Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Abstract

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out‐of‐plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Recent Progress in 1D Contacts for 2D‐Material‐Based Devices

“…[1] However, the use of 2DMs leads to large contact resistance (R C ) when they are in contact with metallic electrodes, since they form a metallic interface that depends on van der Waals (vdW) bonding, which induces the vdW gap and generates metalinduced gap states, ultimately resulting in Fermi-level pinning (FLP). [2,3] As a result, the electronic mobility of the metal-contacted 2DM based devices is found to be much lower than the theoretically expected value, thus preventing their quantum properties from being observed. Therefore, minimizing the R C of the devices using 2DMs represents the most important challenge for exploring the novel quantum electronic properties of the devices.…”

Layer-dependent Schottky contact at van der Waals interfaces: V-doped WSe2 on graphene

“…Doping with nonisovalent transition metals such as V 21 and Nb 30 or Re 18,31 and Mn 32 yielded the expected ptype or n-type behavior, respectively, but it was noted that dopant ionization energies are significantly higher in the monolayer limit due to strong defect state localization 33 . Moreover, for metal-TMDC contacts, Fermi level pinning because of metal-induced gap states (MIGS) at the interface or due to disorder-order induced gap states, which originate from vacancies or substitutional defects, reduce the tunability of the Schottky barrier height and results in high contact resistance, thus limiting device performance 24,27,28,[34][35][36][37] . Van der Waals semimetals like graphene or graphite greatly suppress the formation of MIGS owing to their low density of states at the Fermi level while at the same time reducing interface disorder through the absence of dangling bonds.…”

Two dimensional semiconducting materials for ultimately scaled transistors

“…The ion implantation technique has been widely used to highly dope the contact areas in Si transistors, which is beneficial to realize ultra-low contact resistance below 50 Ω μm ( Taylor et al., 2013 ; Wong et al., 2017 ). In contrast, direct deposition of bulk metal contacts onto the 2D plane generally introduces gap states that lead to the Fermi level pining (FLP) effect, usually leading to a large contact resistance in the magnitude of several kΩ·μm using TMDs as channel materials ( Durán Retamal et al., 2018 ; Liu et al., 2022 ). For this reason, various approaches have been developed to eliminate the FLP effect for low Schottky barrier height and contact resistance, such as the transfer of vdW bonded bulk metal, metallic or semi-metallic 2D materials-based electrodes, as well as the phase engineered seamless contacts ( Huang et al., 2020a ; Schulman et al., 2018 ).…”

“…Additionally, the carrier transport at the metal-2D materials interface is dominated by the covalent bonding in edge-contact, which is distinct from the vdW gap existed in the top-contacted devices. This is essential to eliminate the metal-induced gap states that are responsible for the uncontrollable Schottky barrier and Fermi-level pinning effect in 2D transistors, which could substantially improve the field-effect mobility ( Kim et al., 2017 ; Liu et al., 2022 ). Recently, Won Jong Yoo’s group comprehensively reviewed the 1D edge contacts for 2D material-based devices, in which state-of-the-art techniques for achieving the edge contacts, together with the charge transport and device applications of edge contacts are elaborately discussed ( Choi et al., 2022 ).…”