Abstract
Layer-number-dependent performance of metal-semiconductor junctions (MSJs) with multilayered two-dimensional (2D) semiconductors has attracted increasing attention for their potential in ultrathin electronics and optoelectronics. However, the mechanism of the interaction and the resulting charge transfer/redistribution at the two kinds of interfaces in MSJ with multilayered 2D semiconductors, namely, the metal-semiconductor (M-S) and the semiconductor-semiconductor (S-S) interfaces, have not been well understood until now, although that is important for the overall Schottky barrier height and the energy-band-offset between different layers of the 2D semiconductors. Here, based on state-of-the-art density functional theory calculations, the mechanisms of bonding and asymmetric electron redistribution at the M-S and S-S interfaces of metal-bilayer MoS2 junctions are revealed. Multiple mechanisms collectively contribute to the electron redistribution at the two kinds of interfaces, and the dominant mechanism depends on both the dimensionality (2D vs 3D) and the work function of metal electrodes. For the M-S interface, the pushback effect and metal-induced gap states play a dominant role for MSJs with 3D metal, while the covalent-like quasi-bonding feature appears for MSJs with medium-work-function 2D metals, and charge transfer plays a main role for MSJs with 2D metals that have very large or small work functions. For the S-S interface, it inherits the electron-redistribution behavior of the M-S interface for MSJs with 2D metal, while opposite electron-redistribution appears in MSJs with 3D metal. These mechanisms provide general insights and new concepts to better understand and use MSJs with multilayered 2D semiconductors.
Highlights
The potential applications of two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDs) in electronics[1,2,3,4,5] and optoelectronics[6,7,8] have stimulated intensive research of their fundamental properties as well as the metal–semiconductor junctions (MSJs) based on them
The binding energies in MSJ with 3D metal or 2D metallic Mo2C(OH)[2] are stronger than typical van der Waals interaction, they are far from chemical bonding, and all the systems we studied belong to weak interactions
We have revealed the mechanisms of the electron density redistribution at the M–S and S–S interfaces of bilayer MoS2 contact to metal electrodes via physisorption
Summary
The potential applications of two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDs) in electronics[1,2,3,4,5] and optoelectronics[6,7,8] have stimulated intensive research of their fundamental properties as well as the metal–semiconductor junctions (MSJs) based on them. In contacting MoS2 to metal electrodes, there is an unexpectedly high electrical contact resistance, which reduces the field-effect mobility of charge carriers across the interface of MSJ and degrades device performance.[12] The non-ohmic behavior at scitation.org/journal/jcp metal–MoS2 interfaces indicates the existence of significant Schottky barrier height (SBH), which depends on the energy level alignment between isolated MoS2 and metal in the ideal Schottky–Mott limit.[13] Realistically, there is a serious deviation from the Schottky– Mott limit especially for MoS2 contacts to 3D metal electrodes due to the strong Fermi-level pinning (FLP) caused by the interface dipole formed due to the asymmetric charge redistribution at the metal–MoS2 interface, even for a high-quality interface without defects/disorder.[14] The charge density redistribution at the metal– MoS2 interface is collectively contributed by several factors, including (quasi-)chemical interaction, metal-induced gap states (MIGS), pushback effect that pushes electrons back to metal, and charge transfer induced by the tendency of MoS2 band edges aligning with the metal Fermi-level.[15]
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