# Substrate-Driven Atomic Layer Deposition of High-\(\kappa\) Dielectrics on 2D Materials
## 3 Substrate-Driven ALD of High-\(\kappa\) Dielectrics on 2D Materials
### ALD on Exfoliated MoS\({}_{2}\) on Gold  
More recently, the direct thermal ALD of an ultra-thin and pinhole-free Al\({}_{2}\)O\({}_{3}\) layer has been demonstrated on monolayer (1 L) MoS\({}_{2}\) residing on a gold substrate [22]. During the last few years, several research groups have investigated mechanical exfoliation on gold as an effective way to separate large-area (cm\({}^{2}\)) 1 L MoS\({}_{2}\) membranes from bulk molybdenite crystals by exploiting the strong S-Au interactions [71; 72; 73]. Furthermore, the exfoliated membranes could be transferred from gold to insulating substrates, demonstrating electrical and optical properties comparable to the best-quality MoS\({}_{2}\) crystals [71; 74]. Recently, [PERSON] et al. [22] compared the thermal ALD of Al\({}_{2}\)O\({}_{3}\) on 1 L MoS\({}_{2}\) residing on gold and on 1 L MoS\({}_{2}\), transferred on an insulating Al\({}_{2}\)O\({}_{3}\) substrate. Using identical ALD conditions, inhomogeneous ALD coverage by Al\({}_{2}\)O\({}_{3}\) islands was obtained in the case of the 1 L MoS\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/Si (see Figure 10a). In contrast, pinhole-free Al\({}_{2}\)O\({}_{3}\) and a flat morphology were observed on the 1 L MoS\({}_{2}\)/Au system (see Figure 10b). The authors illustrated the two different growth modes in Figure 10c,d for ALD-Al\({}_{2}\)O\({}_{3}\) on 1 L MoS\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/Si and on 1 L MoS\({}_{2}\)/Au, respectively. They found that the ALD nucleation on 1 L MoS\({}_{2}\)/Au was favored from the early stages of the process, with the Al\({}_{2}\)O\({}_{3}\) surface coverage increasing from \(\sim\)50% (after only 10 ALD cycles) to \(>\)90% (after 40 cycles). Interestingly, less uniform Al\({}_{2}\)O\({}_{3}\) coverage was observed in the bilayer and multilayer MoS\({}_{2}\) regions. The close similarity of the observed behavior to the case of 1 L graphene/Cu suggested that the improved reactivity of Au-supported 1 L MoS\({}_{2}\) toward the ALD precursors could be attributed to the partial transparency of 1 L MoS\({}_{2}\) to the electrostatic field generated by charges at the interface with the gold substrate. Furthermore, the strong interaction between MoS\({}_{2}\) and Au was found to be responsible for the significant tensile strain and p-type doping of 1 L MoS\({}_{2}\), which were relaxed by increasing the number of MoS\({}_{2}\) layers [74]. Hence, these effects of the MoS\({}_{2}\)/Au interaction can also play a role in the enhanced nucleation in the early stages of ALD processes. Figure 8: AFM morphology of the as-grown epitaxial graphene surface (**a**) and after 10 (**b**), 40 (**c**), and 80 (**d**) ALD cycles for Al\({}_{2}\)O\({}_{3}\) deposition, as well as the percentage of the graphene surface covered by the deposited Al\({}_{2}\)O\({}_{3}\) as a function of the number of ALD cycles (**e**). Schematic representation of the evolution of the deposited Al\({}_{2}\)O\({}_{3}\) on epitaxial graphene with an increasing number of ALD cycles (**f**). Images adapted with permission from [27], copyright Elsevier, 2020. Figure 9: C-AFM morphology (**a**) and current map (**b**) acquired on a scratch of the deposited Al\({}_{2}\)O\({}_{3}\) (40 cycles) on EG. Line scans of the height (**c**), from which the dielectric thickness of \(\sim\)2.4 nm has been determined, and of the current (**d**), showing the insulating properties of the deposited film. Images adapted with permission from [27], copyright Elsevier, 2020. The thickness and the insulating properties of the uniform Al\({}_{2}\)O\({}_{3}\) layer on MoS\({}_{2}\)/Au were also evaluated by C-AFM measurements according the schematic in Figure 11a. A 3.6-nm thickness of the Al\({}_{2}\)O\({}_{3}\) on 1 L MoS\({}_{2}\) was evaluated from the morphological map (Figure 11b), whereas the good insulating behavior of the deposited high-\(\kappa\) material was confirmed by the current map in Figure 11c, collected at a bias of 3 V between the tip and the gold substrate.