The structure of 6H-SiC layers grown by sublimation on (0001) dislocation- and damage-free Lely platelets has been investigated by high-resolution transmission electron microscopy (HRTEM). The interface region contained multiple groups of stacking faults consisting of I to 3 mis-stacked Si-C bilayers per group. The total number of mis-stacked bilayers was between 5 and 100 depending on growth conditions. The stacking sequences are consistent with forced two-dimensional nucleation and indicate polytype instability at high growth temperatures.