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One of the major challenges for the semiconductor industry is to continue with the miniaturization of the device features, increasing the integration densities with higher operation frequency. Silicon the material of choice so far, has been arriving at its physical limits which has led the condensed matter researchers to look for alternative new material which can set the foundation for the next generation computing paradigms or lead to applications in spintronics. There has been a rising interest in so-called Dirac materials, characterized by a linear dispersion relation, giving rise to exotic physical phenomena such as high carrier velocities ~ 106 m/s and dissipationless charge transport. In this thesis, we have studied two classes of Dirac materials - graphene and topological insulators (TIs) namely, bismuth selenide (Bi2Se3) and antimony telluride (Sb2Te3). Specifically, we investigate the optical behavior of Dirac materials using terahertz time domain spectroscopy (THz-TDS) contact-free optical technique, used to probe the low-energy excitations in strongly correlated electron gases. Chapter 1 provides a broad introduction to the field of topological insulators and graphene the various optical and electronic methods, which have been employed to explore their response. In particular the focus in on detecting and isolating the response from the topological surface state (TSS) in TIs, which are “robust”, as it is protected against backscattering by spin− momentum locking and time reversal symmetry. Various literature reports describing the current understanding of the TI field are then discussed. This sets the context for understanding the approach undertaken in the rest of the thesis, towards investigating these materials. In Chapter 2 we discuss the intrinsic plasmonic response in chemical vapour deposited (CVD) graphene and its relation to the domain size of graphene. A novel ion gel based top gate is implemented with the possibility of tuning the plasmonic resonances by ~ 70 GHz. We further employ THz-TDS to map the conductivity of graphene film on different substrates such as germanium and sapphire. In chapter 3, we investigate Bi2Se3, a representative TI using THz spectroscopy and magnetotransport measurements. The temperature-dependent optical behavior of a 23-quintuple-thick film of Bi2Se3, is used to deconvolve the surface state response from the bulk resulting in an optical mobility exceeding 2000 cm^2/V·s at 4 K, indicative of a surface-dominated response. Further, a scattering lifetime of ∼0.18 ps and a carrier density of 6 × 10^12 cm^−2 at 4 K is obtained using the THz measurements. The electrical transport measurements reveal weak antilocalization behavior in the Bi2Se3 sample, consistent with the presence of a topological surface state. Chapter 4, discusses the phase transition in a rather less considered TI, Sb2Te3, using THz-TDS. We track through a series of topological phase transitions from 3D-TI to 2D hybrid topological insulator and then a 2D trivial insulator, as function of Sb2Te3 film thickness. Reducing the film thickness further resulted in a reduced mobility suggesting that the formation of a spin-conserving scattering channel characteristic of hybridized topological insulator phase. Finally, the Chapter 5, concludes with a summary of the thesis and presents future opportunities for further research arising from this work.

Layered Van der Waals solids, due to their highly anisotropic structure as well as their availability in mono, few and multi-layer form constitute a perfect playground, where a variety of possibility in structural variation as well as functionalities are expected. This potentially gives rise to a library of unique and fascinating 2D materials systems. These systems appear to demonstrate some spectacular variety of fundamental physics as well as indicate that some of these systems can be beneficial for several niche applications directly or indirectly resulting from their electrical and optical properties.

Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of great interest to contemporary scientists because of their ability to directly convert temperature differences into electricity, and are regarded as a promising mode of alternative energy. The TE conversion efficiency is determined by the Carnot efficiency, η_C and is relevant to a commonly used figure of merit ZT of a material. Improving the value of ZT is presently a core mission within the TE field. In order to advance our understanding of thermoelectric materials and improve their efficiency, this dissertation investigates the low-temperature behavior of the p-type thermoelectric Cu2Se through chemical doping and nanostructuring. It demonstrates a method to separate the electronic and lattice thermal conductivities in single crystal Bi2Te3, Cu, Al, Zn, and probes the electrical transport of quasi 2D bismuth textured thin films. Cu2Se is a good high temperature TE material due to its phonon-liquid electron-crystal (PLEC) properties. It shows a discontinuity in transport coefficients and ZT around a structural transition. The present work on Cu2Se at low temperatures shows that it is a promising p-type TE material in the low temperature regime and investigates the Peierls transition and charge-density wave (CDW) response to doping [1]. After entering the CDW ground state, an oscillation (wave-like fluctuation) was observed in the dc I-V curve near 50 K; this exhibits a periodic negative differential resistivity in an applied electric field due to the current. An investigation into the doping effect of Zn, Ni, and Te on the CDW ground state shows that Zn and Ni-doped Cu2Se produces an increased semiconducting energy gap and electron-phonon coupling constant, while the Te doping suppresses the Peierls transition. A similar fluctuating wave-like dc I-V curve was observed in Cu1.98Zn0.02Se near 40 K. This oscillatory behavior in the dc I-V curve was found to be insensitive to magnetic field but temperature dependent [2]. Understanding reducing thermal conductivity in TE materials is an important facet of increasing TE efficiency and potential applications. In this dissertation, a magnetothermal (MTR) resistance method is used to measure the lattice thermal conductivity, κ_ph of single crystal Bi2Te3 from 5 to 60 K. A large transverse magnetic field is applied to suppress the electronic thermal conduction while measuring thermal conductivity and electrical resistivity. The lattice thermal conductivity is then calculated by extrapolating the thermal conductivity versus electrical conductivity curve to a zero electrical conductivity value. The results show that the measured phonon thermal conductivity follows the e^(Δ_min⁄T) temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. These low-temperature experimental data and analysis on Bi2Te3 are important compliments to previous measurements and theoretical calculations at higher temperatures, 100 – 300 K. The MTR method on Bi2Te3 provides data necessary for first-principles calculations [4]. A parallel study on single crystal Cu, Al and Zn shows the applicability of the MTR method for separating κ_e and κ_ph in metals and indicates a significant deviation of the Lorenz ratio between 5 K and 60 K [3]. Elemental bismuth is a component of many TE compounds and in this dissertation magnetoresistance measurements are used investigate the effect of texturing in polycrystalline bismuth thin films. Electrical current in bismuth films with texturing such that all grains are oriented with the trigonal axis normal to the film plane is found to flow in an isotropic manner. By contrast, bismuth films with no texture such that not all grains have the same crystallographic orientation exhibit anisotropic current flow, giving rise to preferential current flow pathways in each grain depending on its orientation. Textured and non-textured bismuth thin films are examined by measuring their angle-dependent magnetoresistance at different temperatures (3 – 300 K) and applied magnetic fields (0 – 90 kOe). Experimental evidence shows that the anisotropic conduction is due to the large mass anisotropy of bismuth and is confirmed by a parallel study on an antimony thin film [5]. [1] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril Opeil, J. Alloys Compd. 699, 718 (2017). [2] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril P. Opeil, J. Materiomics 3, 150 (2017). [3] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal metals: Al, Cu and Zn, Mengliang Yao, Mona Zebarjadi, and Cyril P. Opeil, under review. [4] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal bismuth telluride, Mengliang Yao, Stephen Wilson, Mona Zebarjadi, and Cyril Opeil, under review. [5] Albert D. Liao, Mengliang Yao, Ferhat Katmis, Mingda Li, Shuang Tang, Jagadeesh S. Moodera, Cyril Opeil, Mildred S. Dresselhaus, Appl. Phys. Lett. 105, 063114 (2014). / Thesis (PhD) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

We are living in an era of digital electronics. The number of robots have already exceeded the human population of the entire earth. An article in the Guardian newspaper dated 30th March 2018 suggests that 10 million UK workers will be jobless within 15 years as they will be replaced by robots. These astonishing facts shed light on the importance of knowledge and how important it is to use it wisely for our benefit without ultimately destroying us. Knowledge in all forms is accessible without going to a library or buying a newspaper. Furthermore to access information, we often use sleek devices such as smart phones, using highly developed multimedia platforms which consume large amounts of power. In 2016, IBM found that humans create 2.5 quintillion bytes of data daily. Since high computing usage is related to large power consumption, the basic building block of electronics i.e. the transistor is required to be more power efficient. This is now possible through spintronics, where the spin of an