From the Buffer Layer to Graphene on Silicon Carbide: Exploring Morphologies by Computer Modeling

Abstract

Epitaxial graphene grown by thermal Si decomposition of Silicon Carbide appears in different morphological variants, depending on the production conditions: the strongly rugged buffer layer, retaining a considerable amount of sp 3 hybridized buffer layer, the softly corrugated graphene monolayer and the rather flat quasi free standing monolayer with sparse small pits pinned to localized electronic states. Therefore, graphene on SiC is not a single material, but a set of materials with different morphologies depending on the environmental conditions during the synthesis. In all cases the distortion from the ideal flat structure seem to follow to some extent specific symmetries, which appear to preserve some memory of the interaction with the SiC bulk, even in the cases in which the sheet is substantially decoupled from it. Defects bear interesting properties, e.g., localized hot spots of reactivity and localized electronic states with specific energy depending on their nature and morphology, while their possible symmetric location is an added value for applications. Therefore, being capable of controlling the morphology, concentration, symmetry and location of the defects would allow tailoring this material for specific applications. Based on ab initio calculations and simulations, we first describe in detail the morphology of the different systems, and subsequently, we formulate hypotheses on the relationship between morphology and the formation process. We finally suggest future simulation studies capable of revealing the still unclear steps. These should give indication on how to tune the environmental conditions to control the final morphology of the sample and specifically design this material.

Influence of Proximity to Supporting Substrate on van der Waals Epitaxy of Atomically Thin Graphene/Hexagonal Boron Nitride Heterostructures

“…Such a remote influence has been recently demonstrated for GaAs grown on graphene-covered GaAs . In EG, the closer proximity to the partially sp 3 -bonded IFL on the supporting SiC substrate results in an atomic corrugation in the SLG. , The relatively larger vertical displacement of C atoms in the SLG as compared to BLG is here responsible for an enhanced chemical reactivity. , Therefore, we assume, that during the vdWE on the corrugated and more reactive SLG, h-BN islands nucleate with a higher density, which in turn induce the formation of further layers via nucleation at grain boundaries, thereby forming multiple, disordered h-BN layers, as they are observed in the HRTEM images in Figure e and Figure S6. The lower growth rates and the smoother formation of the h-BN on BLG, which we observe in Figure d as well as Figure S5, is attributed to screening of the atomic corrugation in the IFL by two layers of graphene, resulting in a lower chemical reactivity and most likely higher desorption rates for B and N adatoms.…”

“…Strain and doping of the graphene on the terraces is governed by a graphene-like IFL, also known as the buffer layer, which is partially sp 3 -bonded to the SiC substrate. − Due to the covalent bonds to the SiC substrates, the graphene-like IFL is highly corrugated. , This vertical displacement of C atoms in the IFL successively induces a corrugation in the first layer of graphene, which is further attenuated for BLG due to the existence of an additional graphene layer. , Furthermore, charge transfer from donor-like states in the IFL to the graphene results in n-type doping of the SLG. , Additional layers of graphene screen this charge transfer, resulting in a lower carrier concentration and thereby a different doping-induced shift of the Fermi energy for BLG as compared to SLG. − This can be imaged via the CPD between the metallic tip and graphene by KPFM. Two different areas can be discerned in the CPD map shown in Figure b, which follows the surface morphology shown in Figure a.…”

“…35−37 Due to the covalent bonds to the SiC substrates, the graphenelike IFL is highly corrugated. 38,39 This vertical displacement of C atoms in the IFL successively induces a corrugation in the first layer of graphene, which is further attenuated for BLG due to the existence of an additional graphene layer. 39,40 Furthermore, charge transfer from donor-like states in the IFL to the graphene results in n-type doping of the SLG.…”

Engineering 3D Graphene-Based Materials: State of the Art and Perspectives

“…We now focus on graphene on SiC, to further explore this concept. It is important to observe that graphene on SiC is not a single material but includes different types of 2D carbons [106] that can be obtained with different procedures (see Figure 2a). Upon Si evaporation from the Si-rich surfaces with hexagonal symmetry, excess carbon produces in the first instance a hexagonal carbon buffer layer (BL) [107], covalently bound to the substrate, and partially sp 3 hybridized.…”

Graphene Transfer: A Physical Perspective

“…A note about terminology: in general, graphene does not explicitly form covalent bonds with its underlying substrates. Several covalently bound carbon morphologies are present during epitaxial growth of graphene on silicon carbide, but even in this case, the carbon overlayer is generally not considered graphene until no covalent bonds exist between it and the underlying "buffer layer" [15]. We will follow this terminology here, and only consider graphene to be a hexagonal two-dimensional form of carbon which does not explicitly form covalent bonds with the underlying substrate material.…”