Dielectric properties of aluminum-doped silcion carbide using chemical vapor deposition

Silicon carbide coatings have been grown onto graphite substrates by normal pressure chemical vapor deposition using methyltrichlorosilane (MTS, CH_sSiCl₃) and trimethylaluminum (TMA, AKCH₃)₃) as source precursors, H₂ as carrier gas and Ar as carrier and diluent gas in the horizontal cold-wall CVD reactor. The coatings phase and chemical composition were studied. The SiC coating consisted of β-SiC with very little free carbon. The Al doped coating was composed of massive β-SiC, a little amount of Al₄SiC₄ and certain free carbon. The dielectric properties of both coatings after removing excessive free carbon and SiO₂ were measured in a frequency range of 8.2-12.4 GHz, The results show that real part and imaginary part of permittivity of undoped SiC are lower than those of Al doped SiC coating. The reason is probably the SiC coatings dissolve a great deal of aluminum. So the charged defects and quasi-free electrons move in the response to the electric field, and a diffusion or polarization current results from the field propagation. The higher imaginary part (ε″) is due to the dielectrical relaxation.