Synchrotron white beam topography studies of 2H SiC crystals

Synchrotron White Beam X-ray Topography (SWBXT) has been used to characterize 2H silicon carbide crystals grown at NASA Glenn Research Center by the chemical reduction of methyltrichlorosilane at 1400 °C. The crystals studied have the form of tapered needles around 0.5 millimeters long and had hexagonal cross-sections 0.2 millimeters in diameter. Transmission Laue patterns confirmed the wurtzite structure of the 2H crystal. SWBXT images recorded in transmission geometry from two different crystals revealed excellent crystal quality, as evidenced by the presence of Pendellosung fringes, a result of the dynamical diffraction processes occurring inside the crystal. Close examination of these topographs and others recorded with different diffraction vectors revealed no evidence for the presence of the axial screw dislocations reported by Setaka and Ejiri, based on their observations of spiral growth steps, and also observed, for example, in (α-Al₂O₃ and AlN whiskers. Nor was any evidence found for the presence of the `Eshelby Twist' associated with the presence of such dislocations. The absence of axial screw dislocations evidently indicates that the growth was not screw dislocation-assisted, and suggests that the stacking sequence was determined by the thermodynamic growth conditions. Isolated inclusions, with characteristic contrast, were also observed in both crystals.