Abstract
Samples of porous silicon, produced electrochemically with a spatially varying current density, have have studied by Raman scattering, by spectral photoluminescence (PL) and by photoluminescence topography. The PL maximum observed at about 1.7 eV shifts to higher energies (2.0 eV) for the strongest structuring, where the highest PL is also found. These results give evidence for an effective direct bandgap in this material, which is a condition for the high PL. For a different kind of sample, produced by reactive ion etching, a very low wavelength PL peak is observed at 550 nm (2.0 eV). A model of spherical crystalline silicon particles (of up to 706 silicon atoms), saturated at their surface with hydrogen, has been studied by density functional theory. The calculation shows that this nanocrystalline silicon has a direct gap which is substantially larger than the bulk (indirect) gap and is consistent with experimental observation.
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