Modification and Scaling of Metal Contacts to 2D Materials Using an In-Situ Argon Ion Beam
Graphene, the wonder material, has captured the spotlight of electronics researchers since its discovery. Even though it has proven to be unfit for use in digital transistors, one of graphene’s lasting contributions is that it ushered in a whole new world of two-dimensional (2D) materials. One sub-family of such 2D materials are transition metal dichalcogenides (TMDs), which comprise ~40 different materials with a range of electronic properties, including insulators, semiconductors, and conductors. Molybdenum disulfide (MoS2) is one of the most studied semiconducting TMDs in recent years because of its air stability, high effective mass and sizable bandgap ranging from 1.2 to 1.9 eV, which make it suitable for transistor applications. Challenges, however, still cloud the promise of unbounded applications for MoS2 and other 2D TMDs. One major difficulty is the formation of high-quality contacts as the performance of the 2D transistors heavily relies on carrier injection through the contact interfaces. Although some progress has been made in recent years, a robust, effective contact scheme is still lacking and keeps the materials from being a viable option for future nanoelectronics.
This dissertation presents systematic studies on the metal interface to 2D materials for both the typical top-contact geometry and a newly developed in-situ edge-contact geometry. First, in the top-contact geometry, two kinds of ion beam sources (broad and convergent) are introduced to create defects in the otherwise dangling bond-free surface of MoS2. Below a certain threshold, these generated defects are shown to promote more efficient carrier transport between the contact metal and MoS2. This ion beam modification approach decreases the contact resistance by 50% and doubles the corresponding current in the device. Second, a pure edge contact scheme is introduced, where an Ar ion beam is used in situ (with the metal deposition system) to etch the MoS2, creating an abrupt edge profile that interfaces with the deposited contact metal (without overlap of the metal on the MoS2). Edge contacts are able to withstand ultimate scaling, down to sub-5 nm, and experimental results from edge contacts with a 20 nm contact length (Lc) yield nominally the same performance as Lc = 60 nm edge contacts. Hence, the in-situ edge contact approach shows great promise for aggressively scaled transistor technology.
New observations on the contact scaling behavior of top contacts is also covered. The transfer length of a typical metal-2D contact is found to be much smaller than previously estimated by studying scaled contacts on the same 2D crystal grain. Other progress in forming contacts to ultrasensitive 2D materials is also presented, where the in-situ ion beam can expose air sensitive materials for deposition of metal contacts without breaking vacuum. Based on these findings, future work is proposed that includes 3D integration of 1D edge-contacted 2D FETs, all-ALD transistors, and metallic CNT contacts to 2D materials. These future projects will push the boundary of our understanding of scaled contact interfaces as well as the development of emergent technologies for future nanoelectronic devices.
2D metal interface
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