In this work, we report on the p-i GaAsSb/AlGaAs nanowires (NWs) ensemble device exhibiting good spectral response up to 1.1 μm with a high responsivity of 311 A W-1, an external quantum efficiency of 6.1 × 104%, and a detectivity of 1.9 × 1010 Jones at 633 nm. The high responsivity of the NWs has been attributed to in situ post-growth annealing of GaAsSb axial NWs in the ultra-high vacuum. The enabling growth technology is molecular beam epitaxy for the Ga-assisted epitaxial growth of these NWs on Si (111) substrates. Room temperature Raman spectra, as well as temperature dependent micro-photoluminescence peak analysis indicated suppression of band tail states and non-radiative channels due to annealing. A similar improvement in in situ annealed p-i GaAsSb NW ensemble with an AlGaAs passivating shell was inferred from a reduction in the Schottky barrier height as well as the NW resistance compared to the as-grown NW ensemble. These results demonstrate in situ annealing of nanowires to be an effective pathway for improving the optoelectronic properties of the NWs and the device thereof.