<p>The miniaturization of a MOSFET is the constant driving force in semiconductor technology over the decades. This scaling enables the realization of the ever complex and functional integration on a single chip where over tens of billions of transistors densely packed. Silicon (Si) is always the golden performer until recent years when the shrinking of a transistor becomes more and more difficult, due to phenomena such as short channel effect and mobility degradation, which is a challenge especially for atomic level scaling. The dawning of low dimensional materials, such as graphene, transition metal dichalcogenides (TMDs), black phosphorus (BP), with their natural atomically thin two-dimension (2D) layered structure and other novel properties, might serve as an alternative solution for ultimate scaling. However, the understanding of the electronic transport in these Van der Waals materials is still lacking. </p><p> In this research, the exploration of this material was first initiated on the vertical heterojunctions where two materials’ interfaces meet. Many previous literatures claimed this hetero-interface creates a P/N junction that results in a diode-like rectification. Yet, by careful analysis and comparing with our “real” vertical structures where the lateral components were eliminated, it is proved this rectification is a direct result from the contact region. The Schottky barrier on the drain side together with the gate effect is the true culprit.</p><p> Realizing how the Schottky barrier could be dominating in these 2D FETs, the second study is the Schottky barrier effect on the contact resistances and furthermore the mobility of the device. Because of the existence of the Schottky barrier between the channel and contact, the contact resistance is not negligible, unlike the ohmic contact for conventional Si MOSFETs. By comparing the intrinsic and extrinsic mobilities of TMD materials, It is found that the contact resistance’s response to the back gate, namely, the rate of how it changes with the back gate has a huge factor in determining whether the extrinsic field-effect mobility underestimates or overestimates its intrinsic mobility. This opens a new insight on the understanding of the transport mechanism under contacts for different TMDs.</p> With the understanding of the Schottky barrier FETs, lastly, the flexibility of these 2D materials is utilized to create high performance three-dimensionally stacked multi-channel FETs, from the inspiration of the Si gate-all-around nanosheet structure. A first-ever 3D integrated high performance MoS₂ device with two channels on top of each other was designed and fabricated, where the current is doubled with an extra layer of channel. The potential of these novel material to be implemented on the future generations of high-performance devices is demonstrated, shedding light on the prospect for extending the Moore’s Law with proper assistance from new materials.

This thesis presents a theory of electron-electron interaction effects and optical properties of nanostructures of two-dimensional (2D) honeycomb crystals - graphene and transition metal dichalcogenides (TMDC). Graphene, a semimetallic hexagonal lattice of carbon atoms can be described by a massless Dirac fermion model, with the conduction band (CB) and valence band (VB) touching in the corners of a hexagonal Brillouin zone, valleys K and -K. TMDC crystals sites host either a transition metal atom or a chalcogen dimer, which opens the energy gap and allows for describing their low-energy nature with massive Dirac fermion (mDf) model. The metal atom in TMDC crystals causes strong spin-orbit (SO) coupling, resulting in large SO splitting in bands at both valleys. For TMDCs it is possible to excite carriers in each valley with oppositely circularly polarised light, which offers promising prospects for devices based on electrons valley index, i.e. valleytronic devices. Additionally, the optical response of TMDCs is enhanced by the presence of secondary CB minima, at Q-points. The dimensionality of 2D crystals can be further reduced to form quantum dots (QDs) - nanostructures con ned in all dimensions. This thesis first discusses hexagonal graphene QDs, which exhibit energy gap oscillation as a function of size, due to the edge type: zigzag or armchair. These QDs are divided into concentric rings, analysed with tight-binding (TB) model. An armchair edged QD is built from a zigzag edged QD by adding a 1D Lieb lattice of carbon atoms on its edge. The energy gap is formed differently for both edges: from the outer ring states for zigzag edge and from the 1D Lieb lattice zero-energy states for armchair edge, which causes the energy gap. The remaining portion of the thesis focuses on TMDC materials. First a TB model is presented for a member of TMDC group, MoS2, using three d orbitals of Mo atom and three p orbitals of the S2 dimers. The tunneling matrix elements between nearest-neighbor and next-nearest-neighbour sites are explicitly derived at K and -K to form a six band TB Hamiltonian. Its solutions are fitted to the bands obtained from the density functional theory ab initio calculations to obtain the correct behaviour of bands around K and additional minima at Q-points, which explains the role of d orbitals in TMDCs. Close to K the TB model is reduced to mDf model, which is then studied in response to light, yielding the valley-dependent selection rules for absorption. The interaction of mDf with light is further studied in the presence of strong external magnetic eld, which leads to the formation of Landau levels (LLs), asymmetric between both valleys, and valley Zeeman splitting. These LLs are populated with electrons to form a Hartree-Fock ground state (GS), which can exhibit valley polarisation due to the LL asymmetry. Quasi-electron-hole excitations out of the GS are then formed and their self-energy, vertex corrections and scattering energy is calculated. The effect of electron-electron interactions on valley Zeeman splitting is demonstrated and the Bethe-Salpeter equation is numerically solved to give magnetoexciton spectrum for both valleys. The results include a valley-dependent absorption spectrum for mDf magnetoexcitons that vary with the valley polarisation. The final part of this thesis discusses the single particle and interacting effects in gated MoS2 QDs. First, I perform a single electron atomistic calculation for a million-atom computational box with periodic boundary conditions based on a TB model developed from ab initio methods for bulk MoS2. Electrons are then con ned with a parabolic electrostatic potential from top metallic gates. They exhibit twofold degenerate harmonic oscillator energy spectrum with shell spacing ω associated with valleys K as well as a sixfold degenerate energy spectrum derived from the Q-points. The degeneracy of electronic shells is broken due to valley contrasting Berry curvature,which acts as an effective magnetic eld splitting opposite angular momentum states in both valleys. I populate up to ve K-derived harmonic oscillator shells with up to six electrons and turn on the electron-electron interactions. The resulting GS phases form two regimes dependent on ω, which are dominated each by a broken-symmetry phase, i.e. valley and spin polarised GS for low ω and valley and spin unpolarised but spin intervalley antiferromagnetic GS for higher ω. This behaviour is explained as an effect of the strong SO splitting, weak intervalley exchange interaction and strong correlations. Means of detecting these effects in experiment based on the spin and valley blockade are proposed. These results advance the understanding of interaction-driven breaking of symmetry for valley systems, crucial for designing of valleytronic devices in the future.

Heat generated by electronic devices must be dissipated in order to ensure reliability and prevent device failure. In order to design devices properly, it is important to have precise knowledge of materials' thermal properties at the nano and micro scales. Here we present a series of experimental studies of heat transport for two different types of material: a two dimensional (2D) material such as MoS2 and micron scale particles. We used frequency domain thermoreflectance (FDTR) to conduct all thermal property measurements. This technique can measure thin film thermal properties as well as characterize the interface between two materials. Molybdenum disulfide (MoS2), a transition metal dichalcogenide, is a 2D material that has potential applications as a transistor in nanoelectronics due to its semiconductor properties. We studied cross plane thermal transport across exfoliated monolayer and few layer MoS2 deposited on two distinct substrates: SiO2 and Muscovite mica. The cross plane direction is critical in layer structure devices since the largest thermal resistances are found along this way. The results show enhanced thermal transport across monolayer MoS2 on both substrates indicating that monolayer MoS2 has superior thermal properties for its use in electronic devices. On the other hand, thermally conductive micro particles are used as fillers in composite materials in order to improve the thermal conductivity of the host or matrix material. They can be embedded in polymers for die attach applications as well as in metals to create more efficient heat sinks. We developed new FDTR based thermal models that apply to isolated particles as well as particles surrounded by another material. We tested the models with isolated diamond and silicon micron size particles and with diamond particles embedded in tin. We were able to obtain the thermal conductivity of individual particles, an effective particle volume and the thermal interface conductance between a particle and its surrounding matrix. This technique could have important applications in industry since it could be used to measure in situ the thermal interface conductance between particles and their matrix, often the highest thermal resistance in composite materials.

