EVIDENCE OF NANOSTRUCTURES OF DIFFERENT SIZE IN POROUS SILICON

Porous silicon layers 7 mum thick with a porosity of 80% have been prepared 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 band increases from 25 to 3000 mus and the lifetime of the 1.5 eV emission 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 presence of classes of wires of different size, multiple of the minimum etchable dimension a/4, being a the silicon lattice parameter.