Breakdown of Impurity Al in SiC polytypes

In this work, we have studied I-V characteristics of Al breakdown in 6H-, 4H- and 15R-SiC in electrical field. As a result there obtained the next original data: I) decreasing dependence of breakdown field due to the concentration increase in the range of N(a) - N(d) = 5x10(17)-10(19) cm(-3); 2) absence of low temperature breakdown when N(a) - N(d)< 10(17) cm(-3); 3) increasing of breakdown field while temperature declines from 77K to 4.2K; 4) at 300K the breakdown field decreases and the breakdown takes place in samples with the absence of low temperature breakdown; 5) gigantic enhancement of breakdown field at F parallel to C. 6) the theoretical analysis based on the theory of a zero radius potential supports the probability of breakdown field enhancement at F parallel to C.