# Substrate-Driven Atomic Layer Deposition of High-\(\kappa\) Dielectrics on 2D Materials
## 2 Atomic Layer Deposition on 2D Materials
### Methods to Promote ALD Growth on 2D Materials
However, from the electrical point of view, poor interface quality between the seed layer and 2D materials can result in a high-density interface or near-interface traps, ultimately compromising the performances of the final device. On the other hand, the metal seed layer deposition has been found to be beneficial for protecting the structural and electrical properties of the 2D material from the following ALD process. In particular, recent reports on the ALD deposition of ZrO\({}_{2}\) on MoS\({}_{2}\)[38; 39] demonstrated that the Al seed layer reduces the oxidation effect of the 2D material by the co-reactant (H\({}_{2}\)O, O\({}_{3}\)) during the ALD process, mainly for the O\({}_{3}\)-based method. Moreover, the introduction of a seed layer limits the minimum achievable thickness of the high-\(\kappa\) film, and this can represent a serious concern for the downscaling of 2D material-based transistors. Chemical pretreatments of a 2D material surface, such as exposure to NO\({}_{2}\)[45], O\({}_{3}\)[33; 46; 47; 48] or O\({}_{2}\)-plasma [50; 51], and N\({}_{2}\)-plasma [52], introduce functional groups or dangling bonds, enabling the ALD nucleation and growth of high-\(\kappa\) dielectrics. These approaches allow, in principle, the growth of ultra-thin insulating films, but at the same time, they can introduce defects in the 2D layer, degrading its electronic properties. Aside from the seeding layer and chemical pretreatments, the ALD process conditions (e.g., the deposition temperature and the pulse and purge times) can play a key role in achieving uniform coverage of a 2D material's surface. ALD depositions of Al\({}_{2}\)O\({}_{3}\) or HfOare typically carried out in a temperature window from 200 to 300 \({}^{\circ}\)C, which ensures the optimal reaction of the precursors. However, lower deposition temperatures allow higher physisorption of the precursors on the graphene or TMD surface, improving ALD nucleation [32; 41; 53]. As an example, [PERSON] et al. [53] demonstrated improved coverage and uniformity of deposited films on graphene within a temperature window of 80-100 \({}^{\circ}\)C for both Al\({}_{2}\)O\({}_{3}\) and HfO\({}_{2}\) materials. In particular, optimized ALD coverage (up to 98%) was demonstrated using prolonged pulse times (>2 s) [53]. Recently, [PERSON] et al. [32] reported a systematic study on the thermal ALD of Al\({}_{2}\)O\({}_{3}\) on the surface of multilayers of TMDs (MoS\({}_{2}\), WS\({}_{2}\), and WSe\({}_{2}\)) using different deposition temperatures (see Figure 2). Inhomogeneous Al\({}_{2}\)O\({}_{3}\) layers (with a large density of pinholes) were obtained using the conventional temperatures of 200-250 \({}^{\circ}\)C, whereas uniform coverage was observed when lowering T\({}_{\text{dep}}\) to 150 \({}^{\circ}\)C. Furthermore, at the same deposition temperature, better uniformity of the deposited Al\({}_{2}\)O\({}_{3}\) (~10 nm) films was observed on WSe\({}_{2}\) and on WS\({}_{2}\) than on MoS\({}_{2}\). This was ascribed to the larger polarizability of the W-and Se-based TMDs, giving rise to a higher adsorption energy of the TMA precursor due to electrostatic interaction. A two-step ALD process with early low-temperature (80 \({}^{\circ}\)C) deposition of an ultrathin AlO\({}_{\text{x}}\) layer and a following deposition at a higher temperature (180 \({}^{\circ}\)C) was also recently employed to obtain a homogeneous Al\({}_{2}\)O\({}_{3}\) film with a <10 nm total thickness on MoS\({}_{2}\)[43]. The low temperature, however, can be responsible for the degradation of the dielectric quality due to the reduced reactivity of the ALD precursor. In these conditions, the insulating films are generally characterized by a lower density and, consequently, by a lower dielectric constant [58].