Remote epitaxial frustration

TL;DR

The study addresses whether remote epitaxy truly governs film registry in graphene-covered substrates, distinguishing it from pinhole or direct epitaxy. It combines MBE growth of GdAuGe on buffer/epitaxial graphene/SiC, in situ STM/XPS, XRD, STEM, and DFT modeling to analyze competing surface potentials. Two signatures of remote frustration emerge: a few-monolayer disordered interfacial layer and a $30^\\circ$ rotated epitaxial domain, consistent with a competition among graphene, the remotely screened substrate, and graphene-induced reconstructions, described by $\varphi_{total} = \varphi_{gr} + T_s \varphi_{sub} + \varphi_{rec}$ with $T_s \approx \exp(-k_{TF} N \Delta z)$. The findings provide direct evidence for remote interactions and suggest tunable interfacial order through graphene layer count, enabling design of interfacial glassy or twisted Moiré structures with clean interfaces.

Abstract

Remote epitaxy relaxes the constraints of conventional epitaxy, to enable low defect density, chemically abrupt heterostructures and exfoliation of single crystalline membranes. However, definitive evidence for a true remote mechanism remains elusive because most experiments can be explained by alternative mechanism that are macroscopically indistinguishable from true remote epitaxy. Using GdAuGe films grown on graphene/SiC (0001), we present two signatures that cannot be explained by the leading alternatives to the remote mechanism: (1) a few atomic layer thick disordered interlayer at the GdAuGe/graphene interface and (2) a $30\degree$ rotated epitaxial relationship between the GdAuGe film and the SiC substrate. Density functional theory calculations indicate these signatures arise from remote epitaxial \textit{frustration}, a competition amongst epitaxy to the remotely screened substrate, to graphene, and to the graphene-induced interfacial reconstruction. Tuning the amplitudes and periodicities of these competing potentials provides new opportunities to intentionally disrupt long-range order.

Figures (5)

• Figure 1: Simulations of the remote frustration potentials. (a) Slab models 6H-SiC (0001), buffer graphene on SiC, epitaxial graphene on SiC, and free-standing graphene. (b) Calculated electrostatic potential $\varphi$ at 3 Å above each surface. (c) Linecuts through the Fourier transform $\hat{\varphi}(\vec{Q})$, showing single frequencies for bare SiC and freestanding graphene, and multiple peaks above buffer and epitaxial graphene on SiC. The $0\degree$ cuts in black are for $\vec{Q}\parallel [11\bar{2} 0]_{SiC}$. The $30\degree$ cuts in blue are for $\vec{Q}\parallel [10 \bar{1} 0]_{SiC}$.
• Figure 2: Lattice mismatch and strain relaxation for GdAuGe films on graphene/SiC (0001). (a) Bulk in-plane lattice parameters and expected alignments. The red unit cells outline the smallest commensurate site lattice matching conditions. Black outlines the primitive surface unit cells. (b) Measured reciprocal space maps (RSM) of the GdAuGe $10\bar{1}6$ reflection, for 20 nm thick GdAuGe grown on SiC, buffer graphene/SiC, epitaxial graphene/SiC, and H-intercalated graphene on SiC. (c,d) In-plane and out-of-plane lattice parameters of GdAuGe films determined from the RSM. Dotted line represents the bulk lattice parameter.
• Figure 3: Remote frustration of GdAuGe on graphene/SiC. (a) Cross-sectional HAADF STEM, showing a few atomic layer thick disordered layer at the graphene interface, for GdAuGe on epitaxial graphene (egr) and on buffer graphene (bgr) and a $30\degree$ rotated structure for GdAuGe on buffer-graphene. Gd (red), Au (yellow), Ge (blue). (b) X-ray azimuthal scan showing GdAuGe on buffer graphene is rotated in-plane by $30\degree$, and all the other samples are aligned with the SiC.
• Figure 4: Wetting and charge transfer on buffer vs epi graphene/SiC. (a,b) Filled states STM images of buffer and epi graphene/SiC (0001), respectively, showing the $(6\times 6)$ quasi periodicity. (c,d) STM images of 2 atomic layers of GdAuGe grown on buffer (filled states) and epi graphene (empty states), respectively, showing smoother wetting on buffer graphene. (e,f) In situ XPS of the C $1s$ core level for varying thicknesses of GdAuGe grown on buffer and epi graphene. $C_B$ component is from bulk Si-C bonding, $S_{1,2}$ from $sp^2-sp^3$ buffer graphene, and $S_{ML}$ from $sp^2$ graphene as identified following Ref. conrad2017structure. (g,h) Au $4f$ core level, showing Au to C charge transfer for GdAuGe on buffer graphene but not on epi graphene.
• Figure 5: Formation of the frustrated interface. (a) STEM images of a 4 nm thick GdAuGe seed layer on buffer graphene/SiC as a function of anneal temperature. The buffer graphene layer is noted by the white arrow. The corresponding RHEED patterns are shown in the insets, where red arrows mark the crystalline reflections. (b) X-ray diffraction around the $0004$ reflection. (c) Normalized RHEED intensity ($I_{diffracted}/I_{background}$) and $c$ lattice parameter from x-ray diffraction, as function of anneal temperature. The bulk lattice parameter is marked by the dotted line.