Interface characteristics affecting electrical properties of Y-doped SiC

Liquid-phase sintered SiC, doped with 3 vol% AlN, Al_2OC, Y_3Al_5O_12, revealed a variation in electrical resistivity of more than five orders of magnitude (<10^2-10^7 Ω cm) upon slight variations in the sintering process. The materials were characterized using various transmission electron microscopy techniques such as high-resolution transmission electron microscopy (HRTEM), Fresnel fringe imaging, analytical electron microscopy, and electron holography. The main focus of this study was to verify whether there is a correlation between interface structure and electrical resistivity. Scanning electron microscopy (SEM) of polished and plasma-etched surfaces showed interface features similar to those observed in Si_3N_4 ceramics containing amorphous grain-boundary films. Such films are expected to act as an insulating barrier for electric current. However, in contrast to the SEM results, HRTEM of SiC grain boundaries revealed no intergranular film in any of the SiC materials studied. Elemental analysis (i.e., energy dispersive x-ray and electron energy loss spectroscopy) of these “clean” SiC interfaces showed the segregation of secondary phase elements at grain boundaries. Electron holography and the Fresnel fringe technique were used to determine the change in the mean inner potential across SiC interfaces, which could be associated with the spatial charge distribution of a double Schottky barrier. The height of the potential barrier correlates with the electrical resistivity recorded via impedance spectroscopy.

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