<p>The importance of simulation is increasing in the researchon semiconductor devices and materials. Simulations are used toexplore the characteristics of novel devices as well asproperties of the semiconductor materials that are underinvestigation, i.e. generally materials where the knowledge isinsufficient. A wide range of simulation methods exists, andthe method used in each case is selected according to therequirements of the work performed. For simulations of newsemiconductor materials, extremely small devices, or deviceswhere non-equilibrium transport is important, the Monte Carlo(MC) method is advantageous, since it can directly exploit themodels of the important physical processes in the device.</p><p>One of the semiconductors that have attracted a lot ofattraction during the last decade is silicon carbide (SiC),which exists in a large number of polytypes, among which3C-SiC, 4H-SiC and 6H-SiC are most important. Although SiC hasbeen known for a very long time, it may be considered as a newmaterial due to the relatively small knowledge of the materialproperties. This dissertation is based on a number of MCstudies of both the intrinsic properties of different SiCpolytypes and the qualities of devices fabricated by thesepolytypes. In order to perform these studies a new full-bandensemble device MC simulator, the General Monte CarloSemiconductor (GEMS) simulator was developed. Algorithmsimplemented in the GEMS simulator, necessary when allmaterial-dependent data are numerical, and for the efficientsimulation of a large number of charge carriers in high-dopedareas, are also presented. In addition to the purely MC-relatedstudies, a comparison is made between the MC, drift-diffusion,and energy-balance methods for simulation of verticalMESFETs.</p><p>The bulk transport properties of electrons in 2H-, 3C-, 4H-and 6H-SiC are studied. For high electric fields the driftvelocity, and carrier mean energy are presented as functions ofthe field. For 4H-SiC impact-ionization coefficients,calculated with a detailed quantum-mechanical model ofband-to-band tunneling, are presented. Additionally, a study oflow-field mobility in 4H-SiC is presented, where the importanceof considering the neutral impurity scattering, also at roomtemperature, is pointed out.</p><p>The properties of 4H- and 6H-SiC when used in short-channelMOSFETs, assuming a high quality semiconductor-insulatorinterface, are investigated using a simple model for scatteringin the semiconductor-insulator interface. Furthermore, theeffect is studied on the low and high-field surface mobility,of the steps formed by the common off-axis-normal cutting ofthe 4H- and 6H-SiC crystals. In this study an extension of theprevious-mentioned simple model is used.</p>

This dissertation consists of six chapters: In the first chapter, electrical and material properties and charge transport in organic semiconductors and graphene based field-effect transistors (FETs) are introduced. In the second chapter, device architectures of indenofluorene-phenanthrene copolymer based thin-film transistors (TFTs) are discussed. The combination of recessed source/drain and surface treatments on electrical contact and low-voltage-operated TFTs with solution-processed high-k dielectric are investigated. In the third chapter, device physics and charge transport of diketopyrrolopyrrole-naphthalene copolymer based TFTs are discussed. Top-gate TFTs with the polymer dielectric exhibit mobilities of ~1 cm2/V-s and charge transport measurements in steady-state and under non-quasi-static conditions reveal device physics in dual-gate configuration. In the fourth chapter, device characteristics and charge transport in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer based TFTs are focused. The ambipolar polymer TFTs possess balanced electron and hole mobilities which are both > 0.5 cm2/V-s. The trap density of states is calculated using two analytical methods developed by Lang et al. and Kalb and Batlogg. In the fifth chapter, charge transport of diketopyrrolopyrrole-thiophene copolymer based TFTs employing 4-point-probe configuration is studied. Such polymer TFTs possess the mobilities of up to 3 cm2/V-s. The activation energy as a function of carrier concentration represents multiple trapping and thermally release model or Monroe-type model of charge transport. In the sixth chapter, transformation of electrical characteristics of graphene FETs with an interacting capping layer of fluoropolymers and pi-conjugated organic semiconductors is investigated. The electrical properties of graphene by wafer-scale chemical vapor deposition can be favorably tuned by fluorocarbon capping methods. / text

