn content, depth-profiled pulsed low-energy positron annihilation
lifetime spectroscopy and Doppler broadening spectroscopy were combined to
investigate GeSn epitaxial layers with Sn content in the 6.5-13.0 at.% range.
The samples were grown by chemical vapor deposition method at temperatures
between 300 and 330 {\deg}C. Regardless of the Sn content, all GeSn samples
showed the same depth-dependent increase in the positron annihilation line
broadening parameters, which confirmed the presence of open volume defects. The
measured average positron lifetimes were the highest (380-395 ps) in the region
near the surface and monotonically decrease across the analyzed thickness, but
remain above 350 ps. All GeSn layers exhibit lifetimes that are 85 to 110 ps
higher than the Ge reference layers. Surprisingly, these lifetimes were found
to decrease as Sn content increases in GeSn layers. These measurements indicate
that divacancies are the dominant defect in the as-grown GeSn layers. However,
their corresponding lifetime was found to be shorter than in epitaxial Ge thus
suggesting that the presence of Sn may alter the structure of divacancies.
Additionally, GeSn layers were found to also contain a small fraction of
vacancy clusters, which become less important as Sn content increases.
