Atomic and electronic structures of p-type dopants in 4H-SiC

Using hybrid density functional calculation, we study the atomic and electronic structures of p-type dopants, B, Al and Ga, in 4H-SiC. For B, depending on the growth condition, it can occupy both Si and C sites. In contrast, Al and Ga on the C sites exhibit too high formation energy to exist in a significant amount. In 4H-SiC, there exist two types of Si sites in wurtzite-like and zincblende-like local coordination, respectively. Our calculations suggest that the dopant atoms have negligible preference occupying the two sites. In neutral charge state, all the dopants exhibit significant distortions from the structure in the negatively charged state. For most cases, our calculations yield three distorted structures, in which the most stable one has the dopant atom displaced along its bond with one of the surrounding equatorial Si or C atoms, lowering the C_3v symmetry to C_s symmetry (i.e., a mirror symmetry only). Among the three dopant elements, Al on Si sites exhibits overall the lowest formation energy and the shallowest acceptor level. Nevertheless, it is not a hydrogenic dopant with the acceptor level 0.12 eV above the valence band maximum based on calculation using a 400-atom supercell. Its corresponding defect state exhibits apparent localization along the 0001 direction, but it is relatively delocalized in the (0001) plane.