Semimetal contacts to monolayer semiconductor: weak metalization as an effective mechanism to Schottky barrier lowering

Recent experiment has uncovered semimetal bismuth (Bi) as an excellent electrical contact to monolayer MoS2 with ultralow contact resistance. The contact physics of the broader semimetal/monolayer-semiconductor family beyond Bi/MoS2, however, remains largely unexplored thus far. Here we perform a comprehensive first-principle density functional theory investigation on the electrical contact properties between six archetypal two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors, i.e. MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, and two representative types of semimetals, Bi and antimony (Sb). As Bi and Sb work functions energetically aligns well with the TMDC conduction band edge, Ohmic or nearly-Ohmic n-type contacts are prevalent. The interlayer distance of semimetal/TMDC contacts are significantly larger than that of the metal/TMDC counterparts, which results in only weak metalization of TMDC upon contact formation. Intriguingly, such weak metalization generates semimetal-induced gap states (SMIGSs) that extends below the conduction band minimum, thus offering an effective mechanism to reduce or eliminate the n-type Schottky barrier height (SBH) while still preserving the electronic structures of 2D TMDC. A modified Schottky-Mott rule that takes into account SMIGS, interface dipole potential, and Fermi level shifting is proposed, which provides an improved agreement with the density functional theory-simulated SBH. We further show that the tunneling-specific resistivity of Sb/TMDC contacts are generally lower than the Bi counterparts, thus indicating a better charge injection efficiency can be achieved through Sb contacts. Our findings reveal the promising potential of Bi and Sb as excellent companion electrode materials for advancing 2D semiconductor device technology.

Automated summary

Recent experiments have demonstrated that semimetal bismuth (Bi) is an excellent electrical contact to monolayer MoS2 with ultralow contact resistance. However, the contact physics of other semimetal/monolayer-semiconductor systems remains largely unexplored. In this study, we investigate the electrical contact properties between six two-dimensional transition metal dichalcogenides (TMDCs) and two representative semimetals, Bi and antimony (Sb), using density functional theory. We find that weak metalization occurs at the semimetal/TMDC interfaces, generating semimetal-induced gap states (SMIGSs) below the conduction band minimum and effectively reducing the n-type Schottky barrier height. We propose a modified Schottky-Mott rule that considers SMIGS, interface dipole potential, and Fermi level shifting, which improves the agreement with density functional theory simulations of the barrier height. We also show that Sb contacts have lower tunneling-specific resistivity than Bi contacts, indicating better charge injection efficiency. Our findings suggest that Bi and Sb are promising companion electrode materials for advancing 2D semiconductor device technology.