3C-SiC was grown by step-free surface heteroepitaxy on 4H/6H-SiC on-axis wafers patterned with mesa structures. Under proper growth conditions, 3C-SiC films without stacking faults (SF's) or double positioning boundaries (DPB's) were achieved on many of the mesas. The growth rate of the 3C-SiC films with extended defects such as SF's and DPB's was considerably enhanced over SF/DPB-free 3C-SiC films. The experimental data, including atomic force microscope (AFM) studies of as-grown surfaces, indicate that defects act as preferred sites for nucleation of new 3C bilayers on the (111) growth surface. Larger growth rate discrepancies between mesas with and without SF defects are observed at higher 3C growth temperatures.