Alternating current electrical properties of island aluminum thin films on polyimide substrates

Electrical resistance and impedance measurements of island aluminum films on polyimide on Si[100] substrates are comparatively discussed over a broad frequency range covering from d.c. to 10⁷ Hz. Studies of both d.c. resistance and film morphology data show that the d.c. resistances of the films exhibit an island gap size dependence, which can be explained on the basis of thermally activated tunneling, so that tunneling among islands appears to be the predominant mechanism in films of thicknesses less than 30 nm. A frequency-dependent impedance is observed which is stronger in the thinner films, i.e. with lower metallic island concentrations. Hopping conduction and different equivalent circuit models that include the inter-island resistance, capacitance, bulk resistance, and a contact-injection pseudo-inductance are used to explain the frequency dependence of the impedance for films with different morphologies.