Modeling and Simulation of Multilayer MoS2 Schottky Barrier Field-Effect Transistors

Abstract

As a member of the transition metal dichalcogenide (TMDCs) family, Molybdenum Disulfide (MoS2) exhibits a layered 2D structure with exceptional electronic and optoelectronic properties, making MoS2 a promising candidate for next-generation nano-devices. However, despite numerous efforts in synthesis, fabrication process, and device structure for few-layer MoS2. A key challenge that remains is the Fermi-pinning effect. Due to defects distributed at the MoS2/metal interface, the Schottky-mott rule fails to predict the Schottky barrier height based on the Fermi-level difference between MoS2 and the metal. Instead, the Fermi level of the metal is pinned near the conduction band edge regardless of the work function of the metal used. This phenomenon results in an inevitable Schottky barrier, which must be recognized in device simulations.

In this thesis, the electrical and optoelectronic performance of multilayer MoS2 field-effect transistors is predicted and analyzed through simulation techniques. Drift-diffusion equations are employed to model electronic properties, utilizing finite element methods (FEM) to solve the corresponding partial differential equations. FEM discretizes space and divides the solution domain into finite elements. For optoelectronic simulations, When solving Maxwell's equation for optical absorption and carrier generation rates. Finite-Difference Time Domain (FDTD) methods are applied, which discretize both space and time, representing fields as discrete values on a grid in both dimensions.

We examine the effect of Schottky barrier height on MoS2-based devices and complete the missing p-branch of MoS2 SBFETs due to the Fermi-pinning effect. Initially, we verify our model against experimental data, results prove the capability of our model to predict the electrical performance of Schottky barrier FETs. By varying the Schottky barrier height from 0 eV to 1.3 eV across the MoS2 bandgap, a transition from n-type transport to p-type transport is observed. However, the ambipolar transport is limited by the relatively large bandgap. Ambipolarity is enabled through asymmetric metal configurations, as evidenced by simulations showing that the Pd-Au configuration can achieve ambipolar behavior with currents comparable to symmetric MoS2-based FETs. Furthermore, we simulate dual-gated FETs by incorporating an additional gate, allowing for reconfigurability between NP and PN configurations. These devices exhibit an outstanding rectification ratio that can be optimized under low gate voltage conditions. Nevertheless, the asymmetry in performance between PN and NP configurations indicates the significant impact of metal choices. The successful establishment of p-n junction in the dual-gated devices illustrates their potential as photodetectors. Simulation results indicate a photoresponsivity of 24.2 mA/W for PN configuration.