Interfacial properties of all-epitaxial Fe–Ge/Ge heterostructures on Ge(111)

Abstract

The molecular-beam-epitaxy growth of Fe–Ge/Ge/Fe–Ge trilayers on Ge(111) wafers has been investigated as a function of three parameters: the Ge spacer coverage, the substrate temperature (TD) and the dynamic atomic hydrogen (H) exposure during the Ge spacer deposition. Morphology and crystal structure have been characterized in situ by means of scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron diffraction and ex situ with high-resolution transmission microscopy. Whatever the H flux, epitaxial growth of Ge spacer requires deposition temperature above ~220°C. At the earliest stages of Ge deposition, the surface periodicity of the bottom Fe1.9Ge epilayer changes from p(2×2) to 3×3R30° for deposition temperature above ~220°C, whatever the H dosing. It results from severe intermixing that modifies the stoichiometry of the whole Fe–Ge layer. However, this layer preserves its hexagonal B82 crystal structure and no additional Fe–Ge compounds formed at the interface. We found also that the H flux drastically modifies the Ge spacer growth mode for deposition temperature above ~220°C. In particular, the Ge morphology evolves from 3D islands without H supply to flat and continuous film for H partial pressure above 10–3Pa. Finally, the top Fe–Ge electrode crystallizes in the same structure as the bottom electrode, and the planar relations of the trilayer are: (111)Ge-wafer||(0001)bottom–Fe1.9Ge||(111)Ge-spacer||(0001)top–Fe1.9Ge and [−110]Ge-wafer||[11–20]bottom–Fe1.9Ge||[1–21]Ge-spacer||[11–20]top–Fe1.9Ge. These fully epitaxial Fe–Ge/germanium hybrid heterostructures with single crystal Ge layer of diamond structure appear therefore as promising candidates for semiconductor spintronics.