# Epitaxial Graphene on 4H-SiC (0001) as a Versatile Platform for Materials Growth: Mini-Review
## 2 Key Properties of Epitaxial Graphene as a Substrate for Materials Deposition  
Before delving into the discussion of issues related to deposition of metals and insulators on epitaxial graphene on 4H-SiC, we would like first to recall the peculiar properties of epitaxial graphene that make it appealing to the growth of diverse functional materials. For all cases of the materials growth, which will be considered at a later stage, the common feature is that we used the same type of substrate: monolayer epitaxial graphene (MLG) prepared by high-temperature (\(\sim\)1900 \({}^{\circ}\)C) thermal decomposition of the Si-face of nominally on-axis 4H-SiC (0001) substrates in Ar atmosphere [41]. The graphene formation process encompasses three successive stages: (i) initial Si sublimation that takes place in inductively heated graphite enclosure in highly controlled conditions (gas, pressure, time), (ii) C-rich 6\(\surd\)3 \(\times\) 6\(\surd\)3 R30\({}^{\circ}\) surface reconstruction of (0001) face of 4H-SiC (buffer layer formation), and (iii) final formation of topmost graphene layer above the buffer layer. The key stages behind MLG synthesis are illustrated in Figure 1.  
MLG quality control is carried by two independent methods: optical reflectance mapping [43] and micro-Raman spectroscopy. Typically, the grown samples are composed  
Figure 1: The experimental steps leading to the formation of monolayer epitaxial graphene on SiC. This scheme has been adapted with permission from [42]. Copyright 2021 The Authors. Published by American Chemical Society (ACS), Washington, United States. Further permissions related to the material excerpted should be directed to the ACS, Washington, United States. of 85% monolayer graphene (and even larger % for recently synthesized samples) and 15% bilayer graphene that is represented by small bilayer patches above the first graphene layer (Figure 2a). The presence of randomly distributed bi-layer graphene islands within MLG sample can be beneficial to provide extra nucleation sites during early growth stages. More information about the intrinsic properties of the epitaxial graphene on SiC can be extracted from the analysis of the Raman mapping data [42]. First, it was determined that the grown samples were defect-free, which was confirmed by the absence of defect-related Raman peaks (mainly \(D\) Raman modes, Figure 2b). Therefore, one may exclude defects (carbon vacancies or \(sp^{3}\) defects) as preferential nuclei. Second, the experimental points in \(2D\)-\(G\) space scattered primarily along the strain line with a slope of 2.4 (Figure 2c), which is indicative of \(n\)-doping and biaxial compression of the topmost graphene layer [43; 44]. Density functional theory (DFT) calculations showed that the charge transfer between SiC and graphene caused the shift of the Dirac cone (0.49 eV) below the Fermi level \(E_{\text{F}}\) (Figure 3), thereby corroborating experimental findings on SiC-substrate-induced \(n\)-type doping of graphene. In this regard, a comparison of the charge redistribution map of free-standing graphene with that of epitaxial graphene revealed a pronounced accumulation of negative charge on SiC-supported graphene sheet (Figure 4). As was shown by [PERSON] et al. [44], there is a direct correlation between the electron density of graphene and the adsorption energy of water molecule. More particularly, the downshift of the Dirac point below the Fermi level is responsible for the adsorption energy increasing from 127 to 210 meV (Figure 5). It was argued that this phenomenon can be beneficial for the ALD process underlying the growth of Al\({}_{2}\)O\({}_{3}\) layers using trimethylal aluminium (TMA) as the Al precursor and H\({}_{2}\)O as the co-reactant. Indeed, the better wettability of graphene by water, the larger number of nuclei for Al\({}_{2}\)O\({}_{3}\) formation. Figure 2: (**a**) The optical reflectance map confirming the prevalence of monolayer graphene on a \(30\times 30\) μm2 area. Bilayer patches are seen in yellow.