Ab-Initio NEGF transport for next generation energy-efficient 2D material transistors including Cold-Source-, Dirac- and Van-der-Waals tunneling FETs

Two dimensional materials and in particular transition metal dichalcogenides (TMDs) are widely investigated by the scientific community nowadays. Their large variety of bandgaps, effective masses, and their excellent electrostatic properties related to their 2D nature hold promise to find the candidate for ultra-scaled CMOS applications. However, even if the electrostatic control were perfect, 2D-material transistors are still limited by the thermionic limit of 60mV/dec for switching the transistor from OFF to ON state, which puts constraints on the minimum operating voltage V_DD and minimum power limit of a CMOS circuit.

2D materials offer interesting candidate to circumvent and breach the thermionic limit constraint, using various energy filtering concepts:  Using a cold metal at the source, a metal that has no or limited high-energy states, the so-called Cold-Source (CS) FET, or its variant the Dirac FET using a semi-metal Dirac material like graphene,  could achieve steep-slope switching by limiting the injection of carriers at energies high enough to cross the channel barrier in off-state [1]. In addition, 2D van der Waals (vdW) heterojunction could be a powerful tool to enlarge their application space, such as in band-to-band tunneling field-effect-transistors (TFETs) for low-power electronics [2].

Today, much is still to be done to explore and fully understand and design the potential of this novel class of intriguing materials and devices. Atomistic modeling and simulations are crucial to understand the physics, practical challenges and proof-of-concept including non-idealities, and select the relevant device architectures and 2D materials for these applications. To get insight and understanding at the microscopic level, a parameter-free or ab-initio atomistic method, such as Density-Functional-Theory (DFT), is required.

During this PhD, you will explore the fundamental physics and properties of 2-D material CS, Dirac and vdW TFET devices, focusing on the modeling and physical understanding of the role of the atomistic structure in the overall device transport properties, i.e., from atoms to devices. These include monolayers or a few layers of mono- or heterojunction materials with various pristine or defective interfaces... You will build and study, using DFT tools (e.g., Quantum Espresso, CP2K), the supercell elements needed for the device simulations. The transport properties will then be studied using ATOMOS, our state-of-the-art dissipative DFT - Non-Equilibrium Green’s functions (NEGF) atomistic modelling solver [2,3]. The device Hamiltonian is created in our simulator using as building blocks DFT-supercell elements of the materials or combination of materials of interest. You will work and collaborate with experts in the field. Possibility to work and interact with experimentalists is also available, as imec has a strong expertise in 2-D materials and devices fabrication and characterization.

[1] R. Duflou, M. Houssa, A. Afzalian, Electron-phonon scattering in cold-metal contacted two dimensional semiconductor devices, 2021 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), pp. 94-97, 2021/9/27, Dallas USA.

[2] A. Afzalian, E. Akhoundi G. Gaddemane, R. Duflou and M. Houssa, "Advanced DFT–NEGF Transport Techniques for Novel 2-D Material and Device Exploration Including HfS2/WSe2 van der Waals Heterojunction TFET and WTe2/WS2 Metal/Semiconductor Contact," in IEEE Transactions on Electron Devices, vol. 68, no. 11, pp. 5372-5379, Nov. 2021, doi: 10.1109/TED.2021.3078412

[3] Afzalian, A. Ab initio perspective of ultra-scaled CMOS from 2D-material fundamentals to dynamically doped transistors. npj 2D Mater Appl 5, 5 (2021). https://www.nature.com/articles/s41699-020-00181-1