# Substrate-Driven Atomic Layer Deposition of High-\(\kappa\) Dielectrics on 2D Materials
## 1 Introduction  
Two-dimensional (2D) materials such as semi-metallic graphene and the semiconducting transition metal dichogenides (TMDs) have been the object of intensive research investigations in the last 15 years due to their wide range of potential applications in electronics, optoelectronics, and sensing. As an example, the excellent carrier mobility (10\({}^{3}\)-10\({}^{5}\) cm\({}^{2}\)/Vs) [1] and micrometer electron mean free path [2] of graphene have been exploited in the demonstration of high-frequency (100-300 GHz) transistors [3, 4], for which the low on/off current ratio (due to the lack of an energy bandgap) represents a minor issue. On the other hand, semiconducting TMDs (including MoS\({}_{2}\), WS\({}_{2}\), MoSe\({}_{2}\), and WSe\({}_{2}\)) are currently regarded as the most suitable channel materials for ultra-thin body field effect transistors to be employed in next-generation logic or switching applications [5, 6]. As an example, transistors with high on/off current ratios (10\({}^{6}\)-10\({}^{8}\)) and low subthreshold swings (SS \(\approx\) 70 meV/ decade) [7, 8] have been fabricated using monolayer (1 L) MoS\({}_{2}\) (with a direct bandgap of 1.8 eV) or multilayer MoS\({}_{2}\) (with an indirect bandgap of 1.2 eV) [9, 10]. Practical implementation of electronic devices based on 2D materials requires their combination with high-\(\kappa\) dielectrics, such as Al\({}_{2}\)O\({}_{3}\), HfO\({}_{2}\), and ZrO\({}_{2}\). In the case of top-gated field effect transistors with graphene or MoS\({}_{2}\) channels, high-\(\kappa\) gate insulators are necessary for efficient electrostatic control of the lateral source-to-drain current while minimizing the gate leakage phenomena [11, 12, 13]. Furthermore, the encapsulation of the monolayer or multilayers of MoS\({}_{2}\) with high-\(\kappa\) dielectrics has been demonstrated to behighly beneficial in the improvement of the field effect mobility up to 200-500 cm\({}^{2}\) V\({}^{-1}\)s\({}^{-1}\) due to the efficient reduction of charged impurity scattering [7; 8; 14]. Obviously, with the scaling down process which invests the electronic field, most of these applications require high-\(\kappa\) dielectrics in the form of uniform and ultra-thin layers. Aside from conventional field effect transistor structures, novel device concepts and architectures also require the integration of high-quality and ultra-thin insulators on the surfaces of 2D materials [15; 16]. As an example, the recently investigated graphene-base hot electron transistors (GBHETs) for THz electronics require the integration of an ultra-thin insulator on graphene working as a base collector tunnel barrier for vertical transport [17; 18; 19]. Furthermore, graphene sensors may involve high-\(\kappa\) deposition on graphene as a functional layer toward specific chemical and biological molecules [20]. In this context, the atomic layer deposition (ALD) is the most appropriate technique for the growth of ultra-thin layers, with accurate control of the thickness and surface coverage. However, the surface of pristine 2D materials is chemically inert toward the chemical reactions involved in the ALD process, typically resulting in inhomogeneous coverage by the deposited films. To overcome this inherent issue, several approaches have been investigated in order to promote ALD growth on 2D crystals, including the predeposition of seed layers and surface activation via chemical treatments. As a matter of fact, these methods present advantages and disadvantages, ultimately affecting the final performance of graphene or MoS\({}_{2}\) devices. Hence, the direct (i.e., pre-functionalization and seed layer-free) ALD growth of high-\(\kappa\) dielectrics on the pristine surface of 2D materials still remains highly desirable and has been the object of investigation in recent years.