Epitaxial graphene-encapsulated surface reconstruction of Ge(110)

Understanding and engineering the properties of crystalline surfaces has been critical in achieving functional electronics at the nanoscale. Employing scanning tunneling microscopy, surface x-ray diffraction, and high-resolution x-ray reflectivity experiments, we present a thorough study of epitaxial graphene (EG)/Ge(110) and report a Ge(110) “6 × 2” reconstruction stabilized by the presence of epitaxial graphene unseen in group-IV semiconductor surfaces. X-ray studies reveal that graphene resides atop the surface reconstruction with a 0.34 nm van der Waals (vdW) gap and provides protection from ambient degradation.

Figure 1

XRR data of the as-grown (top) and annealed 6 × 2 (bottom) EG/Ge(110), corresponding electron density, and model structure. (a) High-resolution XRR data and model fit. The dashed line is the simulated XRR for an ideal bulk terminated Ge(110) surface. (b) The model fit derived electron density profile along the [110] direction for the EG/Ge(110) structure. In the background is shown a sideview of a ball-and-stick representation of the model with coverages consistent with the electron density profile.

Figure 2

(a) STM of annealed EG/Ge(110) showing the characteristic 6 × 2 reconstruction with lattice parameters of ${\mathbf{a}}_{\mathbf{s}}$ and ${\mathbf{b}}_{\mathbf{s}}$ underneath EG $\left(V=-1\mathrm{V},I=400\mathrm{pA}\right)$. The EG hexagonal basis vector lies along the Ge [001] direction. (b) Reciprocal space STM data displaying relative orientations of Ge(110) 6 × 2 in red and EG in brown.

Figure 3

In-plane SXRD surface characterization. (a) SXRD measured values of $|F_{h_S{k}_S}{|}^2$ in 2D reciprocal space indexed in $h_s$ and $k_s$ of the Ge(110) 6 × 2 basis. The measured values are proportional to the areas of the hollow circles. The statistical error (red) is indicated by the perimeter width of each circle. Bulk Ge(110) reflections are marked with an x (blue). (b) The contour map of the direct space Patterson function for the Ge(110) 6 × 2 surface from Eq. (6). All peaks with values greater than 1/4 the origin peak are shown.