Method for making semiconductor device including a superlattice and enriched silicon 28 epitaxial layer

A method for making a semiconductor device may include forming a first single crystal silicon layer having a first percentage of silicon 28, and forming a superlattice above the first single crystal silicon layer. The superlattice may include a plurality of stacked groups of layers, with each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base silicon portions. The method may further include forming a second single crystal silicon layer above the superlattice having a second percentage of silicon 28 higher than the first percentage of silicon 28.