Abstract
Ultrathin films of manganese silicides on silicon are of relevance as a possible material system for building spintronics devices with silicon technology. In order to achieve insight into epitaxial growth of such films on Si001, total-energy calculations are presented using density-functional theory and the full-potential augmented plane wave plus local orbital method. For adsorption of a single Mn atom on Si001, we find that binding at the subsurface sites below the Si surface dimers is 0.9 eV stronger than on-surface adsorption. There is an energy barrier of only 0.3 eV for adsorbed Mn to go subsurface, and an energy barrier of 1.2 eV for the reverse process. From the calculated potential-energy surface for the Mn adatom, we conclude that the most stable site on the surface corresponds to the hollow site where Mn is placed between two Si surface dimers. For on-surface diffusion, both along and perpendicular to the Si dimer rows, the Mn atoms have to overcome energy barriers of 0.65 eV. For deposition of 0.5 monolayers ML or more, we find that the Si dimers of the Si001 surface are broken up, and a mixed MnSi layer becomes the energetically most favorable structure. For coverages above 1 ML, the lowest-energy structure changes to a full Mn subsurface layer, capped by a layer of Si adatoms. We identify this transition with the onset of Mn-silicide formation in an epitaxially stabilized CsCl-like crystal structure. Such MnSi films are found to have sizable magnetic moments at the Mn atoms near the surface and interface, and ferromagnetic coupling of the Mn magnetic moments within the layers. Layer-resolved electronic densities of state are presented that show a high degree of spin polarization at the Fermi level, up to 45% and 27% for films with two or three Si-Mn layers, respectively.
Highlights
There has been growing interest in magnetic materials in semiconductor technology
For adsorption of a single Mn atom on Si001͒, we find that binding at the subsurface sites below the Si surface dimers is ϳ0.9 eV stronger than on-surface adsorption
Fig. 1͒, as well as surface diffusion, is obtained from DFT calculations of the potential energy surfacePESof a Mn atom, whose position is denoted by EPITAXY OF Mn ON Si001͒: ADSORPTION
Summary
There has been growing interest in magnetic materials in semiconductor technology. From a technological point of view, it would be highly desirable to grow heterostructures made from a ferromagnetic material and silicon This goal can be achieved essentially in two ways, either by depositing a structurally well-defined thin film of a ferromagnetic metal on silicon, or by turning silicon into a magnetic semiconductor. Band structure theory predicts them to be magnetic half metals, i.e., ideally up to 100% spin polarization could be possible Both these properties make heterostructures of Heusler alloys on silicon promising for efficient spin injection. 205305-1 have been reported, with particular focus on Mn-doped Si as a possible ferromagnetic semiconductor.8 We extend these investigations to systematic studies of the potential energy surface for Mn diffusion, to higher coverages of Mn, and to MnSi thin films on Si001͒. A summary and an assessment of the applicability of Mn/ Si as a system for spintronics applications is attempted
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