abstract: A highly uniform and repeatable method for synthesizing the single-layer transition metal dichalcogenides (TMDs) molybdenum disulfide, MoS2, and tungsten disulfide, WS2, was developed. This method employed chemical vapor deposition (CVD) of precursors in a custom built cold-wall reaction chamber designed to allow independent control over the growth parameters. Iterations of this reaction chamber were employed to overcome limitations to the growth method. First, molybdenum trioxide, MoO3, and S were co-evaporated from alumina coated W baskets to grow MoS2 on SiO2/Si substrates. Using this method, films were found to have repeatable coverage, but unrepeatable morphology. Second, the reaction chamber was modified to include a pair of custom bubbler delivery systems to transport diethyl sulfide (DES) and molybdenum hexacarbonyl (MHC) to the substrate as a S and Mo precursors. Third, tungsten hexacarbonyl (WHC) replaced MHC as a transition metal precursor for the synthesis of WS2 on Al2O3, substrates. This method proved repeatable in both coverage and morphology allowing the investigation of the effect of varying the flow of Ar, varying the substrate temperature and varying the flux of DES to the sample. Increasing each of these parameters was found to decrease the nucleation density on the sample and, with the exception of the Ar flow, induce multi-layer feature growth. This combination of precursors was also used to investigate the reported improvement in feature morphology when NaCl is placed upstream of the substrate. This was found to have no effect on experiments in the configurations used. A final effort was made to adequately increase the feature size by switching from DES to hydrogen sulfide, H2S, as a source of S. Using H2S and WHC to grow WS2 films on Al2O3, it was found that increasing the substrate temperature and increasing the H2S flow both decrease nucleation density. Increasing the H2S flow induced bi-layer growth. Ripening of synthesized WS2 crystals was demonstrated to occur when the sample was annealed, post-growth, in an Ar, H2, and H2S flow. Finally, it was verified that the final H2S and WHC growth method yielded repeatability and uniformity matching, or improving upon, the other methods and precursors investigated. / Dissertation/Thesis / Doctoral Dissertation Physics 2019

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This past decade, 2D materials beyond graphene, and most specifically transition metal dichalcogenides (TMDCs) have gained remarkable attention due to their novel applications in electronics and optoelectronics applications. This work reports large-area growth and structural, optical, and electronic transport properties of few-layer MoS2 thin films fabricated using a hybrid approach based on the magnetron sputtering method. In the first part of this dissertation, properties of optimally annealed MoS2 on different substrates such as amorphous BN, SiO2, Si, Al2O3 are discussed using diffraction, spectroscopic, and transport techniques. Later, we show that the physical properties of large-area sputtered MoS2 thin films can be dramatically improved by an ex-situ high-temperature sulfurization process as it leads to the formation of defect-free MoS2 by removing sulfur vacancies. Sharp film-substrate interface along with high bulk structural order is demonstrated as inferred from diffraction and spectroscopic methods. We show that sulfur vacancies can obscure the MoS2 A-B exciton peaks along with a sharp increase in dc conductivity of MoS2. In the last part of my dissertation, we outline the growth of a novel thermoelectric material (SnSe) and new magnetic inverse-Heuslers (of nominal composition MnxFeSi) using the co-sputtering method. These are some of the first attempts, to our knowledge, to grow such materials in thin-film form. Detailed structure-property relations are thoroughly discussed.