electron is exploited instead of the charge. A new class of exotic materials called topological insulators are predicted to exhibit efficient spintronic applications. These materials can conduct spin polarised current on their surface while remaining completely insulting from the inside. Moreover, doping topological insulators with magnetic impurities unlocks new avenues for spin memory devices in the form of a single spin polarized dissipationless conduction channel. In topological insulators, there is always a contribution from the inside (bulk) in addition to surface conduction, thereby yielding charge transport rather than spin transport. On this basis, the aim of my PhD was to explore techniques to grow, characterize, fabricate and measure devices on topological Dirac materials, with the hope to experimentally distinguish the bulk from the surface states and also exploit their exotic properties arising from opening of the bulk band gap by intentional magnetic doping. Samples consisted of thin films of Bi2Se3, Sb2Te3, Cr doped Sb2Te3, bilayers of Dy doped Bi2Te3/Cr doped Sb2Te3 and Cd3As2 nanowires. It was found that a seed layer of an undoped topological insulator was a crucial first step to ensure high quality growth by molecular beam epitaxy, followed by the desired stoichiometry. By physically doping Sb2Te3 with Cr, a successful control of the magnetic and electrical properties such as coercivity, anomalous Hall resistance RA xy, Curie temperature Tc, carrier density and mobility were achieved. A substitutional Cr doping ranging from 7.5% to 38% was attained revealing a Tc reaching up to 186 K. Gated electrical measurements displayed a change in RA xy and carrier density by ~ 50% on applicating of just -3 V gate bias in a sample with 29% doping. A comparison between electrical transport, Magneto-optical Kerr effect and terahertz time domain spectroscopy measurements revealed that the mechanism of magnetization was RKKY mediated. Furthermore, the bilayer structure displays a clear exchange bias coupling arising from the proximity of the antiferromagnetic Dy doped Bi2Te3 layer with the ferromagnetic Cr doped Sb2Te3 layer. Electrical transport measurements on Bi2Se3 Hall bars fabricated using Ar+ milling and wet chemical etching were compared. The results showed a more bulk type response in the chemical etched sample even though Ar+ milling was responsible for creating more disorder in the system leading to a higher carrier density and lower mobility. A thickness dependent study on Sb2Te3 thin films revealed a single conducting channel associated with a coupled surface and bulk state for a 12 nm sample, compared to, two conducting channels associated with the top and bottom surfaces for the 25 nm sample. Electrical transport on Dirac semimetal Cd3As2 nanowires reveal an ultra-high mobility of 56884 cm2V-1s-1 at 1.8 K from analysis of Shubnikov-de Haas oscillations. By studying various Dirac materials, new avenues for practical device applications can be explored.

Les couches minces du silicium présentent de nombreux avantages dans la course à la production de modules solaires à grande échelle de part leur consommation très réduite de matière, leur faible coût de production et leur pertinence dans la technologie solaire flexible. Le silicium microcristallin hydrogéné (c-Si:H), préparé par dépôt chimique en phase vapeur (PECVD), a suscité un intérêt croissant grâce à sa stabilité contre la dégradation induite par la lumière et sa meilleure absorption comparées à celles du silicium amorphe. La structure mixte de ce matériau constituée du silicium amorphe et de grains cristallins arrangés sous forme d'agrégats coniques ou colonnaires influe sur les mécanismes du transport électrique.Dans cette thèse, un modèle tridimensionnel de croissance du c-Si:H est utilisé pour reproduire les principales caractéristiques de la dynamique de croissance et la microstructure du c-Si:H : une forme conique ou colonnaire des grains, une zone de transition amorphe nanocristalline, une rugosité de surface et une fraction cristalline qui évoluent avec l'épaisseur.Un modèle de transport électrique tridimensionnel utilisant les matériaux générés est développé. Ce modèle met en jeu des paramètres électriques correspondant au transport dans la phase amorphe, cristalline et au travers des joints de grains. Les résultats de la simulation sont comparés aux mesures de conductivité électrique montrant un excellent accord et permettant d'extraire les caractéristiques de la barrière de potentiel formée entre les grains. Cette modélisation numérique, à la fois du processus de la croissance et du comportement électrique permet de contribuer à une meilleure compréhension des phénomènes de transport dans ces matériaux fortement hétérogènes.Une caractérisation en bruit basse fréquence des couches microcristallines ayant différentes fractions cristallines est menée dans le but de mieux appréhender les mécanismes de transport. Le comportement en bruit trouvé est typique d'un phénomène de percolation.Les contacts métalliques utilisés lors des caractérisations électriques sont étudiés par la méthode TLM. La modélisation numérique de la structure de test a permis d'extraire la résistivité de contact et la résistance carrée des couches. Nos résultats suggèrent un processus de percolation à l'interface métal/c-Si:H. / Silicon thin films present many advantages in the production of large scale solar cells due to their very low material consumption, low production cost and their relevance in the flexible solar technology. The hydrogenated microcrystalline silicon (c-Si:H) prepared by chemical vapor deposition (PECVD ), has attracted increasing interest due to its stability against degradation induced by light and its better absorption compared to amorphous silicon . The mixed structure of this material consisting of amorphous silicon and crystalline grains arranged in the form of conical or columnar aggregates affects the