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 silic on in contact with an aqueous electrolyte are investigated under polar isation conditions where efficient visible electroluminescence can be observed. A photo-induced current is obtained by electron-hole pairs g eneration simultaneously in the porous silicon skeleton and in the und erlying silicon substrate using illumination at suitable wavelengths. Experiments are based on the analysis of the impedance response of thi s 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 experimen ts suggests the following points: (1) The analysis of the impedance re sponse shows that the potential drop occurs primarily in the silicon s ubstrate across the space charge region. (2) Only the carriers generat ed in the space charge region give rise to a photo-induced current. (3 ) The electrons which are photo-generated in the space charge region a re accumulated in the initially depleted porous skeleton as can be evi denced by a visible EL emission. In the light of these results the ele ctron transport mechanisms in the porous silicon skeleton are clarifie d, The major conclusion is that the liquid-impregnated porous silicon skeleton is under equipotential conditions and that the substrate-supp lied electrons penetrate the porous layer by a diffusion process. The relation between these findings and the high EL efficiency shown by th is system is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.