  The hexagonal-2H crystal phase of Ge recently emerged as a promising direct
bandgap semiconductor in the mid-infrared range providing new prospects of
additional optoelectronic functionalities of group-IV semiconductors (Ge and
SiGe). The controlled synthesis of such hexagonal (2H) Ge phase is a challenge
that can be overcome by using wurtzite GaAs nanowires as a template. However,
depending on growth conditions, unusual basal stacking faults (BSFs) of
I$_3$-type are formed in the metastable 2H structure. The growth of such
core/shell heterostructures is observed in situ and in real-time by means of
environmental transmission electron microscopy using chemical vapour
deposition. The observations provide direct evidence of a step-flow growth of
Ge-2H epilayers and reveal the growth-related formation of I$_3$-BSF during
unstable growth. Their formation conditions are dynamically investigated.
Through these in situ observations, we can propose a scenario for the
nucleation of I$_3$-type BSFs that is likely valid for any metastable hexagonal
2H or wurtzite structures grown on m-plane substrates. Conditions are
identified to avoid their formation for perfect crystalline synthesis of
SiGe-2H.
