Visible electroluminescence (EL) from cathodically biased n-Si in an electrolyte solution containing S 2 O 8 2Ϫ has been studied. EL from the porous n-Si samples fabricated by photoelectrochemical etching over a wide range of current densities (J a ) shows both blueshift and peak broadening with increasing J a , which can be attributed to the progressive participation of more silicon hydride species, Si-H n , with larger n in the emitting progress. Based on analysis of spectral changes and resolution of the EL peak into components, a revised mechanism of the EL, where silicon hydrides are considered to donate electrons to sulfate radical anions, is proposed. EL is produced by recombination of the resulting radical, Si-H n• ϩ , with an accumulated electron in the conduction band. The radicals would lead to competing reactions that strip off hydrogens from the surface silicon hydrides. Si-H n with larger n is subject to the earlier EL.

The spectral shifts of visible electroluminescence from chemically modified porous Si under cathodic bias in an electrolyte containing S 2 O 8 2Ϫ have been studied by luminescence and Fourier transform infrared measurements. Modification of porous Si with soluble polyaniline and silane compounds results in blueshifts, whereas redshifts are observed with H ϩ and O ϩ ion implantations. Based on a recently proposed mechanism that silicon hydride acts as an electron injector to sulfate radical anion, the blueshift is linked to the lowering of the energy levels of silicon hydrides on the porous Si surface with the bandedges being invariant with the chemical modifications. The redshift is seen as a result of the conversion of silicon dihydride to silicon monohydride, with the energy levels of silicon monohydride lying higher than silicon dihydride.