The Dielectric Properties of Two-dimensional Materials and Their Applications in Electronic Devices: a First-principles Study




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Recent developments in the field of two-dimensional (2D) van der Waals (vdW) materials have captured great interest for their possible applications in the next generation of complementary Metal-Oxide Semiconductor (CMOS) technologies. Easy cleavage along the layer planes, naturally passivated surfaces, and intriguing anisotropic electrical, thermal, and optical properties make vdW materials ideal candidates to reach the ultimate scaling limit of transistors. Moreover, vdW transistors could add great functionality in the backend-of-line or for other novel applications like neuromorphic computing. In this doctoral dissertation, I will identify and study novel 2D vdW dielectric candidates to address some drawbacks of currently available non-van der Waals (non-vdW) dielectrics, e.g., HfO2 and Al2O3 . Although traditional three-dimensional (3D) dielectrics provide high-k solutions for silicon-based semiconductor technologies, they cannot be easily scaled and therefore deteriorate device performance in vdW channel transistors. Moreover, the covalent bonds in non-vdW dielectrics may destroy the naturally passivated bonds in a vdW channel material. To address this concern, we investigate 40 alternative 2D vdW dielectrics and evaluate their performance in conjugation with six Transition Metal Dichalcogenide (TMD) channels. We employ Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) to calculate the electronic and dielectric properties of a wide range of 2DvdW dielectrics. We perform highly accurate calculations to investigate the electronic band structure, thermodynamic properties, structural stability, and dielectric properties of 2D vdW structures. We perform precise calculations at high DFT and DFPT levels to obtain the bandgap, electron affinity, out-of-plane electron effective mass, as well as in-plane and out-of-plane dielectric constant. We calculate the band offset (conduction and valence band edge) of each material to evaluate their insulating properties for the potential application in n-MOS and p-MOS technologies. We compute the Equivalent Oxide Thickness (EOT) for each compound and use direct tunneling and thermionic emission equations to examine the performance of a device made by a 2D dielectric and a TMD channel. We eventually introduce the most promising candidates that can satisfy the low-power limit introduced by the 2020 International Roadmap for Devices and Systems (IRDS) by having an acceptable leakage current. We eventually compare our results with the industry’s most desirable dielectrics (SiO2 , and HfO2) and 2D hexagonal Boron Nitride (h-BN). We explicitly show how some materials in our list outperform the available high-k dielectrics. Given the growing interest in the 2D layered dielectric materials and their possible use in future scaled electronic devices, we believe that 2D vdW dielectric candidates identified in this PhD dissertation could pave the way for the future design of advanced n-MOS and p-MOS transistors.



Engineering, Materials Science, Physics, Condensed Matter, Physics, Theory