A method of forming a semiconductor device includes forming a trench in a semiconductor body; at least partially filling the trench with a filling material; introducing dopants into a portion of the filling material; and applying a first thermal processing to the semiconductor body to spread the dopants in the filling material along a vertical direction of the filling material by a diffusion process. The vertical doping profile of the dopants within the doped filling material is shaped during the first thermal processing. Additionally, the dopants are substantially confined to within the trench and substantially do not diffuse from the doped filling material into the semiconductor body during the first thermal processing. A second thermal processing is applied to the semiconductor body after the first thermal processing to cause diffusion of the dopants from the doped filling material into the semiconductor body adjoining the trench.