Direct-gap photoluminescence from germanium nanowires

We report observation of near-infrared photoluminescence from free-standing, vertically aligned germanium nanowires grown on a (111)-oriented silicon substrate. The energy of the photoluminescence peak is very close to that of the bulk crystalline germanium direct band gap. The intensity shows an approximately quadratic dependence on excitation laser power and decreases with decreasing temperature. The peak position exhibits a redshift with increasing laser power due to laser-induced heating of the wires. These observations indicate that the photoluminescence originates from the direct band-gap recombination in the germanium nanowires.