A method of fabricating semiconductor device includes forming a recess having a substantially rectangular section and forming an oxide layer on sidewalls and an oxide layer on a bottom of the recess by anisotropic oxidation, wherein the oxide layer on the sidewalls is thinner than the oxide layer on the bottom of recess. The method further includes completely removing the oxide layer on the sidewalls and partially removing the oxide layer on the bottom of the recess. The method also includes performing an orientation selective wet etching on the recess using a remaining oxide layer of the recess as a stop layer to shape the sidewalls into a Σ shaped section. The method includes removing the remaining oxide layer using an isotropic wet etching.