CRITERIO DE ORIGEN IDENTIFICACIÓN DEL CRITERIO EN EL CERTIFICADO

1. Summary of Computational Studies

The first half of this chapter presented a systematic and rigorous study of the physical nature of metal-TMD interfaces. The electron injection efficiency of the interfaces is shown to be characterized by three key criteria—tunnel barrier, Schottky barrier, and orbital

overlap. In order to accurately capture each of those criteria, DFT simulations incorporating semiempirical vdW potential are employed for the first time for metal-1L-TMD interfaces, and optimized geometries, effective potential, band structures, PDOS, valence electron densities, and bond Mulliken populations of metal-1L-TMD contacts are calculated. We find that Ti and Mo are the best top-contact metals for monolayer and multilayer intrinsic MoS2

and are n-type contact metals. Pd is the best p-type top-contact metal for monolayer intrinsic WSe2 while W can achieve high-quality n-type top contacts with WSe2 due to the strong

orbital overlaps and vanishing of Schottky barriers. While none of the metals studied in this work indicate the capability of forming good p-type contacts to MoS2, from the basic

interface physics revealed in this study, materials with strong orbital overlaps with MoS2

have the potential to lead to such contacts. Such properties can possibly be found in molybdenum oxide compounds (MoOx).

It is also shown that edge-contacted configurations can improve the contact by lowering tunnel barriers and strengthening the orbital overlaps. With the right metal and certain contact area, in order to achieve the lowest contact resistance, it is desirable to combine edge contact with top contact for monolayer TMDs. It can be inferred that inducing of edge contacts can be more significant for multilayer 1L-TMDs. For more-than-ten-layer TMDs, it is necessary to ensure that all of the edges are contacted to the metal using the tilt deposition technique [282]. On the other hand, it is possible to increase the edge contact length for lower contact resistance, for example, by cutting 1L-TMD edges into jagged edges. The results obtained in this study not only reveal the types of metals and configurations that can be employed for achieving low contact resistance with MoS2 and WSe2, but also highlight

that the properties of contacts cannot be intuitively predicted by solely considering WF values (e.g., Au versus Pd; In versus Ti; Mo or W versus other metals).

Moreover, the significance of the developed framework, which features vdW interactions and bond Mulliken population analysis is apparent not only for contacts to various 2D materials, but also for understanding the nature of interfaces to a wide variety of 2D materials, which will be a key issue in optimizing the performance of all emerging 2D materials-based devices including the proposed concept of “all-2D devices and circuits”

[148] (where graphene is used as gate electrodes and interconnects, MoS2 and WSe2 are used

as channel materials in the FETs, and insulating h-BN is used as a gate dielectric). By combining our framework and transport simulations, quantitative values of contact resistances can be calculated in the future.

2. Experimental Review of Contact Resistances

Figure 73 gives the summary of contact resistances for 2D semiconductors found in the

literature. From an experimental point of view, the contact resistance depends mainly on three parameters: contact metal, 2D

contact

 and the number of layers. This makes comparing contact resistance values found in the literature difficult because available data sets often differ by more than one parameter. Although results obtained using different metals are available, it is difficult to draw clear conclusions as to which metal yields the best contact to any given material from this meta-analysis. We nonetheless indicate for each study the contact metal. Figure 73 shows the minimal RC values from several studies on MoS2 as a

function of the number of layers [5], [6], [42], [217], [218], [258], [283]–[287]. Despite the scatter in the data, there is a clear trend of decreasing RC with increasing thickness. This

comes as no surprise, since the larger band gap in thinner flakes (red dashed line in Figure

Figure 73. Contact resistance for 2D semiconductors.

Minimum contact resistance as a function of the number of atomic layers in several

studies on MoS2 as well as some other 2D semiconductors [5], [6], [42], [217], [218],

[258], [283]–[287].

3. Chapter Summary

Realizing good electrical contacts is a prerequisite to harness the full potential of two- dimensional semiconductors. The atomic-scale thickness and pristine surfaces of 2D materials make it difficult to reduce the contact resistance. New theoretical models and

experimental approaches better suited to the low-dimensionality of the semiconducting material need to be developed. Recent years have shown impressive progress towards solving this problem. Several routes towards high-quality electrical contacts have been identified, the most promising of which is the realization of “seamless” electrical contacts, in which “native” chemical bonds allow much easier charge transport, and thereby lower contact resistances. For example, metallic TMDs can be used as covalently bonded electrical contacts to semiconducting TMDs, or sp2 _{carbon-carbon covalent bonding is retained at the} graphene-GNR junctions. However, most of the results so far were obtained on graphene and MoS2. Material-specific properties such as the types of atoms and atomic defects can

strongly influence the electrical properties. In this respect, our understanding of these contacts is still very limited and more systematic studies are needed, particularly in other TMDs.

IV. Other Interfaces of 2D Materials