We report the effect on SiO2/SiC interface state density near the conduction band of different Si:C ratios (0.12 to 0.55) used during the growth of n-4H and n-6H-SiC epitaxial layers. We also report the effect of a post-growth re-oxidation anneal on the interface state density for both n- and p-4H-SiC. The interface trap density near the conduction band and the effective oxide charge increase with increasing Si:C ratio for both polytypes; however, the n-4H polytype is found to have an order of magnitude higher interface trap density near the conduction band compared to n-6H-SiC. The effective oxide charge is also higher for n-4H polytype. The distribution of interface states for 4H-SiC (measured using n- and p-type material) is asymmetric, with a higher trap density near the conduction band. Post-growth annealing in wet O-2 at 950 degreesC increases the interface trap density near the conduction for n-4H-SiC.