In this thesis, colloidal nanocrystalline silicon (ncSi) capped with allylbenzene (AB) groups, is created using a sol-gel method. This novel material, AB-ncSi, is size separated and its optical, structural, and electrical characteristics are investigated in detail. For optical characterization, the photoluminescence (PL) peak is located for each fraction of the nanoparticles to determine its diameter. The AB-ncSi samples have diameters ranging from 2.89 nm to 7.65 nm. Ellipsometry and scanning electron microscopy (SEM) are used to estimate the film thickness and average distance between the particles, respectively, for structural characterization. No correlation was found between AB-ncSi size and film thickness however the estimated average distance between the particles decreased with decreasing diameter. Finally, for electrical characterization, conductivity of size-separated samples is measured and the temperature dependent conductance is analyzed. The results emerging from these analyses suggest that the charge transport mechanism for AB-ncSi is nearest-neighbor hopping (NNH) albeit VRH is also a potential contributor.

In this thesis, colloidal nanocrystalline silicon (ncSi) capped with allylbenzene (AB) groups, is created using a sol-gel method. This novel material, AB-ncSi, is size separated and its optical, structural, and electrical characteristics are investigated in detail. For optical characterization, the photoluminescence (PL) peak is located for each fraction of the nanoparticles to determine its diameter. The AB-ncSi samples have diameters ranging from 2.89 nm to 7.65 nm. Ellipsometry and scanning electron microscopy (SEM) are used to estimate the film thickness and average distance between the particles, respectively, for structural characterization. No correlation was found between AB-ncSi size and film thickness however the estimated average distance between the particles decreased with decreasing diameter. Finally, for electrical characterization, conductivity of size-separated samples is measured and the temperature dependent conductance is analyzed. The results emerging from these analyses suggest that the charge transport mechanism for AB-ncSi is nearest-neighbor hopping (NNH) albeit VRH is also a potential contributor.

In this thesis, photo luminescent diodes with the device structure of ITO/PEI/(CdTe/PDDA)*n/Al were fabricated using the method of layer-by-layer self assembly. The film thicknesses were varied from 150 nm to 380 nm. The films were characterized through X-ray diffraction (XRD), optical absorption and photoluminescence (PL) measurements. The XRD results on the film indicated a cubic crystalline structure (111) for the nano-CdTe particles. The band gap of the nano-particles were evaluated to be 2.1 eV in solution and 2 eV in films, which was further confirmed by the PL measurements as the solution exhibited a yellow luminescence while the film exhibited orange luminescence. The J vs. V curves revealed that the diodes exhibited rectifying behavior in both the forward and reverse biasing. Two models of current transport, one based on a Schottky mechanism and the other based on a tunneling mechanism were developed and were compared with the experimental values. The tunneling model developed could simulate the experimental currents up to four orders of magnitude. The tunneling mechanism of charge transport was further proved by the capacitance vs. voltage curves, which were identical to that of ITO/MEH-PPV/Al devices, where tunneling mechanism was the dominant method of charge transport.

Charge transport, charging, and subsequent electrostatic discharge due to interactions with the space environment are primary concerns of spacecraft designers. Developing a physical understanding of the interactions of charge with the multitude of materials that spacecraft are composed of is a critical step in understanding and mitigating both short-term and long-term spacecraft degradation. In particular, the study of charge transport in highly insulating materials is critical as they store charge longer, with higher capacity, and with greater destructive capability than other materials.The Utah State University Materials Physics Group, with the funding of the NASA James Webb Space Telescope project and Rocky Mountain Space Consortium, have developed a complete and consistent theoretical model that predicts short-term and long-term storage capabilities based on physical material parameters. This model is applicable across a wide range of experimental systems designed to test specific behaviors that characterize charging phenomena.Modeling and understanding the complex relationships between the spacecraft and its surroundings are fundamentally based on detailed knowledge of how individual materials store and transport charge. The ability to better understand these effects will help make exploring the edges of the universe more stable, reliable, and economic.

