Techniques for Polytypic Transformations in Silicon Carbide

Abstract

Two main polytype transformations in silicon carbide, namely, 2H → 6H and 3C → 6H, have been studied by ab initio methods. It has been shown that the intermediate phases with trigonal symmetry P3m1 and monoclinic symmetry Cm make it much easier to move the close-packed layers in such transitions by breaking them up into separate stages. It has been found that these two polytype transformations proceed in completely different ways. The links being relocated noticeably tilted compared to their initial position at the transition 2H → 6H, which allows the compression of the SiC links in the plane (\(11\bar {2}0\)). The transition 3C → 6H is carried out through the formation of Si–Si and C–C auxiliary links, living for a short time and helping densely packed layers to swap places. As a result, the activation barrier of the transformation 2H → 6H (1.7 eV/atom) is significantly less than the activation barrier of the transformation 3C → 6H (3.6 eV/atom), which means that the second transition should occur at the temperatures by 750–800°C higher than the first one. The energy profiles of this polytypic transformations, as well as the geometry of all intermediate and transition phases have been calculated. It has been shown that all the transition states have monoclinic symmetry.