To date, a wide variety of processes has been reported to promote ALD nucleation on 2D layered materials, which finally results in the deposition of uniform dielectric layers. Table 1 reports a selection of the literature results for different types of 2D materials, with the indication of the used surface activation for ALD, the deposited high-κ insulator, its thickness and uniformity, and eventual damage introduced in the 2D material by the ALD process.

From the historical point of view, most of these approaches have been developed for graphene [35,37,55,56], and many of them have been adapted to other layered materials, such as TMDs [36]. These processes can be grouped into two categories:

(i)

Predeposition of a seed-layer;

(ii)

Chemical pre-functionalization.

The seed layers can be of both an organic nature, such as self-assembled monolayers (SAM) [34,57] or spin-coated polymers layers [35,36,55], and an inorganic nature, such as sputtered or evaporated metal or metal oxide films [37,38,39,56] or SiO₂ nanoparticles [40]. They can be pre-deposited “ex situ” (i.e., outside the ALD reactor chamber) and also “in situ” (i.e., within the reactor), like in the case of low temperature H₂O-assisted dielectric seed layer growth on the surface of graphene [41,42].

The seed layer approach proved to be an efficient and widely used method to promote the subsequent ALD growth process on graphene and TMDs. Typically, the spin-coating, self-assembling, and physical deposition methods (e.g., evaporation) used to deposit seed layers result in limited damage to the 2D membranes. 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 ZrO₂ on MoS₂ [38,39] demonstrated that the Al seed layer reduces the oxidation effect of the 2D material by the co-reactant (H₂O, O₃) during the ALD process, mainly for the O₃-based method. Moreover, the introduction of a seed layer limits the minimum achievable thickness of the high-κ 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 NO₂ [45], O₃ [33,46,47,48] or O₂-plasma [50,51], and N₂-plasma [52], introduce functional groups or dangling bonds, enabling the ALD nucleation and growth of high-κ 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 Al₂O₃ or HfO₂ are typically carried out in a temperature window from 200 to 300 °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, Aria et al. [53] demonstrated improved coverage and uniformity of deposited films on graphene within a temperature window of 80–100 °C for both Al₂O₃ and HfO₂ materials. In particular, optimized ALD coverage (up to 98%) was demonstrated using prolonged pulse times (>2 s) [53].

Recently, Park et al. [32] reported a systematic study on the thermal ALD of Al₂O₃ on the surface of multilayers of TMDs (MoS₂, WS₂, and WSe₂) using different deposition temperatures (see Figure 2). Inhomogeneous Al₂O₃ layers (with a large density of pinholes) were obtained using the conventional temperatures of 200–250 °C, whereas uniform coverage was observed when lowering T_dep to 150 °C. Furthermore, at the same deposition temperature, better uniformity of the deposited Al₂O₃ (~10 nm) films was observed on WSe₂ and on WS₂ than on MoS₂. 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 °C) deposition of an ultra-thin AlO_x layer and a following deposition at a higher temperature (180 °C) was also recently employed to obtain a homogeneous Al₂O₃ film with a <10 nm total thickness on MoS₂ [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].

The quality of grown high-κ dielectrics and of their interface with the 2D material strongly influences the electrical performances of the final devices. The seed layer deposition, typically carried out ex situ, can introduce impurities and particles, which act as charge traps compromising the reliability of the high-κ dielectrics subsequently grown by ALD. Fallahazad et al. explained that the HfO₂ deposited on graphene by the evaporated Al seed layer presents a relatively low permittivity (ε = 5.5) and a low-quality interface, which is characterized by impurities and defects responsible for mobility degradation [37,59]. Alternatively, a method based on the in situ deposition of a metal oxide H₂O-assisted seed layer has been proposed [41,42]. This approach involves the physical adsorption (by Van Der Walls interactions) of a continuous layer of H₂O molecules, which acts as a site of nucleation for a seed layer of high-κ dielectrics at a low temperature (100 °C), followed by conventional deposition at 200–250°C. Fisichella et al. [42] demonstrated that this approach ensures the optimal electrical features of the deposited dielectric on graphene, with a favorable permittivity (ε = 7.45) and breakdown field (E_BD = 7.4 MV/cm). On the other hand, significant charge trapping at the Al₂O₃/graphene interface has been observed during the operation of top-gated field effect transistors (Figure 3a). In particular, the density of negative trapped charges up to ~4 × 10¹² cm⁻² was evaluated by capacitance voltage (C-V_TG) measurements when increasing the forward and backward gate bias sweeps (Figure 3b,c).

Additionally, high-κ layers grown on 2D materials pre-functionalized by reactive gas species are subjected to charge trapping phenomena both at the interface and inside the dielectrics. As an example, the HfO₂ top gate dielectric grown on a MoS₂ surface after UV-O₃ treatment [60] shows significant charge trapping, which can be ascribed to impurities and defects generated during the reactive gas exposure. Additionally, O₂-plasma pretreatments have been shown to have an effect on the interfacial properties and, consequently, the performance of the dielectric [51,52]. As an example, Zhou et al. [61] demonstrated that remote O₂-plasma is effective for the ALD growth of Al₂O₃ on graphene, but on the other hand, they found the formation of a defective interfacial layer, resulting in a low breakdown field for the oxide (1.2 MV/cm). As previously discussed, the temperature used during the ALD process plays an important role in the uniformity of the dielectric coverage. In particular, as demonstrated in [32,53,54], growth temperatures lower than 150 °C allow for achieving better uniformity, but at the same time, they result in a significant increase of trapping phenomena and, consequently, a degradation of the passivation degree [60,62].