Theoretical Study of Electronic Transport in Two-dimensional Materials

Theoretical methods are critical for evaluating two-dimensional materials as possible channel materials in future field-effect transistors (FETs). The majority of modeling attempts concentrate on electronic transport in ideal free-standing layers, ignoring the dielectric environment’s effect on transport characteristics. We study the carrier transport in two-dimensional (2D) transition metal dichalcogenides (TMDs) by extending our Monte-Carlo model for a free-standing monolayer to include the effect of dielectric environment on the 2D layer. The major effects of the dielectric environment are the dielectric screening and the scattering of electrons with the coupled (hybrid) optical-phonon/plasmon excitations that are present at the interfaces in a field-effect transistors with a channel consisting of a monolayer of a polar semiconductor (TMDs) with top and bottom gates in a double-gate geometry. We perform an extensive study of the electron mobility in supported and/or gated monolayer TMDs in the presence of dielectric environment, to study of the carrier mobility of six TMDs (MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2) with all TMDs assumed to be supported by SiO2 and with SiO2, HfO2, Al2O3, AlN, and hBN as gate insulators. The carrier mobility improves significantly when considering only the effect of dielectric screening by the free carriers and the surrounding dielectrics. However, these positive effects are countered by the detrimental effects due to the interface plasmon/phonon (IPP) scattering. The carrier mobility decreases significantly below its value in free-standing monolayers with increasing dielectric constant of the insulator(s). The carrier mobility decreases almost monotonically with increasing dielectric constant of the gate insulator, with two exceptions:

1. The beneficial effects of dielectric screening of the ‘out-of-plane’ field lines are seen for hBN, thanks to its relatively low ionic polarization and the high phonon frequencies resulting from the light weight of the B and N ions.
2. On the contrary, resonance effects among the optical phonons of the substrate, of the TMD layer, and of the top oxide result in a low carrier mobility when AlN and/or Al2O3 are taken as gate insulators. We also evaluate the temperature dependence, carrier density dependence and the dependence of the dielectric constant of the 2D layer on the carrier mobility in the presence of the dielectric environment. We then extend our model to evaluate the high-field characteristics and simulate a 2D material based field effect transistor (FET), considering the TMD as the channel. Despite the TMDs having a low electron mobility, in most cases, under the effect of dielectric environment. Though we see a comparable carrier mobility with hBN as gate- insulator, the comparatively low dielectric constant of hBN negates the scaling benefits of high-κ insulators. Unfortunately, the transport properties of the ideal stack of gate-insulator of HfO2 with SiO2 as substrate and the 2D TMD between them, remain disappointing.