The silicon porous layers used in the experiments are realized onto 3-inches diameter (100) silicon wafers ( 400 micrometer-thick,p-type and electrical resistivity -0.01 Aohm.cm). The mesoporous character of the layers is formed by an anodisation process during 35 min in a mixture of 27% fluorhydric acid (HF), 38% deionized water and 35% ethanol with a current density of 100 mAcm