Carrier transport mechanisms in porous silicon in contact with a liquid phase: a diffusion process

The carrier transport mechanisms in reverse-biased p-type porous silicon in contact with an aqueous electrolyte are investigated under polarisation conditions where efficient visible electroluminescence can be observed. A photo-induced current is obtained by electron–hole pairs generation simultaneously in the porous silicon skeleton and in the underlying silicon substrate using illumination at suitable wavelengths. Experiments are based on the analysis of the impedance response of this system, on the evolution of the photo-induced current vs. potential as a function of the porous layer thickness and on the time-evolution of the electroluminescence (EL) signal. The results of these experiments suggests the following points: (1) The analysis of the impedance response shows that the potential drop occurs primarily in the silicon substrate across the space charge region. (2) Only the carriers generated in the space charge region give rise to a photo-induced current. (3) The electrons which are photo-generated in the space charge region are accumulated in the initially depleted porous skeleton as can be evidenced by a visible EL emission. In the light of these results the electron transport mechanisms in the porous silicon skeleton are clarified. The major conclusion is that the liquid-impregnated porous silicon skeleton is under equipotential conditions and that the substrate-supplied electrons penetrate the porous layer by a diffusion process. The relation between these findings and the high EL efficiency shown by this system is also discussed.