Boron forming a deep P+ layer within a semiconductor substrate upwardly diffuses during subsequent heat treatment operations such as annealing. A method for retarding this upward diffusion of boron includes implanting nitrogen to form a nitrogen barrier layer near the upper boundary of the P+ layer and well below transistor source/drain regions. One embodiment includes a lightly doped epitaxial layer formed upon an underlying P+ substrate. In another embodiment, a deep boron implant forms a P+ layer within a P− substrate, and affords many of the advantages of an epitaxial layer without actually requiring such an epitaxial layer. The nitrogen implant is performed at a preferred energy of 1-3 MeV to form the implanted nitrogen barrier layer at a depth in the range of 1-5 microns. Oxygen may also be implanted to form a diffusion barrier layer to retard the upward diffusion of arsenic or phosphorus forming a deep N+ layer.