Engineering of metal-MoS2 contacts to overcome Fermi level pinning

Epitaxy of NiTe2 on WS2 for the p-Type Schottky Contact and Increased Photoresponse

“…Because these two devices were fabricated by the same process on the same WS 2 nanosheet, the different conduction polarities can be attributed to the different electrode contacts, rather than the different electron density levels in the channel materials. When the Au electrodes directly contact with the monolayer TMDCs, besides the defects generated by the bombardment of the metal atoms or clusters, many gap states can even result from the interaction of metal atoms with a 2D semiconductor. , These are the main origin of the FLP effect. ,− , As a result, the Fermi level of WS 2 is pinned far away from the valence band at the Au/WS 2 interface, which results in a lower potential barrier for electrons than for holes and hence an n-type FET [see the energy band diagrams in the Figures f (upper) and S3a,b in the Supporting Information]. By contrast, when the electrodes are deposited on the NiTe 2 layer, the WS 2 underneath retains its intrinsic status due to the protection of the upper NiTe 2 layers, which can avoid the FLP effect at the source and drain contacts.…”

“…When the Au electrodes directly contact with the monolayer TMDCs, besides the defects generated by the bombardment of the metal atoms or clusters, 33 many gap states can even result from the interaction of metal atoms with a 2D semiconductor. 42,43 These are the main origin of the FLP effect. 33,[41][42][43]56 As a result, the Fermi level of WS 2 is pinned far away from the valence band at the Au/WS 2 interface, which results in a lower potential barrier for electrons than for holes and hence an ntype FET [see the energy band diagrams in the Figures 4f (upper) and S3a,b in the Supporting Information].…”

“…42,43 These are the main origin of the FLP effect. 33,[41][42][43]56 As a result, the Fermi level of WS 2 is pinned far away from the valence band at the Au/WS 2 interface, which results in a lower potential barrier for electrons than for holes and hence an ntype FET [see the energy band diagrams in the Figures 4f (upper) and S3a,b in the Supporting Information]. By contrast, when the electrodes are deposited on the NiTe 2 layer, the WS 2 underneath retains its intrinsic status due to the protection of the upper NiTe 2 layers, which can avoid the FLP effect at the source and drain contacts.…”

“…This is ascribed to the Fermi level pinning (FLP) at the interface of TMDCs/metal electrodes as the result of the formation of the metal-induced gap states during the high-energy metal deposition processes. − Recently, Liu et al demonstrated that the interface chemical disorder and hence the FLP could be successfully eliminated by transferring metal films onto MoS 2 to construct the van der Waals (vdW) contact, due to which the device polarities were modulated from the n-type to p-type when the electrodes were transferred from Au to Pt . Layered semi-metals, such as NiTe 2 , VSe 2 , VTe 2 , NiSe, and so forth, not only possess excellent conductivity but also have the atomically flat surface free of dangling bonds, which are considered as the ideal vdW electrodes. ,, More importantly, these 2D semi-metals can be directly grown on TMDC layers via vdW epitaxy, which is expected to achieve a metal/semiconductor contact with higher quality and efficiency relative to the transfer routes. − Although it has been proved that the work performance of the TMDC-based devices could be significantly enhanced through improving the heterointerface quality with layered semi-metal electrodes, there are still no reports on the p-type polarity devices. − …”

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS2

“…Efficient carrier injection across the contact interface requires a strong orbital overlap between the contact and channel material, a low Schottky barrier height (SBH), and narrow tunnel barriers [33,34]. For MoS 2 , the states near the conduction and valence band edges are dominated by Mo-4d states [35][36][37][38][39][40][41]. The inherent van der Waals (vW) gap present in these systems limits the orbital overlap between the topcontact metal and these states.…”