Dissipative quantum transport simulations in two-dimensional semiconductor devices from first principles

In this thesis a simulation framework for efficient and accurate atomic-level treatments of electron transport in the presence of electron-phonon interactions in nanoscale devices is developed. It is based on the non-equilibrium Green's function (NEGF) formalism with every building blocks of the considered systems determined from ab initio density-functional theory (DFT) calculations. The strength of this approach is demonstrated through the investigation of transistors made of single- and few-layer semiconductors as well as van der Waals heterostructures. DFT simulations represent a robust technique in the modeling of nanostructures, but their practical use is restricted to scales below realistic device sizes due to the heavy computational burden associated with them. This limitation is circumvented here by transforming the delocalized crystal electron states into maximally localized Wannier functions (MLWFs) that serve as the basis for the employed NEGF quantum-transport solver. The electron-phonon coupling strengths are also extracted from DFT calculations where the atoms are displaced from their original positions by small numerical values. The analysis of MoS2 field-effect transistors (FETs) reveals that the inclusion of electron-phonon scattering in the computational model is essential for transition metal dichalcogenide (TMD) devices, the ballistic approach leading to unphysical negative differential resistance phenomena. It is also found that due to the strong screening effects a double-gate design is required to benefit from the improved carrier mobility in triple-layer MoS2 compared to a single- or double-layer device. The performance of other TMDs as well as black phosphorus FETs are studied and compared to each other in order to support the on-going experimental efforts in the pursuit for ultimate high-performance logic devices. The proposed framework has been extended to simulate heterostructures in addition to homogeneous FET channels. A MoTe2-SnS2 van der Waals heterojunction tunneling transistor is investigated as possible efficient subthermionic low-power switch. The effect of metal contacts on two-dimensional semiconductors is also examined. The thesis provides detailed explanations with step-by-step tutorials on the application of the MLWF technique in transport problems. Such approaches have recently started to gain increasing attention from the device modeling community.