Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors

Abstract

Among various 2D materials, monolayer transition-metal dichalcogenide (mTMD) semiconductors with intrinsic band gaps (1-2 eV) are considered promising candidates for channel materials in next-generation transistors. Low-resistance metal contacts to mTMDs are crucial because currently they limit mTMD device performances. Hence, a comprehensive understanding of the atomistic nature of metal contacts to these 2D crystals is a fundamental challenge, which is not adequately addressed at present. In this paper, we report a systematic study of metal-mTMD contacts with different geometries (top contacts and edge contacts) by ab initio density-functional theory calculations, integrated with Mulliken population analysis and a semiempirical van der Waals dispersion potential model (which is critical for 2D materials and not well treated before). Particularly, In, Ti, Au, and Pd, contacts to monolayer MoS 2 and WSe 2 as well as Mo-MoS 2 and W-WSe 2 contacts are evaluated and categorized, based on their tunnel barriers, Schottky barriers, and orbital overlaps. Moreover, going beyond Schottky theory, new physics in such contact interfaces is revealed, such as the metallization of mTMDs and abnormal Fermi level pinning. Among the top contacts to MoS 2 , Ti and Mo show great potential to form favorable top contacts, which are both n-type contacts, while for top contacts to WSe 2 , W or Pd exhibits the most advantages as an n-or p-type contact, respectively. Moreover, we find that edge contacts can be highly advantageous compared to top contacts in terms of electron injection efficiency. Our formalism and the results provide guidelines that would be invaluable for designing novel 2D semiconductor devices.

Single-layerMoS2on Au(111): Band gap renormalization and substrate interaction

“…This value significantly underestimates the electronic gap of 2.8 eV determined by more sophisticated quasi-particle (GW) calculations [6]. Interestingly, calculations with the local density approximation (LDA) [44,45] or with another exchange-correlation functional (PW91) within the generalized gradient approximation (GGA) lead to slightly (≈ 0.2 eV) larger values [46] than the 1.58 eV calculated here. Band gap underestimation is a well-known shortcoming of the LDA and GGA approaches, which can be partially remedied by including a fraction of Hartree-Fock exchange, leading to the so-called hybrid functional formalism.…”

Tellurium: Fast Electrical and Atomic Transport along the Weak Interaction Direction

“…The band gap (calculated using the HSE functional) decreases as thickness increases, also following a linear relation with 1/n or 1/d, and becomes 0.4 eV for infinitelythick (bulk) Te, close to the experimental value 0.35 eV 28 . The VBM/CBM energy (calculated using the HSE functional) also decreases/increases linearly with 1/n or 1/d, as shown in 19 , which suggests a lower Schottky barrier for hole injection than for electron [29][30][31] . This could be one reason why Te is p-type 7 .…”

Influence of interface coupling on the electronic properties of theAu/MoS2junction

“…The triangular shaped structures result from the initial stages of growth where threedimensional triangular islands first nucleate and grow with a (111) orientation before eventually forming a continuous coverage as more Au is deposited [11,[15][16][17]. The interface is known to be atomically smooth and only weakly bonded [15,[17][18][19] such that it is possible to drag entire Au islands across the MoS 2 surface using an STM tip [20]. Thicker films exhibit a combination of hexagonal and irregular-shaped structures such as those seen on the surface of the 20 nm film in Fig.…”

“…Instead, we turn our attention to scattering at the Au/MoS 2 interface. Recent calculations predict a narrow tunneling barrier at the Au/MoS 2 interface [18,19] that would act to hinder transmission. If our previous conclusion is correct, we would expect transmission to decrease at a greater exponential rate as the Au/MoS 2 spacing steadily increases, consistent with the sudden drop in transmission at roughly 16 nm seen in Fig.…”