STRUCTURAL AND ELECTRONIC-PROPERTIES OF SIC POLYTYPES

To study the structural and electronic properties of SiC, we have perf ormed self-consistent pseudopotential calculations for three SiC struc tures, namely 6H, 8H and a supercell containing thick slabs of the wur tzite and cubic forms. The supercell calculations we performed with an d without including the structural relaxation of the wurtzite structur e. Our main findings are: (i) the -dipole set up at the stacking bound ary is not centred on the central bond but on the adjacent C atom, and the disturbance to the charge density ranges about 3.5 angstrom on ei ther side; (ii) the spontaneous polarization in SiC polytypes can be i nterpreted as a superposition of localized dipoles due to each of the stacking boundaries, and it is mainly due to the charge density redist ribution; (iii) the valence-band offset between the cubic and wurtzite forms, determined using the same supercell, is 0.13 eV, with the vale nce band edge of the wurtzite structure being higher in energy; (iv) f rom (i), one can easily explain the MAS-NMR data concerning the inequi valent Si and C sites and the structural relaxation of SiC polytypes; (v) the effect of the macroscopic electric fields on the relative stab ility of SiC polytypes is discussed and found to be negligible.