PhD Defense of Jiang CAO

The successful isolation of graphene in 2004 has attracted great interest to search for potential applications of this unique material and other newborn members of the two-dimensional (2D) family in electronics, optoelectronics, spintronics and other fields. Compared to graphene, the 2D transition metal dichalcogenides (TMDs) have the advantage of being semiconductors, which would allow their use for logic devices. In the past ten years, significant developments have been made in this area, where opportunities and challenges co-exist.
This thesis presents the results of quantum transport simulations of novel 2D-material-based tunnel field-effect transistors for ultra-low-power digital applications. Due to their size, such devices are intrinsically dominated by quantum effects. This requires the adoption of a fairly general theory of transport, such as the nonequilibrium Green's functions (NEGF) formalism, which is a method extensively used for the simulation of electron transport in nanostructures.
In the first part of this thesis, a brief introduction about the 2D materials, their synthesis and applications is presented. Then, the NEGF formalism is concisely reviewed. This approach is applied to the simulation of two different models of vertical tunnel field-effect transistors based on 2D-TMD van der Waal heterojunctions (MoS2 and WTe2). To properly describe the system, a coupled effective mass Hamiltonian has been implemented and carefully calibrated to experimental measurements and density functional theory to reproduce the band structure in the energy range of interest for the simulations.
This thesis not only demonstrates the ultra-steep subthreshold slope potentially expected for these devices, but also provides a physical insight into the impact of the transistor geometry on its performances. In the last and more exploratory part of the manuscript, the effect of rotational misalignment within the two layers of the heterostructure is investigated. Experimentally, such a disorder is difficult to avoid, and it can substantially affect the device performances.
Through accurate quantum simulations and deep physical analysis, this study sheds light on the design challenges to be addressed for the development of efficient tunnel field-effect transistors based on 2D materials.