Theoretical investigation of a Mn-doped Si∕Ge heterostructure

We investigate, through ab initio density-functional theory calculations, the electronic and structural properties of neutral Mn impurities at tetrahedral interstitial and substitutional sites in both Si and Ge layers of a Si∕Ge heterostructure. We conclude that substitutional Mn at the Ge layers is more stable than interstitial Mn at the Si layers by approximately 0.45 eV, and we estimate an energy barrier of at least 1.12 eV to diffuse away from these most stable substitutional sites. Mn has a magnetic moment in the heterostructure that is similar to that in the bulk, and for the compressed Ge layer the Mn-Mn exchange interaction is always weakly antiferromagnetic. Varying the lattice constant of the substrate, the Mn-Mn ground state becomes ferromagnetic. This result opens up the possibility of manipulating the interaction between Mn impurities at Ge layers grown over a Si_1−xGe_x substrate by changing x.