The methods of X-ray topography and diffractometry have been applied to characterize the structure of epilayers grown on porous layers. Two geometrical configurations of defects determined to be stacking faults (SF) were revealed: i) with the images of triangular shape with the edge size 560 gm along the < 10-10> directions; ii) linear shape along the [11-20] direction. The sources of SFs are located within the epilayer and start from the epilayer / porous layer interface. We propose that the source of SFs is connected with graphitization of porous layer at the temperature of epitaxy.