Disclosed is a semiconductor device and a method for fabricating the semiconductor device. The method for fabricating the semiconductor device comprises steps of: forming a side cliff in a substrate in accordance with a gate mask pattern, the side cliff being substantially vertical to a substrate surface; forming a dielectric layer on the substrate that comprises the side cliff; etching the dielectric layer to have the dielectric layer left only on the side cliff, as a dielectric wall; and burying the side cliff by a substrate growth, the burying is performed up to a level higher than the upper end of the dielectric wall.