MECHANISM OF AC ELECTRICAL-TRANSPORT OF CARRIERS IN FRESHLY FORMED AND AGED POROUS SILICON

Electrical impedance of aging porous silicon layers (PS) was studied a pplying 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 pr operties at temperatures above 50 degrees C, presumably associated wit h the escape of electrolyte from the pores. A.C. electrical conductivi ty of PS is very sensitive to its postanodizing treatments (heat-treat ment, pore filling by inert electrolyte, evacuation at increased tempe ratures, 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 sample s. 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 cond uction is assumed to be a charge carrier migration through the localiz ed states associated with the surface impurities (hydrides and/or oxid es of silicon). The obtained impedance data show the strong influence of residuals of the electrolyte inside the pores of freshly prepared P S on its aging stability.