Charge transport in silicon carbide: Atomic and microscopic effects

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Abstract
It is shown that charge transport in SiC ceramics includes atomic mechanisms as well as phenomena which depend on the microstructure of the material. Both aspects are revealed by the analysis of temperature-dependent 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 of the dc conductivity. At low temperatures 3D variable range hopping between boron impurity states or point defects takes place whereas at higher temperatures Arrhenius-like carrier activation becomes dominant. The ac behavior depends on the dc conductivity, but it reflects phenomena on a larger microscopic scale as well. The real part of the DF has huge values of up to 104. Two polarization processes have been identified. The low-frequency process is related to a conduction 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-Sillars interfacial polarization in crystalline SiC grains with a size of several μm.