Structural, electrical and optical characteristics of commercial 4H and 6H p-SiC substrate crystals doped with Al were studied. Radial non-uniformity in extended defect distribution over the substrates was found to be a typical feature of the p-SiC substrates grown by modified Lely method. In the central region of the wafers a relatively uniform distribution of dislocations with a density less than 10(5) cm(-2) was observed. While in the peripheral regions of the crystals the dislocation density was found to exceed 10(6) cm(-2) and was highly non-uniform. Domain structure of the investigated crystals was confirmed. Ionization energy of the Al accepters were measured to be 0.18 eV. Some electrical transport and luminescence characteristics of the crystals are also presented.