On the Impact of Materials and Processes on Edge-contacted 2D Transition Metal Dichalcogenide Transistors

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2022

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Two-dimensional (2D) materials, such as semiconducting transition metal dichalcogenides (TMDs), have emerged as channel material candidates for aggressively scaled field-effect transistors (FETs). Unlike silicon, 2D TMDs are stable down to their monolayer form, which is merely three atoms thick. The chemically inert surface of TMDs, which is free of dangling bonds, minimizes charge carrier scattering at the different channel interfaces. Unfortunately, that same advantageous property results in an undesired van der Waals (vdW) tunnel barrier when source/drain metal contacts are deposited on the TMD surface. To make matters worse, TMD transistors suffer from strong Fermi-level pinning – a phenomenon that can cause a sizable Schottky barrier for carriers regardless of the contact metal work function. The vdW and Schottky barriers result in 2D FETs exhibiting relatively high contact resistance and thus poor on-current (ION).Researchers have recognized the inadequate metal contact performance in 2D transistors and exerted considerable effort to understand and alleviate that issue using a variety of approaches, including exploring different metals and inserting additional interfacial layers between the metal and TMD. While significant progress has been made in improving the metal-TMD interface quality, there is a more fundamental issue with the typical top-contact configuration. In a technologically viable 2D FET, the contact length (distance over which metal interfaces with the TMD) will be scaled to sub-20 nm, yet experimental demonstrations employ large contact lengths that sometimes reach the order of several microns. At these unrealistically large dimensions, unwanted effects such as current crowding that will degrade contact performance in scaled devices do not manifest. For proper scaling of the whole transistor, simply improving the performance of relatively large top contacts is not sufficient. The edge-contact configuration is a promising alternative to typical top contacts. In this configuration, the TMD is contacted via its reactive edges to render a pure lateral injection mode. This completely eliminates the vdW tunnel barrier because the metal is covalently bonded to the channel. More importantly, it enables maximum scalability since there is no vertical injection for current crowding to occur. However, forming edge contacts is more challenging due to the need for etching edges to the TMD crystal and realizing pure metal-TMD junctions without unwanted interlayers. The work in this dissertation includes investigations that utilize various metal contact materials, TMDs, and processing conditions to uncover the most suitable approach to fabricate edge-contacted TMD FETs. First, an extensive benchmarking exercise was undertaken to arrive at better ways to gauge the potential of 2D materials to fulfill the requirements of specific technology applications. The performance of reported devices with dissimilar structures and bias conditions was normalized by considering the impact of relative electric fields. The analysis indicated that 2D FETs show promise as high-performance (HP) transistors when properly scaled. The results also highlighted the high-temperature growth and subsequent transfer of 2D materials as the best route to feature them in back-end-of-line (BEOL) integrations. Overall, the targeted benchmarking study confirmed the potential of 2D materials to compete with various incumbent transistor technologies. More particularly, edge contacts fit perfectly as enablers in the HP and BEOL transistor categories given the contacts optimal scalability and compatibility with vertical integration. The second study addressed a gap in previously demonstrated edge-contact work. Edge contacts are a less-investigated contact scheme in general, but more specifically, nearly all reported edge-contacted FETs used MoS2 as the channel material, ignoring other promising TMDs like WS2 and WSe2. Important questions regarding how the different TMD will react and perform with edge contacts remained to be answered. Our work demonstrated one of the first edge-contacted WS2 and WSe2 transistors ever reported. Clean edge contacts were created with three distinct metals on each TMD by utilizing an in-situ Ar+ ion source that is integrated with an electron beam (e-beam) evaporation chamber. The ion source was used to etch the TMD selectively from the contact region followed immediately by deposition of the contact metal while remaining under ultrahigh vacuum. It was discovered that the tungsten-based TMDs exhibited a unique etching effect where residual tungsten remained under the contacts despite substrate over-etching. This distinction from Mo-based TMDs revealed critical differences in processing that must be accounted for in edge-contact schemes. The residual W in the contact regions contributed to an unanticipated polarity shift in the Ti-WS2 device and a consistent performance across the use of three distinct metals (Ti, Ni, and Pd) on WSe2. Consideration of this intriguing etching effect could be essential for tungsten-based, edge-contacted TMD transistors. After studying the influence of metal and TMD selection on edge-contact formation and performance, the influence of varying edge-contact etch conditions was explored. It was found that in-situ ion beam processing of Ni-monolayer MoS2 transistors significantly improved yield and on-state performance compared to ex-situ¬ processed devices. In particular, transistors with edge contacts formed using an in-situ Ar+/N2+ process had the highest ION both in a direct comparison of four transistors with different edge-contact formation conditions on the same channel and in a statistical analysis involving all transistors on a chip. The superior performance was attributed to nitrogen atoms passivating sulfur vacancy defects at the exposed edge and reducing carrier scattering at the interface. It was also found that the contamination of MoS2 edges in ex-situ processed devices formed using reactive ion etching (RIE) is nonreversible even with subsequent in-situ Ar+ cleaning after etching and prior to contact metal deposition. While the incorporation of nitrogen in in-situ processing of Ni-MoS2 edge-contacted transistors is beneficial, further investigation into the usefulness of this approach in other metal-TMD systems is required to extend the scope of this recommendation. The last presented study in this dissertation describes the development of a fabrication process for selective deposition of atomic-layer-deposited (ALD) TiN on TMDs. It is of great interest to investigate whether a nucleation-based metallization approach offers any advantage over typical e-beam evaporation for edge contacts. One suggested future direction is to use in-situ Ar+/N2+ ions to create ALD TiN edge contacts on MoTe2 (sulfur and selenium are highly reactive to Ti). This is only one promising metal-TMD combination that could be investigated among the broad range of exciting possibilities with the novel edge contact scheme. Overall, this dissertation work has generated several key findings related to edge-contacted TMDs for future transistor technologies. First, the role of the transition metal in the TMD in terms of edge-contact formation processing, where the heavier W-based TMDs result in residual tungsten under the etched contact region. Second, the impact of in-situ versus ex-situ formation of the edge contacts where the yield and performance of devices processed in situ was markedly better. This advantage of in-situ processing, particularly using nitrogen, was attributed to the passivation of etch-generated defects at the edges of the TMD. Finally, the cumulation of these results indicates that material selection and processing both contribute critically to the resultant structure, morphology, and performance of edge-contacted TMD transistors.

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Abuzaid, Hattan (2022). On the Impact of Materials and Processes on Edge-contacted 2D Transition Metal Dichalcogenide Transistors. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/25209.

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