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Author ORCID Identifier

https://orcid.org/0000-0002-0780-7486

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Electrical and Computer Engineering

Year Degree Awarded

2021

Month Degree Awarded

May

First Advisor

Zlatan Aksamija

Subject Categories

Electrical and Electronics | Electronic Devices and Semiconductor Manufacturing | Heat Transfer, Combustion | Semiconductor and Optical Materials

Abstract

”Smaller is better” is the mantra that has driven semiconductor industry for the past 50 years. The on-going quest for faster electronic switching, higher transistor density, and better device performance, has been driven by a self-fulfilling prophecy popularly known as Moore’s law, according to which the number of transistors per unit area of a chip doubles itself approximately every two years. A modern smartphone has about 8 billion transistors, which is as large as current earth’s population. Although each transistor dissipates negligible power, but the collective power dissipation from all the transistors in an electronic gadget and inefficient heat removing capability of ultrathin silicon have led to the formation of hotspots—which degrade device performance and, ultimately, lead to their failure. Since the breakthrough of graphene, two-dimensional (2D) materials have drawn tremendous research attention due to their ultra-thin nature and yet high carrier mobility and are, thus, envisioned as potential materials to replace silicon in future electronic devices. vii Among 2D materials, transition metal dichalcogenides (TMDs) and black phosphorene (a 2D analogue of black phosphorus) exhibit high carrier mobility and on-off ratio— necessary requirements for high-speed switching applications. 2D materials, in suspended form, exhibit extremely high carrier mobility, but when they are placed on a substrate, as would be the case in a transistor, their mobility drops by one to two orders of magnitude due to strong Coulomb scattering from the charged impurities in the substrate. In few-layered (FL) devices, where the top layers are shielded from the charged impurities by bottom layers, the mobility has been found to improve significantly. Due to high surface-tovolume ratio, heat removal in FL devices occurs mainly in the cross-plane direction. Weak vdW forces cause weak thermal coupling between layers, and therefore limits the h causes the current to route itself to the bottom layers, where the TBC is higher, resulting in better heat removal and less temperature rise which limits the severity of self-heating and its impact on carrier mobility in FL WSe2 devices. Our electro-thermal simulations also revealed that a FL 2D material with smaller λT F and high TBC will be a suitable candidate for next-generation electronic material. The second part of my dissertation aims at improving thermal management in 2D materials from fundamental perspective. The microscopic interactions between phonons and electrons play a major role in influencing electrical properties of solids and are therefore well studied. However, their impact on phonon transport has received far less attention because they are generally weak except at cryogenic temperatures. Recent studies suggest that phonon-electron scattering can significantly reduce the in-plane thermal conductivity in materials like silicon and its 2D counterpart, silicene. However, the heat removal in devices made up of 2D materials takes place in the cross-plane direction. For cross-plane thermal conduction, the internal resistance due to phonon-phonon interactions is found to be the bottleneck. A significant phonon-electron scattering can reduce the internal resistance and boost its TBC. To examine it, I calculate phonon-electron scattering from the phonon dispersion calculated from first principles and then studied its impact on TBC between a 2D material and a substrate. Our results show that among TMDs, the TBC of MoS2 exhibits the maximum enhancement of about 36% when the carrier density is increased from 1014 to 1018 m−2 . This provides a novel tool to dynamically tune TBC between a 2D material and a 3D substrate via electrostatic gating. Thermal conduction in solids is mainly considered to be diffusive in nature where the quasiparticles carrying heat, called phonons, lose their momenta due to collisions with the underlying lattice. These momentum-relaxing scattering processes are called umklapp scattering. A strong fraction of momentum-relaxing collisions is a signature of diffusive transport. However, under special conditions such as cryogenic temperatures, a large fraction of phonons might undergo momentum-conserving collisions, therefore carrying heat ix over a longer characteristic length called mean-free-path, and hence it may lead to a higher thermal conductivity. Such fluid-like thermal transport in solid is referred to as hydrodynamic transport. Recently, researchers have found graphene to exhibit hydrodynamic transport at around 100 K, which is much higher than cryogenic temperatures. Phonon hydrodynamics at higher temperatures has received significant attention because it may now be used for practical applications. Since hydrodynamic transport is a wave-like phenomenon, it is more rational to study thermal conductivity in the frequency domain. In my dissertation, I derived an expression for thermal conductivity in 2D materials by solving Boltzmann Transport equation, which governs heat conduction in solids, in frequency domain. We quantified the range of frequencies of temperature gradient, where normal scattering is more dominant than umklapp scattering, which supports hydrodynamic transport in graphene ribbons. The method also provides an unique opportunity to perform phonon lifetime spectroscopy. The principle ”Smaller is better is not only applicable for transistors, but for interconnects as well. Along with shrinking the dimensions of transistors, the dimension of interconnects and wires, typically made of copper, has also shrunk. Current state-of-the-art technology uses copper interconnects as thin as 10 nm. However, the resistivity of copper increases exponentially below 100 nm. On the other hand, graphene ribbons exhibit lower resistance at similar linewidths, which makes them promising candidates for interconnects in the next-generation integrated circuits. Mass production of graphene is achieved via chemical vapor deposition technique (CVD). Unfortunately, CVD-grown graphene are, inherently, polycrystalline in nature. The large film consists of several grains, each having different crystal orientation and size, which are separated from each other by grain boundaries (GBs). The relative mismatch in the crystal orientations of the adjacent grains, typically referred to as misorientation angle ΘM, is expected to play a significant role in determining the electrical properties of a polycrystalline material. x In this dissertation, I first studied the impact of misorientation angles on the GB resistivities (ρGB) of 2D materials where the GBs were assumed to be straight lines. We found that ΘM alone does not determine the GB resistivity rather it is the pair of angles that the physical GB makes with the crystal orientations of each grain which governs ρGB. Symmetric GBs are found to be highly conductive approximately equal to the ballistic conductance. Asymmetric GBs are resistive—the resistivity increases exponentially with the degree of asymmetry. In graphene, they exhibit stronger angle dependence than those in MoS2. In practice, GBs are straight lines only in microscopic regions, however, over an extended region, GBs are never straight lines. Consequently, the prediction of ρGB differ significantly from experimental measurements, especially for asymmetric GBs. Here I used two different approaches to simulate extended GBs in graphene and then integrated them with the electrical transport model developed by me to compute GB resistivity. Our results revealed that the resistivity is inversely proportional to the effective slope of the GB, which is given by the ratio of the rms roughness and correlation length. The segments which symmetrically divide the grains destroy the correlation between the resistivity and misorientation angles which we found in our previous study on straight-line GBs. We calculated the resistivity for extended GB to lie between 102 to 104 Ω µm, which aligns well with the measured resistivity values found in the literature.

DOI

https://doi.org/10.7275/22139731.0

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