Remote Epitaxial Frustration Demonstrated in GdAuGe Films with Disordered Interlayers and Rotated Relationships

The creation of perfect, layered materials relies on a process called epitaxy, where crystalline films grow with precise atomic alignment, but achieving this over large areas and with different materials remains a significant challenge. Taehwan Jung, Nicholas Hagopian, and Anshu Sirohi, along with colleagues at the University of Wisconsin-Madison and Sandia and Pennsylvania State Universities, now demonstrate definitive evidence for a phenomenon called remote epitaxy, where the underlying substrate guides growth even when separated by intervening layers. The team’s work with GdAuGe films grown on silicon carbide reveals a disordered interlayer and a rotated crystalline relationship, signatures that conventional explanations cannot account for. These findings, which the researchers attribute to ‘epitaxial frustration’ arising from competing growth forces, not only confirm the existence of true remote epitaxy but also suggest a pathway to intentionally control and disrupt long-range order in layered materials, opening up new possibilities for advanced materials design.

Graphene Substrates Influence GdAuGe Film Growth

This study details the experimental and computational methods used to investigate the growth of GdAuGe films on various graphene/SiC substrates. The research aims to understand how different graphene layers, including buffer layers, epitaxially grown graphene, and hydrogen-intercalated graphene, influence the growth and properties of the resulting GdAuGe film. Graphene layers were grown on SiC substrates using thermal decomposition, varying annealing temperatures and times to create different graphene structures, which were then characterized using Raman spectroscopy and atomic force microscopy. GdAuGe films were then grown on these prepared substrates using molecular beam epitaxy, beginning with a 5 nanometer seed layer at room temperature to promote wetting, followed by continued growth to approximately 20 nanometers.

Elemental fluxes during growth were carefully monitored using Rutherford backscattering spectrometry, and surface analysis techniques, including in-situ scanning tunneling microscopy and x-ray photoemission spectroscopy, provided atomic-scale imaging and elemental composition analysis. Ex-situ x-ray diffraction, scanning transmission electron microscopy, and atomic force microscopy revealed the crystal structure, orientation, and surface topography of the films. Computational modeling using density functional theory complemented the experimental work, employing the QUANTUM ESPRESSO package with appropriate basis sets and exchange-correlation functionals to simulate the growth process on SiC with passivating hydrogen and either vacuum or graphene layers. Careful attention was paid to cross-section preparation, calibration of elemental fluxes, and maintaining a high-vacuum environment to ensure accurate and reliable results.

Remote Epitaxy of GdAuGe on Graphene

This research introduces a novel approach to epitaxial growth, exploring remote epitaxy and its potential for creating high-quality heterostructures. Researchers successfully grew 20 nanometer thick GdAuGe films on graphene supported by 6H-SiC (0001) using molecular beam epitaxy, beginning with a 4 nanometer GdAuGe seed layer at room temperature to enhance wetting, followed by annealing and continued growth at 450°C. To approximate freestanding graphene, the team employed hydrogen-intercalated buffer graphene on SiC, effectively decoupling the graphene from the underlying substrate. Symmetric 2θ-ω x-ray diffraction measurements confirmed the high quality of the films, revealing sharp Kiessig fringes and expected 000L reflections without impurity phases.

Detailed characterization revealed a 30° in-plane rotation between the GdAuGe film and the SiC substrate when grown on buffer graphene, a rotation not observed with direct or van der Waals epitaxy, providing strong evidence for a unique remote epitaxial mechanism. In-situ x-ray photoemission spectroscopy and scanning tunneling microscopy demonstrated smoother wetting of GdAuGe on buffer graphene compared to epitaxial graphene. High-angle annular dark-field scanning transmission electron microscopy revealed a striking signature of remote frustration at the interface, with the first 2-3 atomic layers of GdAuGe highly disordered when grown on buffer and epitaxial graphene, in contrast to crystalline interfaces observed with direct epitaxy. Tracking the formation of these disordered layers as a function of annealing revealed two distinct transitions, with solid phase epitaxy initiating at approximately 200°C, suggesting the disordered interface arises from competing epitaxial constraints and interfacial reconstruction potentials.

Remote Epitaxy Reveals Interlayer Frustration

Scientists have demonstrated definitive evidence for remote epitaxy, where film growth is guided by a substrate hidden beneath an intervening layer. The work centers on growing GdAuGe films on silicon carbide covered with graphene, revealing the formation of a disordered interlayer only a few atomic layers thick at the GdAuGe/graphene interface, and a rotated epitaxial alignment between the grown film and the underlying silicon carbide substrate. These observations strongly suggest “remote epitaxial frustration,” a competition arising from the film attempting to align with both the remotely screened substrate and the graphene layer itself, confirmed by density functional theory calculations. Researchers found that decoupling the graphene from the silicon carbide through hydrogen intercalation suppresses the interfacial reconstruction and eliminates the disordered layer, further supporting the role of remote interactions.

Measurements indicate that the graphene-induced reconstruction of the silicon carbide substrate significantly contributes to the electrostatic potential influencing film growth. This work provides a new pathway to intentionally disrupt long-range order in films by designing surface potentials, potentially leading to novel interfacial structures and previously inaccessible heterostructures, and offers a route towards direct synthesis of twisted Moire heterostructures without exfoliation. The graphene layer passivates the surface, enhancing the robustness and designability of the atomic-scale structure.

Remote Epitaxy via Graphene Frustration

This research demonstrates compelling evidence for true remote epitaxy, where film growth is guided by a distant substrate through an intervening layer, in this case, graphene. Scientists observed a disordered atomic layer and a rotated alignment between the grown film and the underlying silicon carbide substrate, signatures that cannot be explained by alternative growth mechanisms. Density functional theory calculations confirm these observations arise from ‘remote epitaxial frustration’, a competition between different epitaxial influences, and highlight the role of graphene-induced substrate reconstructions in mediating these interactions. These findings not only strengthen the understanding of remote epitaxy but also reveal a pathway to intentionally disrupt long-range order in thin films. By tuning the surface potential through graphene layers, researchers can engineer unique interfacial structures and potentially synthesize heterostructures that are otherwise difficult to achieve.