The ferromagnet Fe3Si forms Schottky contacts with semiconductors like Si, Ge and GaAs. A triple layer structure ferromagnet-semiconductor-ferromagnet may therefore be suitable for Schottky barrier tunneling transistors adding a spin sensitive functionality to integrated circuits. Such Fe3Si/Ge/Fe3Si thin film stacks were grown by a combination of molecular beam epitaxy (MBE) and solid phase epitaxy (Ge on Fe3Si). Solid phase epitaxy allows for overcoming principal difficulties of MBE in growing semiconductors on top of metals. The Ge films unexpectedly crystallize in the well oriented, layered tetragonal structure FeGe2 with space group P4mm. This kind of structure does not exist as a bulk material and is stabilized by solid phase epitaxy of Ge on Fe3Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.
The structures of the epitaxial Ge and Fe3Si films on GaAs substrates match well to the known structures of their bulk materials. However, when the Fe3Si film is used as a substrate for epitaxial growth of Ge, the influence of the Fe3Si structure on the growing epitaxial Ge film unexpectedly turns out to be stronger and ordering phenomena occur. These ordering phenomena are induced by the epitaxial growth and were not observed in bulk material up to now. First, molecular beam epitaxy was applied to achieve perfect Ge films on top of ferromagnetic Fe3Si layers. Recently, we utilized the method of solid-phase epitaxy of Ge in order to achieve a high crystallinity of the film and superior interface quality. However, the interdiffusion of Fe, Si, and Ge was not entirely prevented during the annealing process. Therefore the Ge film contained some amount of Fe and Si, leading to a shift of the x-ray diffraction peak of the Ge(Fe,Si) film and the formation of a superlattice-like structure inside the Ge(Fe,Si) film. The aim of the present work is the investigation of the structure of the thin Ge(Fe,Si) films.
According to energy dispersive x-ray spectroscopy the Ge(Fe,Si) film contains concentratios of 60 at. % Ge, 35 at. % Fe and 5 at. % Si. For further understanding of the structure we performed Z-contrast imaging (see Figure). The Z-contrast mode in the scanning transmission electron microscope (STEM) is an incoherent imaging method. A high angle annular dark field (HAADF) STEM micrograph exhibits Z-contrast: The intensity diffracted by an atomic column is IHAADF ~ Z1.7...2. Heavier atoms give brighter image contrast. In the Figure, we recognize the superstructure in the Ge(Fe,Si) film. The D03 structure of Fe3Si is known and can serve as a reference. On the basis of the Z-contrast of our HAADF micrographs obtained along the two crystal orientations  and , we are able to propose a structural model for the Ge(Fe,Si) film: It is the FeGe2 structure with space group P4mm shown on the left side of the Figure. Four unit cells are depicted for better correspondence with the Fe3Si lattice. The structural models of Fe3Si (below, given as a reference) and FeGe2 (above, our proposal) are drawn. The experimental micrographs (Exp.) are compared to the structural models and corresponding simulations (Sim.) giving an illustration of our proposal of the FeGe2 structure.
|1||Author||B. Jenichen , M. Hanke , S. Gaucher , A. Trampert , J. Herfort , H. Kirmse , B. Haas , E. Willinger , X. Huang , S. C. Erwin|
Ordered structure of FeGe2 formed during solid-phase epitaxy
|Source||Phys. Rev. Mater. , 2 , 051402 ( 2018 )|
: 10.1103/PhysRevMaterials.2.051402 |