NMR Spin Lattice Relaxation in NITROGEN-Doped 6H Silicon Carbide

ABSTRACTNuclear magnetic resonance (NMR) spin lattice relaxation is highly sensitive to the nature, amount, and homogeneity of unpaired-electron-containing impurities in silicon carbide and similar solids, and is a promising tool for the study of dopants and impurities. In nitrogen-doped 6H silicon carbide, 13C and 29Si NMR spin lattice relaxation is highly site-dependent. Not only do carbon sites relax much more rapidly than the corresponding silicon sites, but also there are unprecedented differences in relaxation efficiency among the different carbon (and silicon) sites, consistent with much higher unpaired electron density at the higher-symmetry (Types A and B) sites than at the lowest-symmetry (Type C) site. In contrast, all sites relax at equivalent rates in undoped samples and in commercial abrasive grade material with high levels of impurities, although there are large differences in relaxation efficiency among samples. The change in the nature of the relaxation process, from exponential in high-purity to stretched exponential in lower-purity samples, is apparently related to changes in unpaired-electron mobility.