Influence of humidity on transport in porous silicon

Abstract

We measured systematically the influence of relative humidity on the d.c. electrical conductance of porous silicon (PS) prepared by electrochemical etching. The measurements were taken at room temperature and atmospheric pressure in the range of relative humidity 0%–100%. Between 25% and 75% relative humidity a steep increase in the conductance (of three orders of magnitude) was observed. At higher humidity the dependence saturated. Similarly, below 25% the conductance was only a weak function of relative humidity. It is worth noting that on changing the humidity, typical time constants of the process are of the order of several days. The S-shaped curve obtained is analogous with those for other porous or layered inorganic materials, for example porcelain or mica, respectively, where the enhanced conductance is due to transport via water films condensed in capillaries. Hence a transport mechanism specific to porous silicon can be expected only in samples kept at a relative humidity lower than 20% or previously evacuated. A sudden increase in the conductance at 25% relative humidity is interpreted as the beginning of wetting of the inner surfaces of porous silicon by a continuous water film.