Room temperature properties of electrical contacts to alumina composites containing silicon carbide whiskers

Abstract

The electrical properties of contacts to hot-pressed alumina composites having variable volume fractions of SiC whiskers (Vw) were investigated with impedance spectroscopy and current-mode atomic force microscopy at room temperature. Three different contact materials were studied: sputtered Pt, sputtered Ag, and conductive Ag paint. Electrode properties were characterized while fresh due to instability over long times (∼100 h). For percolated samples with Vw≥0.10, the electrical response of the electrodes is manifested in the complex impedance plane by a semicircle whose properties are rooted in Schottky barrier blocking at metal contacts to surface whiskers participating in percolated linkages. This process was modeled with a parallel resistor-capacitor (RcCc) equivalent circuit having a relaxation frequency ωc. The dependences of the specific contact resistance (ρs) and specific capacitance (Cs) on Vw were in agreement with the microlevel interpretation above. The three contact materials exhibited significantly different properties, indicating sensitivities to metal work function and metal-whisker interfacing. Both Rc and ωc have exponential dependences on dc bias (Vdc) which are related through the equivalent circuit model. The strengths of these exponentials and the degree of nonlinearity in I-Vdc curves increase with the fraction of Vdc dropping at the electrodes. The dependence of Cs on Vdc also indicated symmetrical Schottky blocking and was used to estimate charge carrier concentration in the whiskers as 1017–1019 cm−3 and a barrier height of 0.2–1.6 eV for sputtered Ag.