Abstract
Two-dimensional transition metal dichalcogenides (2D TMDs) is one of the promising materials for future electronics since they have, not only superior characteristics, but also a versatility that conventional materials do not have with a few nanometer thickness. One of the prerequisites for applying these materials to device fabrication is to deposit an ultrathin film below 10 nm with excellent uniformity. However, TMD has quite a different surface chemistry and is fragile to external conditions compared to conventional materials. Thus, thin film deposition on 2D TMD with excellent uniformity using conventional deposition techniques is quite challenging. Currently, the most adequate deposition technique for sub-10 nm-thick film growth is atomic layer deposition (ALD). A thin film is formed on the surface by the reaction between chemical and surface species based on the self-limiting growth manner. Owing to its unique and superior growth characteristics, such as excellent uniformity and conformality, ALD is an essential deposition technique for nanoscale device fabrication. However, since 2D TMD has a lack of reaction sites on the surface, various studies have reported that ALD on 2D TMDs surfaces without any treatment showed an island growth mode or formation of clusters rather than continuous films. For this reason, recent studies have been focused on the deposition of an ultrathin film on 2D TMDs with excellent uniformity. For a decade, there have been various approaches to obtain uniform films on 2D TMDs using ALD. Among them, the authors focus on the most frequently researched methods and adsorption control of chemical species by modifying the process parameters or functionalization of new chemical species that can assist adsorption on the chemically inert 2D TMD surface. In this review, the overall research progress of ALD on 2D TMD will be discussed which would, in turn, open up new horizons in future nanoelectronics fabrication using 2D TMDs.
Highlights
Two-dimensional (2D) materials have been highlighted as promising materials that can exceed the current limitation of the three-dimensional (3D) bulk materials
Since 2D transition metal dichalcogenide (TMD) has a lack of reaction sites on the surface, various studies have reported that atomic layer deposition (ALD) on 2D TMDs surfaces without any treatment showed an island growth mode or formation of clusters rather than continuous films
Direct to indirect bandgap transition of ultrathin 2D TMDs depending on the number of layers12,13 and excellent mechanical durability14–16 is used for the purpose of highperformance optical or mechanical device fabrication
Summary
Two-dimensional (2D) materials have been highlighted as promising materials that can exceed the current limitation of the three-dimensional (3D) bulk materials. From the perspective of surface chemistry, a high level of defect densities on 2D TMD imply that a reaction with the following chemical species would be more favorable on the CVD-grown TMD than on the exfoliated one. A previous report investigated adsorption of various molecules on 2D MoS2.44 The authors calculated adsorption energies between various molecules (CO2, CO, O2, H2O, NO, N2, and H2) and basal plane or defects (S vacancy, S divacancy, Mo vacancy, one or two Mo atoms on S mono- and divacancy, and one or two S atoms on Mo monoand divacancy) of 2D MoS2 by using density functional theory (DFT) calculation They revealed that all the molecules have larger adsorption energy on defect sites than that of on the basal plane, and the adsorption energy varied depending on the surface species and the type of chemical species. Defects on 2D TMDs can play a role as highly reactive sites and trap sites for different molecules by exposure to chemical functional groups.
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