Up-diffusion suppression in a power MOSFET

A method includes forming an ion-implanted capping layer in a first epitaxial layer disposed on a silicon substrate. The ion-implanted capping layer is doped with a second dopant of a same conductivity type as a first dopant in the silicon substrate. The second dopant has a lower diffusivity than the diffusivity of the first dopant. The ion-implanted capping layer has a thickness configured to contain up-diffusion of the first dopant from the silicon wafer in the first epitaxial layer in thermal processes for fabricating a vertical MOSFET device in the substrate. The ion-implanted capping layer is configured to limit up-diffusion of the first dopant from the silicon wafer through the ion-implanted capping layer into a second epitaxial layer such that a concentration of the first dopant in the second epitaxial layer is lower than a concentration of the first dopant in the first epitaxial layer.