Electrical conduction in double-layer P-SiO2/polymer dielectric structures

The dc electrical conduction in MI₁M and MI₁I₂M composite specimens has been studied, where I₁ is polyimide (PI) or polyester (PET), and I₂ is a thin plasma deposited silicon compound film (oxide, P-SiO₂, or nitride, P-SiN). The current-electric field (j(E,T)) characteristics have been measured over a wide range of E values (O⩽E⩽2×10 ⁵ V cm^-1) and temperatures (20⩽T⩽170°C). The observed transport characteristics are in broad agreement with data reported in the literature, except for PI based MI₁I₂M specimens, for which polarity dependent transport behavior occurs for T>60°C and E>2×10³ V cm^-1. The observed behavior of all specimen types can be explained in terms of protonic hopping conduction; the charge carriers are created by thermal dissociation and ionization of carboxylic acid (-COOH) groups, present in PI as several percent of unimidized polyamic acid, and in PET at chain ends. When the polarity is +MI₁I₂M-, the relatively abundant free protons in PI flow to the I₁/I₂ interface, where they can accumulate and give rise to a space charge-induced potential barrier of ~0.2 eV. By modifying Lewis´ molecular dipole model so as to include this potential barrier, we derive expressions for j(E,T) which describe the experimentally observed polarity dependence in PI rather well. The effect is not observed in PET based MI₁I₂M specimens on account of the much lower concentration of free protons in this polymer