A method for forming a semiconductor device includes forming a hard mask layer over a substrate comprising a semiconductor material of a first conductivity type, and forming a plurality of trenches in the hard mask layer and extending into the substrate. Each trench has at least one side wall and a bottom wall. The method further includes forming at least one barrier insulator layer along the at least one side wall and over the bottom wall of each trench, removing the at least one barrier insulator layer over the bottom wall of each trench, and filling the plurality of trenches with a semiconductor material of a second conductivity type.