Interstitial Mn in Si: half-metallic heterostructures studied by density-functional theory

Adding magnetic functionality to the most common semiconductor, Si, is in its infancy. So far, research on Mn-doped Si has concentrated on substitutional Mn (Mn$_{sub})$ as done for Mn-doped GaAs and Ge, although Mn$_{sub}$ impurities in Si are energetically less stable than interstitial Mn (Mn$_{int})$. In this work, we investigate the role of Mn$_{int}$ impurities for ferromagnetism in Si, and propose a novel type of heterostructures with Mn$_{int} \quad \delta $-doping. Using density-functional theory within the generalized gradient approximation, we show that Si-based heterostructures with 1/4 layer $\delta $-doping of Mn$_{int}$ are half-metallic. For Mn$_{int}$ concentrations of 1/2 or 1 layer, the $\delta $-doped heterostructures still display a high spin-polarization of conduction electrons, about 85{\%} and 60{\%}, respectively. The proposed heterostructures are more stable than previously assumed $\delta $-layers of Mn$_{sub}$. Contrary to wide-spread belief, the present study demonstrates that interstitial Mn can be utilized to tune the magnetic properties of Si, and thus provides a new clue for Si-based spintronics materials.