forming first and second superlattices adjacent a semiconductor layer and each comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base semiconductor monolayers defining a base semiconductor portion, and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions, the second superlattice having a greater thermal stability with respect to thermally induced migration of non-semiconductor atoms from positions within the second superlattice than thermally induced migration of non-semiconductor atoms from positions within the first superlattice; and

heating the first and second superlattices to cause non-semiconductor atoms from the first superlattice to migrate toward the at least one non-semiconductor monolayer of the second superlattice.