Mechanism of AC electrical transport of carriers in freshly formed and aged porous silicon

Electrical impedance of aging porous silicon layers (PS) was studied applying the regime df temperature cycling (from -50 to +150 degrees C) in a range of alternating current (a.c.) frequencies 0.2 to 10(5) Hz (amplitude 1 V). Freshly obtained PS exhibits changes of electrical properties at temperatures above 50 degrees C, presumably associated with the escape of electrolyte from the pores. A.C. electrical conductivity of PS is very sensitive to its postanodizing treatments (heat-treatment, pore filling by inert electrolyte, evacuation at increased temperatures, annealing in nitrogen). Activation energy for the electrical conduction ranges from 0.1 to 0.23 eV depending on a.c. frequency and postanodizing treatments. An electric equivalent circuit was designed to fit the experimental data acquired at differently treated PS samples. The components of the equivalent circuit were assigned to physical elements of PS (a surface phase on top of PS and a layer of underlying bulk material). Based on the measurements bf the electrical impedance and chemical composition of PS films the mechanism of electrical conduction is assumed to be a charge carrier migration through the localized states associated with the surface impurities (hydrides and/or oxides of silicon). The obtained impedance data show the strong influence of residuals of the electrolyte inside the pores of freshly prepared PS on its aging stability.