(Invited) Reduction of Contact Resistance through the Application of Cylindrical Contacts to WSe2
Authors: Lee Seungyun , Lee Dongryul , Kim Jihyun 
Published: December 2024

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are gaining attention as the next-generation semiconductor materials instead of silicon, owing to their atomic thickness and thickness-dependent energy bandgap. Moreover, because of their atomic thickness, TMDs show effective electrostatic gate modulation, thereby short channel effects can be suppressed. Among various TMDs, tungsten diselenide(WSe2) has an ambipolar conductivity which can be tuned to n- or p-type via various processing and shows excellent electrical properties and mechanical stability. However, achieving Ohmic contact in 2D material-metal junction is challenging due to high Schottky barrier and Fermi-level pinning. Therefore, research on the improvement of contact resistance is crucial to maximizing the full potential of 2D materials and utilizing their superior electrical properties. Edge contact, where the junction is formed on the edge of the 2D material layers, has been researched as an effective method to minimize the contact resistance in 2D materials. Unlike the traditional surface contact method, which results in high tunnel barrier and out-of-plane current flow, the edge contact method provides strong orbital overlap and allows in-plane current flow through a narrower tunnel barrier, offering advantages in terms of carrier transport at the contact interface [1]. Yang et al. demonstrated conventional edge contacts to MoS2 using hexagonal boron nitride (h-BN) to encapsulate the surface of the MoS2. This method ensures that current flows only between the edge of the MoS2 and metal [2]. However, this approach necessitates additional process that requires h-BN to completely cover the entire upper part of the contact area and cannot control the area of the edge contact.
In this work, edge contact using cylindrical contacts of WSe2 transistors is demonstrated. WSe2 flakes were fabricated through mechanical exfoliation and were dry-transferred on Si/SiO2 substrates. Spin-coating of electron-beam resist and electron-beam lithography (EBL) was performed to prepare the etch mask for cylindrical etching at the contact areas of WSe2. Reactive ion etching was conducted using 15 sccm of Ar, 15 sccm of SF6 with 20 W for 20 s. Spin-coating and EBL process was performed again to define the area for ultraviolet (UV)-ozone treatment and deposition of Pt/Au electrodes. UV-ozone treatment oxidizes the top few layers of WSe2 into WOx, resulting in the p-doping of the underlying WSe2 layer of the contact area. Pt/Au contact metal was deposited by electron-beam evaporation.
The fabricated WSe2 FETs shows p-type dominant characteristic and Ohmic behavior, as shown in Fig. 1. Cylindrical contact and UV-ozone treatment process for WSe2 FETs enabled low contact resistance (4 kΩ·μm), high field-effect mobility (149 cm2/V·s) and high on-off ratio (>107). As the perimeter of the cross-sectional circle of the etched cylinder increases, the area of the edge contact becomes larger. The WSe2 FETs with cylindrical contacts exhibited decrease in the contact resistance and increase in the field-effect mobility as the ratio of the etched perimeter to the top contact area increased (Table 1). P-type WSe2 FETs with low contact resistance and high field-effect mobility were demonstrated using cylindrical contacts. With this fabrication, edge contact area can be controlled with the variation of the etched perimeter and electron transport can be improved using both top contact and edge contact.
Reference:
[1] J. Kang, W. Liu, D. Sarkar, D. Jena, K. Banerjee, Phys. Rev. X 2014,4, 1.
[2] Z. Yang, C. Kim, K. Y. Lee, M. Lee, S. Appalakondaiah, C.-H. Ra, K.Watanabe, T. Taniguchi, K. Cho, E. Hwang, J. Hone, W. J. Yoo,Adv.Mater.2019,31, 1808231

Figure 1