Control of the Schottky Barrier and Contact Resistance at Metal–WSe₂ Interfaces by Polymeric Doping

Abstract

devices, which can accommodate various materials via stacking. With respect to future applications, WSe 2 exhibits strong potential to be used in field-effect transistors (FETs), photodetectors, light-emitting diodes, and solar cells. [5-9] The metal-semiconductor (MS) interface is a critical factor influencing the electronic performance of 2D WSe 2 devices in that it is strongly correlated to device polarity. [10] Thus, the Schottky barrier height (SBH) is also an important parameter of the MS interface. In principle, the SBH can be determined according to the Schottky-Mott rule as the difference between the metal work function and the conduction-band edge or valenceband edge for n-type or p-type transistors, respectively. [11,12] However, the SBH in an actual device deviates from the Schottky-Mott rule because of the interfacial energy states. [13] This phenomenon is known as Fermi-level pinning (FLP), and the extent

Fermi‐Level Pinning Free High‐Performance 2D CMOS Inverter Fabricated with Van Der Waals Bottom Contacts

“…Moreover, it is reported that the defects, unintentional doping and charge trap states are also induced during the direct metal deposition which causes FLP and degrades device performance. [12,[24][25][26] Our vdWs BC approach overcomes the FLP, and controls the polarity of the WSe 2 FETs by contact engineering. First, the transfer of a 2D material onto the pre-patterned electrode avoids the high-energy involved direct deposition process which leads to the generation of defects and residues in the crystal structure of 2D materials.…”

“…The demonstration of p-type intrinsic WSe 2 is more challenging and seldomly realized because of the FLP induced from interface states. [24][25][26] Herein, we test Au (5.31 eV) as the bottom electrodes, from which the fabricated device exhibits clear ambipolar behavior with an on-state hole current of approximately 1 µA µm −1 . Meanwhile, we found that the device presents Schottky contact, judging from the non-linear output curves (see Figure 2h).…”

“…Finally, a local Ti/Au (5/100 nm) top gate is formed.In contrast to our devices, the 2D devices with contact metal formed by direct physical evaporation are subject to the defects resulting from resist residues remaining from the subsequent lithography steps. Moreover, it is reported that the defects, unintentional doping and charge trap states are also induced during the direct metal deposition which causes FLP and degrades device performance [12,[24][25][26]. Our vdWs BC approach overcomes the FLP, and controls the polarity of the WSe 2 FETs by contact engineering.…”

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

“…[75] Much lower contact resistance has also been reported for transistors with their channels doped using dopants such as chloride molecules, [76] gold chlorides, [77] and polyvinyl alcohols. [56] For instance, Yang et al have reported low contact resistances of 0.7 and 0.5 kΩ µm measured for chloride molecular-doped WS 2 and MoS 2 with nickel contact, respectively. [76] Here, an additional mechanism is responsible for the reduced contact resistance.…”

“…[11] Weaker FLP was experimentally observed in WSe 2 with a relatively high pinning factor of 0.38. [56] This can be ascribed to weaker interactions with contact metals that would normally lead to the n-type or electron-dominated ambipolar behavior. [19,29,57] A high pinning factor of 0.3-0.5 has been suggested for monolayer WS 2 based on DFT calculations.…”