The increased deployment for renewable energy sources to mitigate the climate crisis has accelerated the need to develop efficient energy storage devices. Batteries are at the top of the list of the most in-demand devices in the current decade. Nowadays, research is in full swing to develop a battery that meets the needs of today’s renewable energy systems, which are intermittent by nature. Within the framework of improving the performance of batteries, there are parameters in the composition of the battery that play an important role in its performance: electrode materials, electrolytes, separators, and other factors. The key to battery development is the manufacture of electrode materials with optimal properties. Two-dimensional (2D) materials have led to advances in this field, firstly, using graphite as the anode in lithium-ion batteries (LIBs). However, when using the standard graphite as the anode for sodium-ion batteries (NIBs), the large ionic size and energetic instability of Na+ limit intercalation, resulting in a low storage capacity. Therefore, other 2D materials with large interlayer spacing need to be identified for use as electrodes. In this dissertation, our approach is focus on optimizing anode electrode materials by in situ conversion of 2D materials to obtain hybrid materials. These hybrids materials will synergistically improve the performance of LIBs and NIBs by combining the advantages of individual 2D materials. Starting with converted Ti0.87O2 nanosheets to the TiO2/TiS2 hybrid nanosheets. Then, taking advantage of the properties of MXene, we developed hybrid electrodes based on MXenes by converted V2CTx MXene into V2S3@C@V2S3 heterostructures. Finally, we boosted the redox kinetics and cycling stability of Mo2CTx MXene by using a laser scribing process to construct a multiple-scale Mo2CTx/Mo2C-carbon (LS-Mo2CTx) hybrid material.

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The development of digital technology permits the storage and processing of binary data at high rates, with high precision and density. Therefore, over the past few decades, Moore's law has pushed the development of scaling semiconductor devices for computing hardware. Although the current downward scaling trend has reached its scaling limits, a new "More-than-Moore" (MtM) trend has been emphasized as a diversified function of data collection, storage units, and processing devices. The function diversification defined in MtM can be viewed as an alternative form of "scaling down" for electronic systems, as it incorporates non-computing functions into digital ones, allowing digital devices to interact directly with the environment around them. Two-dimensional (2D) materials display promising potential for combining optical sensing and data storage with broadband photoresponse, outstanding photoresponsivity, rapid switching speed, multi-bit data storage, and high energy efficiency. In this work, in-solution 2D materials flakes (Hafnium Diselenide (HfSe2) and Germanium Selenide (GeSe) have been studied as a charge-trapping layer in non-volatile memory through the seamless fabrication process. Furthermore, the behavior of fabricated non-volatile memories under light illumination has been investigated towards in-memory light sensing. Atomic Force Microscopy, RAMAN spectroscopy, and X-ray Diffraction Spectroscopy characterized the charge-trapping materials. The electrical characterization of Metal Oxide Semiconductor (MOS) Capacitor memory revealed a memory window of 4V for the HfSe2 device under ±10V biasing. Intriguingly, the GeSe device exhibited an extraordinarily wide memory window of 11V under the same electrical biasing. Furthermore, the memory endurance for both materials as charge trapping layer (CTL) exceeds the standard threshold of electrical programming and erasing cycles. The accelerated retention test at different temperatures showed the memory device's stability and reliability for both materials. Under light stimuli with electrical readout voltage, the MOS memory exhibited wavelength and intensity-responsive behavior. The MOS memory of HfSe2 has demonstrated remarkable capabilities in storing the detected light signal, while also exhibiting a noteworthy increase in the memory window of approximately 1.8 V when subjected to a laser wavelength of 405 nm. Meanwhile, the GeSe device's CV measurement revealed a similar trend with the greatest memory window enhancements occurring in relation to 465 nm laser wavelength. Under ±6 V biasing in the absence of light, the memory window was found to be 8.3 V. However, following exposure to a 465 nm laser, this value increased significantly to 9.9 V, representing an increment of 1.6 V. In addition, both devices exhibited distinct sensing of various light intensities and an enhanced memory window as a result of the observable Vt shift caused by altering the levels of illumination. This memory enhancement suggests that photoexcited carriers in the CTL layer were responsible for the optical memory behavior. The 2D materials as CTL pave the way for a reconfigurable optical memory with multilevel optical data storage capacity. This research represents a significant step towards the development of a new generation of memory devices that can store and retrieve data using light signals.