AC CONDUCTIVITY OF POROUS SILICON - A FRACTAL AND SURFACE TRANSPORT MECHANISM

In this paper we report on the frequency dependence of the AC conducti vity of porous silicon in the range 10 Hz-100 kHz. Two types of testin g devices have been fabricated on three different series of samples fo rmed electrochemically using as a starting material p-type, n(-)-type and n(+)-type silicon substrates. For frequencies less than 20-40 kHz the conductivity is found to follow a sublinear frequency dependence. This behaviour is typical of a carrier transport mechanism determined by an anomalous diffusion process. At higher frequencies me find that sm face states influence the transport mechanism. This suggests a doub le-channel transport mechanism: one related to porous-silicon ''volume '' properties and the other more connected to the ''surface'', itself.