Engineering the Performance of 2D Transition Metal Dichalcogenide Nanotransistors through Quantum Transport Simulations

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are a class of stable, atomically-thin monolayer materials with unique mechanical and electronic properties, leading to several proposed applications in electronics and optoelectronics. This thesis presents two studies in which ab-initio numerical simulations based on the Non-Equilibrium Green's Function method were used to model the performance of these materials in photodetector and tunnel field-effect transistor (TFET) devices.

The first study presents the design and operation of a MoS2 photodetector, in which the superior electrostatic control provided by the atomically thin device channel allows for the design of a unique, `grounded-gate' device where the source and gate controls are permanently connected. This is done in order to introduce a rectification effect which suppresses the dark current to increase the device's sensitivity. Numerical simulations using an effective-mass approach to model the electronic states of the channel, along with an analytical model for photoconductivity, were used to explain the device operation and reproduce trends in the experimental data. The resulting experimentally-fabricated device shows a high sensitivity of 1.01 A/W and detectivity above 6e10 Jones. Using the simulations, it is then suggested that engineering the gate metal work function can lead to an additional increase in sensitivity by three orders of magnitude.

The second study presents the numerical design and performance analysis of a strained PtSe2 TFET which can deliver high ON-currents. Though they can provide the high level of electrostatic control required to achieve steep-switching, monolayer TMD TFETs typically do not have high ON-currents due to high bandgaps, high effective masses, and/or lack a direct path to facilitate band-to-band tunneling. However, these materials are highly flexible, and mechanical strain is able to modulate the electronic bandstructure of PtSe2 to an extent where it can show ideal properties for use in TFETs. Under biaxial tensile strain, its bandgap and effective mass can be reduced significantly, and a direct path for tunneling emerges. Simulated results show that a double-gated PtSe2 TFET device strained to +5% can drive an ON-current above 100 uA/um while maintaining an OFF-current below 1e-7 uA/um and a sub-thermionic subthreshold swing. This improvement comes at a reasonable cost of OFF-current degradation and minimal effect on the switching characteristics down to 10 nm channel lengths. These results present the flexibility of monolayer TMDs as a powerful tuning parameter towards their use in tunneling devices.