ELECTRICAL BEHAVIOR OF ALUMINUM - POROUS SILICON JUNCTIONS

In order to analyse the electrical behaviour of aluminium-porous silicon junctions, we studied the current-voltage and temperature I(V, T) characteristics of a series of junctions, having a typical porosity of 45% and a layer thickness ranging from 2 to 30 mu m. Under forward bias, the current can be fitted by the law I = I-s(exp(q(V - R,I)/nkT)), with R(s) a serial resistance ranging from a few kiloohms to a few tens of megaohms as the thickness increases. Moreover, analysis of I, vs. the temperature alows us to determine a built-in potential phi(b0) of the order of 0.40 eV, showing pinning of the Fermi level on a density of interface states associated with dangling bonds. Under reverse bias, the logarithm of the current follow s a quite linear law, showing that the conduction is limited by a surface mechanism associated with hopping of the carriers from site to site, each site corresponding to a dangling bond, in agreement with theoretical results (in a porous layer, the carriers are not on the dopants but are localized on dangling bonds). Each site can be modelled as a square well potential and the electrons. to escape this well. have typically to overcome a barrier of the order of 0.10-0.30 eV, giving an extension of the associated wave function of the order of a few reciprocal angstroms. Finally, the possibility of variable range hopping is also considered and ruled out.