On the lattice parameters of silicon carbide

The thermal expansion coefficients of the hexagonal SiC polytypes 4H and 6H and with Al and N dopants have been determined for temperatures between 300 and 1770 K. Further, a set of the room temperature lattice parameters in dependence on doping with N, Al, and B has been obtained. Data for the thermal expansion were taken on a triple axis diffractometer for high energy x rays with a photon energy of 60 keV, which allows the use of large single crystals with a volume of at least 6x6x6 mm(3) without the need to consider absorption. The room temperature measurements for samples with different dopants have been performed on a four-circle diffractometer. The thermal expansion coefficients along the a- and c-directions, alpha(11) and alpha(33), increase from 3x10(-6) K(-1) at 300 K to 6x10(-6) K(-1) at 1750 K. It is found that alpha(11) and alpha(33) are isotropic within 10(7) K(-1). At high temperatures both coefficients for doped samples are similar to 0.2x10(-6) and 0.3x10(-6) K(-1) lower than for the undoped material.