Ultra-thin transition-metal silicide films on Si: trends with chemical composition and substrate orientation

Ferromagnetic films of monosilicides, if grown epitaxially on Si, could be an interesting materials system for spin injection into silicon. While the natural P2$_1$3 structure of the compounds $M$Si ($M$=Mn,Fe,Co,Ni) is incompatible with Si surfaces, silicides with a CsCl-like crystal structure could be grown epitaxially. We study such films on Si(001) systematically by varying the transition metal species, and find ultrathin (2 or 3 monolayer) MnSi and CoSi films to be ferromagnetic, while FeSi and NiSi films turn out to be non-magnetic. For all films, capping by a Si layer is found to be energetically favorable. While MnSi shows a layered magnetic structure with sizable magnetic moments ($\sim 2\mu_B$) in the surface and interface layers, the CoSi films show homogeneous magnetization with magnetic moments of Co $\sim 0.5\mu_B$. Both findings are interpreted in terms of the electronic structure of the films. Comparing CsCl-like MnSi films on either Si(001) or Si(111), we find both to be thermodynamically stable with respect to bulk Mn, but metastable with respect to bulk P2$_1$3-MnSi. The increase in stability with film thickness is monotonic for Si(111), but passes through less stable intermediate structures at $<1$~ML Mn for Si(001). Hexagonal phases of Mn-silicides, which could conceivably be grown on Si(111) as well, are found to be energetically less favorable than the CsCl-like MnSi films proposed by us.