  We address nonradiative recombination pathways by leveraging surface
passivation and dislocation management in micron-scale arrays of Ge crystals
grown on deeply patterned Si substrates. The time decay photoluminescence (PL)
at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in
band-gap engineered Ge micro-crystals. This investigation provides compelling
information about the competitive interplay between the radiative band-edge
transitions and the trapping of carriers by dislocations and free surfaces.
Furthermore, an in-depth analysis of the temperature dependence of the PL,
combined with capacitance data and finite difference time domain modeling,
demonstrates the effectiveness of GeO2 in passivating the surface of Ge and
thus in enhancing the room temperature PL emission.