electrical transport mechanisms. In this thesis, a three-dimensional model is used to reproduce the main features of the growth dynamics and the microstructure of c-Si:H: conical or columnar grains, an amorphous/nanocrystalline transition zone, a surface roughness and a crystalline fraction evolving with the thickness. A three-dimensional model of the electrical transport using the generated structures is developed. This model involves electrical parameters corresponding to the transport in the amorphous phase, crystalline phase and through the grain boundaries. The simulation results are compared to the electrical conductivity measurements showing an excellent agreement and allowing to extract the characteristics of the potential barrier formed between the grains. The numerical modeling of both the process of growth and the electrical behavior contributes to a better understanding of transport phenomena in these highly heterogeneous materials.A low frequency noise characterization of microcrystalline silicon layers with different crystalline fractions has been performed in order to understand the transport mechanism. The noise behavior is found to be typical of a percolation phenomenon.The metallic contacts used in the electrical characterizations are studied by the TLM method. Numerical modeling of the test structure allows extracting the contact resistivity and the sheet resistance of the films. Our results suggest a percolation process on the metal / c-Si:H interface.

<p>Electrical transport properties of nanoparticle gold films made by the gas evaporation method were analysed using resistivity measurements. Low temperature electrical transport measurements showed a cross-over from a temperature range dominated by inelastic scattering to a temperature range dominated by elastic scattering, presumably by grain boundaries. This cross-over shifted towards lower temperatures with increasing grain size. </p><p>High temperature in-situ electrical transport measurements were carried out in isothermal annealing experiments. Four types of samples, prepared at different deposition rates, were analysed. Samples prepared at low deposition rate displayed a higher thermal stability than samples prepared at high deposition rate. A relaxation model was fitted to the in-situ electrical transport data. The model included an activation energy, which was found to increase with increasing annealing temperature for all samples, thus pointing at the presence of pinning mechanisms in the samples.</p><p>Optical properties of nanoparticle gold films were investigated in the 0.3 < λ < 12.5 µm wavelength range. A model taking grain boundary scattering into account was successfully fitted to the experimental data and it was shown that the infrared reflectance decreased with decreasing grain size as a consequence of increased grain boundary scattering.</p><p>Nanoparticle tin-doped indium oxide films were made by spin-coating a dispersion containing the nanoparticles onto a substrate. The tin-doped indium oxide particles were prepared by a wet-chemical method. Optical properties were investigated in the 0.3 < λ < 30 µm wavelength range by reflectance and transmittance measurements. Effective medium theory was employed in the analyses of the optical data and information regarding film porosity and charge carrier concentration and mobility within the individual nanoparticles was obtained. It was found that ionized impurity scattering of the conduction electrons dominates within the particles. The temperature-dependent film resistivity was found to be governed by insulating barriers between clusters containing a large number of nanoparticles, thereby giving a negative temperature coefficient of resistivity.</p>

Electrical transport properties of nanoparticle gold films made by the gas evaporation method were analysed using resistivity measurements. Low temperature electrical transport measurements showed a cross-over from a temperature range dominated by inelastic scattering to a temperature range dominated by elastic scattering, presumably by grain boundaries. This cross-over shifted towards lower temperatures with increasing grain size. High temperature in-situ electrical transport measurements were carried out in isothermal annealing experiments. Four types of samples, prepared at different deposition rates, were analysed. Samples prepared at low deposition rate displayed a higher thermal stability than samples prepared at high deposition rate. A relaxation model was fitted to the in-situ electrical transport data. The model included an activation energy, which was found to increase with increasing annealing temperature for all samples, thus pointing at the presence of pinning mechanisms in the samples. Optical properties of nanoparticle gold films were investigated in the 0.3 &lt; λ &lt; 12.5 µm wavelength range. A model taking grain boundary scattering into account was successfully fitted to the experimental data and it was shown that the infrared reflectance decreased with decreasing grain size as a consequence of increased grain boundary scattering. Nanoparticle tin-doped indium oxide films were made by spin-coating a dispersion containing the nanoparticles onto a substrate. The tin-doped indium oxide particles were prepared by a wet-chemical method. Optical properties were investigated in the 0.3 &lt; λ &lt; 30 µm wavelength range by reflectance and transmittance measurements. Effective medium theory was employed in the analyses of the optical data and information regarding film porosity and charge carrier concentration and mobility within the individual nanoparticles was obtained. It was found that ionized impurity scattering of the conduction electrons dominates within the particles. The temperature-dependent film resistivity was found to be governed by insulating barriers between clusters containing a large number of nanoparticles, thereby giving a negative temperature coefficient of resistivity.

