Atomic-scale origin of rotated epitaxy and its impact on spin-electronic properties at Co2FeSi0.5Al0.5/Ge interface

Controlling interfacial atomic structure via epitaxial growth is essential for the spin polarization in half-metal/semiconductor heterostructures. Here, we use density functional theory to analyze a ‘rotated’ epitaxial relationship at the Co2FeAl0.5Si0.5/Ge(111) interface, experimentally realized on films grown by molecular beam epitaxy. The calculations indicate that a 180°-rotated epitaxy exhibits the lowest formation energy due to the bonding configuration of the initial Co layer. In this configuration, interfacial Ge atoms occupy positions corresponding to Si and Al sites in the extended Co2FeAl0.5Si0.5 lattice. The lowest-energy atomic interface structure results in a bond angle change of the interfacial Ge dumbbell layer, which is confirmed by atomic-resolution electron microscopy. We also show that the interface structure of the observed 180°-rotated epitaxy reduces the formation energy and considerably increases the interfacial spin polarization, which is further enhanced by an energetically favorable Fe–Ge interface intermixing and selective Ge out-diffusion into the film. Furthermore, we demonstrate that the rotated epitaxy is preserved when Co2FeAl0.5Si0.5 is grown on Si, despite the significant interdiffusion, and that such epitaxy should be preserved at the Co2FeAl0.5Si0.5/GaAs(111) interface, confirming the structural origin of the rotated epitaxy between Co-based Heusler alloys and Ge, Si, and GaAs substrates.