# Epitaxial Graphene on 4H-SiC (0001) as a Versatile Platform for Materials Growth: Mini-Review
## 5 Atomic Layer Deposition of Insulators on Epitaxial Graphene  
Apart from the growth of pure metals on epitaxial graphene, the development of controllable and reliable synthesis technology of high-\(k\) dielectrics (for example, Al\({}_{2}\)O\({}_{3}\)) is of paramount practical importance. It is due to the multifaceted role they play in graphene-based devices as gate insulators in field-effect transistors, tunneling barriers, and protective layers [55; 56; 57; 58]. In this context, the integration of epitaxial graphene with insulators can bring this material closer to real electronic applications. In a series of three papers [44; 59; 60], we presented and critically discussed the atomic layer deposition technique as a promising approach to grow high-quality Al\({}_{2}\)O\({}_{3}\) layers of different thicknesses. It was revealed that for a deposition temperature of 250 \({}^{\circ}\)C, the transition from island-based growth to layer-by-layer growth mode occurs after 40 ALD cycles that correspond to \(\sim\)2.4 nm (Figure 13). Figure 12: Current transients recorded on epitaxial graphene electrodes during mercury [51] (**a**,**b**) (Copyright 2019 by the authors) and copper [52] (**c**,**d**) (Copyright 2020 by the authors) electrodeposition. (**b**,**d**) also encompass the comparison of the normalized current transient curves with theoretical curves determined by the Hills–Scharifiker methodology. Such a fast transition of the growth mode can be interpreted by the existence of a high density of nucleation centers on the epitaxial graphene surface during early growth stages, which is uncommon for other types of graphene, especially for free-standing exfoliated graphene. This experimental finding is reinforced by the current maps (at a bias Vtip = 1 V) measured by conductive-atomic force microscopy (C-AFM) on epitaxial graphene before and after deposition of Al2O3. The surface of the as-grown (pristine) epitaxial graphene on SiC demonstrates largely uniform conductivity over the scanned area, while the sample subjected to 40 ALD cycles exhibits close-to-zero current, indicating the formation of an insulating film. Obviously, epitaxial graphene on 4H-SiC makes it possible to form highly uniform Al2O3 thin films without pre-functionalized or seed layers [44]. A cross-sectional TEM image (Figure 14a) of the Al2O3/epitaxial graphene/SiC structure clearly shows uniform contrast over the whole amorphous Al2O3 layer, pointing out a uniform density of the material formed during seed-layer-free ALD. AFM measurements (Figure 14b,c) showed that the topography of the formed Al2O3 layer resembles the terracet-stepped morphology of pristine epitaxial graphene, indicating a small roughness of the ALD-grown film. It is interesting to note that the growth of Al2O3 on bilayer inclusions was also identified (Figure 14d,e). However, in comparison to the densely packed grains on monolayer epitaxial graphene regions, Al2O3 on bilayer graphene regions develops into larger grains with obvious separations. Such an effect can be ascribed to higher reactivity of monolayer graphene than that of bilayer or multilayer graphene. Additional theoretical work has been performed to fully understand the growth mechanism of Al2O3 on epitaxial graphene [44]. The initial idea was to link the adsorption energies of the precursor molecules (trimethyl aluminium, TMA, and water, H2O) and graphene doping levels. However, no sufficient credible evidence to prove that TMA adsorption depends on the Fermi level position in graphene was provided. Figure 13: (**a**) Surface coverage (in %) of ALD-deposited Al2O3 on epitaxial graphene/SiC versus a number of ALD cycles. (**b**) Dependences of the height of Al2O3 islands (left scale) and Al2O3 film thickness (right scale) on the number of ALD cycles [59]. Copyright 2020 Elsevier Ltd., Amsterdam, Netherlands. On the other hand, the adsorption energy of water is more sensitive to the doping of graphene and was found to increase with increasing the negative charge on graphene (Figure 5).