CHARGE-TRANSPORT IN SILICON-CARBIDE - ATOMIC AND MACROSCOPIC EFFECTS

It is shown that charge transport in SiC ceramics includes atomic mech anisms as well as phenomena which depend on the microstructure of the material. Both aspects are revealed by the analysis of temperature-dep endent dc and ac measurements. The complex dielectric function (DF) of boron-doped SiC ceramics with various additives has been measured at frequencies from 5 Hz to 2 GHz and at temperatures between 100 and 330 K. In addition, the dc conductivity was measured between 40 and 220 K . A transport mechanism on an atomic scale determines the temperature dependence on the dc conductivity. At low temperatures 3D variable ran ge hopping between boron impurity states or point defects takes place whereas at higher temperatures Arrhenius-like carrier activation becom es dominant. The ac behavior depends on the dc conductivity, but it re flects phenomena on a larger microscopic scale as well. The real part of the DF has huge values of up to 10(4). Two polarization processes h ave been identified. The low-frequency process is related to a conduct ion current relaxation, i.e. to a partial interfacial polarization in conducting paths. The Barton-Nakajima-Namika relation holds, relating dc conductivity, relaxation time, and relaxator strength. On the other hand, the high-frequency process is attributed to Maxwell-Wagner-Stil ars interfacial polarization in crystalline SiC grains with a size of several mu m. (C) 1996 American Institute of Physics.