Porous silicon layers 7 mum thick with a porosity of 80% have been pre
pared for 1 OMEGA cm, (100) oriented, polished silicon wafers. As the
measurement temperature decreases from 300 to 10 K the width of the PL
spectra remains almost constant, while the lifetime of the 2 eV PL ba
nd increases from 25 to 3000 mus and the lifetime of the 1.5 eV emissi
on changes from 100 to 200 mus. At all the temperatures, the broad PL
bands show distinct peaks. We have interpreted these results in terms
of the confined quantum wire model recalling for the first time the pr
esence of classes of wires of different size, multiple of the minimum
etchable dimension a/4, being a the silicon lattice parameter.