In this work, we have investigated temperature dependent electrical transport properties of carbon based two-dimensional (2D) nanomaterials. Various techniques were employed to synthesize the samples. For instance, high quality large area graphene and boron, nitrogen doped graphene (BNC) were grown using thermal catalytic chemical vapor deposition (CVD) method. Liquid phase exfoliation technique was utilized to exfoliate graphene and graphitic carbon nitride samples in isopropyl alcohol. Chemical reduction technique was used to reduce graphene oxide (rGO) by utilizing ascorbic acid (a green chemical) as a reducing agent. Detailed structural and morphology characterization of these samples was performed using state of the art microscopy as well as spectroscopic techniques (for example; Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), UV-Visible spectroscopy as well as Raman Spectroscopy). The low temperature (5 K< T <400 K) electrical transport properties of these materials show substantial difference from sample to sample studied. For instance, CVD grown graphene film has displayed metallic behavior over a wide range of temperature (5 K < T <300 K). At higher temperatures, resistivity followed linearly with the temperature (ρ(T) ~T). A power law dependence (ρ(T) ~ T4) observed at lower temperatures. Where as liquid phase exfoliated graphene and graphitic carbon nitride samples displayed nonmetallic nature: increasing resistance with decrease in temperature over a wide range (8 K < T < 270 K) of temperature. Electrical transport behavior in these samples was governed by two different Arrhenius behaviors in the studied temperature range. In the case of rGO and BNC layers, electrical conduction show two different transport mechanisms in two different temperature regimes. At higher temperatures, Arrhenius-like temperature dependence of resistance was observed indicating a band gap dominating transport behavior. At lower temperatures, Mott's two dimensional-Variable Range Hopping (2D-VRH) behavior was observed.

In today's "silicon age" in which we live, field-effect transistors (FET) are the workhorse of virtually all modern-day electronic gadgets. Although silicon currently dominates most of these electronics, layered 2D transition metal dichalcogenides (TMDCs) have great potential in low power optoelectronic applications due to their indirect-to-direct band gap transition from bulk to few-layer and high on/off switching ratios. TMDC WSe2 is studied here, mechanically exfoliated from CVT-grown bulk WSe2 crystals, to create a few-layered ambipolar FET, which transitions from dominant p-type behavior to n-type behavior dominating as temperature decreases. A high electron mobility μ>150 cm2V-1s-1 was found in the low temperature region near 50 K. Temperature-dependent photoconduction measurements were also taken, revealing that both the application of negative gate bias and decreasing the temperature resulted in an increase of the responsivity of the WSe2 sample. Besides TMDCs, Group III-VI van der Waals structures also show promising anisotropic optical, electronic, and mechanical properties. In particular, mechanically exfoliated few-layered InSe is studied here for its indirect band gap of 1.4 eV, which should offer a broad spectral response. It was found that the steady state photoconduction slightly decreased with the application of positive gate bias, likely due to the desorption of adsorbates on the surface of the sample. A room temperature responsivity near 5 AW-1 and external quantum efficiency of 207% was found for the InSe FET. Both TMDC’s and group III-VI chalcogenides continue to be studied for their remarkably diverse properties that depend on their thickness and composition for their applications as transistors, sensors, and composite materials in photovoltaics and optoelectronics.