Direct bandgap measurements in a three-dimensionally macroporous silicon 9R polytype using monochromated transmission electron microscope

We demonstrate a three-dimensionally macroporous Si 9R polytype that exhibits a different electronic structure than bulk diamond-structured Si. Unlike the latter one which has an indirect-bandgap transition close to the zone boundary, the conduction band minimum in this material, as revealed by valence electron energy-loss spectroscopy in a monochromated transmission electron microscope, significantly shifts toward the Γ point within a range from 0.6 to 5.6 nm−1, indicating substantially less momentum transfer required to fulfill the bandgap transition.