(Invited) Two-Dimensional Materials for Electronic Devices: Transition-Metal Dichalcogenides and Topological Insulators

Abstract

As scaling of electronic devices has reduced dimensions into the nanoscale regime, investigations towards intrinsically two-dimensional (2D) materials are being pursued. Examples of 2D materials under investigation are monolayers of graphite, transition-metal dichalocogenides (TMDs), tellurium, and 2D topological insulators (TIs). To enable practical applications however, 2D materials must outperform currently employed three-dimensional materials. Properties of interest are mobilities, device drive current, and dielectric response but these properties are not yet fully known and understood in 2D materials. We present a theoretical investigation of the dielectric response in transition-metal dichalcogenides. We determine a method to define and extract, from first principles, the dielectric constant of a two-dimensional material. We apply this method to calculate the low-frequency and high-frequency dielectric constant of semiconducting TMDs, both those in the hexagonal phase, e.g. MoS2, WSe2, as well as those in the tetragonal phase, e.g. ZrS2 and HfSe2. We find that the in-plane static dielectric constant for the TMDs in the tetragonal phase is very large, which will give rise to large polar-optical phonon scattering and low mobilities in tetragonal TMDs. [1] We also present a new strategy to deal with both the high concentration of defects found in 2D materials as well as the difficulty of making perfectly straight edges. The strategy proposes to make transistors out of 2D TIs since 2D TI materials host topologically protected edge states that are robust against perturbations. In fact, we show how 2D TI field-effect transistor performance is actually improved in the presence of defects. The high on-current, related to the high electron velocity in these devices gives 2D field-effect transistors a predicted performance that exceeds that of conventional field-effect transistors. [2, 3] [1] A. Laturia, M. L. Van de Put, and W. G. Vandenberghe, "Dielectric properties of hexagonal boron nitride and transition metal dichalcogenides: from monolayer to bulk," npj 2D Materials and Applications, vol. 2, no. 1, p. 6, 2018/03/08 2018. [2] W. G. Vandenberghe and M. V. Fischetti, "Imperfect two-dimensional topological insulator field-effect transistors," Nature Communications, vol. 8, 2017. [3] S. Tiwari, M. L. Van de Put, B. Sorée, and W. G. Vandenberghe, "Carrier Transport in a Two-Dimensional Topological Insulator Nanoribbon in the Presence of Vacancy Defects," in 2018 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2018, pp. 92-96: IEEE.