The properties and behaviors of charge transport mechanisms in highly insulating polymers are investigated by measuring conduction currents through thin film samples of low density polyethylene (LDPE). Measurements were obtained using a constant voltage method with copper electrodes inside a chamber adapted for measurements under vacuum and over a wide range of temperatures and applied fields. Field-dependent behaviors, including Poole-Frenkel conduction, space charge limited current (SCLC), and Schottky charge injection, were investigated at constant temperature. These field-dependent mechanisms were found to predict incorrect values of the dielectric constant and the field dependence of conductivity in LDPE was not found to be in agreement with SCLC predicted behavior. A model of thermally assisted hopping was a good fit at low applied fields and produced activation energies within the accepted range for LDPE. Low applied field measurements over the range of 213 K to 338 K were used to investigate two prominent hopping conduction mechanisms: thermally assisted hopping and variable range hopping. The observed temperature dependence of LDPE was found to be consistent with both thermally assisted hopping and variable range hopping. Activation energies determined for the range of temperatures were consistent with values reported in the literature for LDPE under similar conditions. A third aspect of charge transport behavior is a bulk response with time dependence. Conductivity behavior is examined in relation to transient current behavior, long time decay currents, and electrostatic discharge. Comparing charging and discharging cycles allowed qualitative separation of polarization and multiple trapping behaviors.

Metal-Organic Frameworks (MOFs), a class of porous materials realized via reticular construction of a plethora of organic linkers and metal nodes, have emerged as excellent candidates for light-harvesting compositions (LHC), photo or electrocatalysis. This is due to their ability to organize chromophores and metal nodes with desired functionalities, and remarkable porosity that allows efficient mass transfer of reactants and electrolytes. Recent studies have shown intriguing delocalized excited state of the orderly organized pigments in MOFs. Furthermore, the accessible pores/channels allow it to host complementary optical/redox active species within the frameworks by means of de novo or postsynthetic functionalization, as a manner for MOF compositions to integrate functionalities beyond photosensitizer, such as photo/electrocatalytic sites. In such multi-component assemblies, profound understanding of charge transfer and separation process is crucial to make the designed LHC efficient. Therefore, we could adopt chromophoric MOFs as a scaffold to systematically investigate photoinduced charge transfer by installing judiciously selected redox moieties into MOFs, whose unique electronic properties could define distinct electronic interplay with MOFs. From an aspect of further applications, photo-generated electrons can be utilized more efficiently by an external electric field applied on MOF films, which prolongs the charge-separation lifetime. For this purpose, sufficient electrical conductivity is necessary to allow charges delivered across the MOF film. Considering a large energy mismatch between the majority of traditional metal nodes including metal oxo clusters and carboxylic based struts, charge transport is defined by a slow hopping process, which hinders the harvesting of relatively short-lived separated charges. Hence, developing neoteric linkage chemistry is critically needed to overcome the charge-transport challenge.Keeping these points in mind, the scope of this dissertation mainly focuses on unraveling the fundamental principles of photoinduced charge transfer and separation, ground-state charge transport boosted by nontraditional coordination chemistry and incorporation of complementary redox species, and their substantial correlation with MOF-based photocatalysis, electrocatalysis and photoelectrocatalysis. The first chapter lays the foundational knowledge regarding generic properties (chemical and physical) of MOFs, and adopted typical postsynthetic functionalization method, namely, solvent-assisted ligand incorporation (SALI), and other physical processes including photoinduced charge and energy transfer among components within MOFs, and mechanism of electron transport, that has so far been understood, in MOFs driven by an external electric field and commonly used approaches to measure that. Chapter two and three reveal the rule to control photoinduced charge transfer in MOF compositions prepared by the installation of a series of zinc porphyrins possessing gradient excited-state and frontier-orbital energy that can define distinct charge-transfer driving force into the mesopore of a photosensitizing MOF, NU-1000. These compositions show potential for their utilization as artificial light-harvesting assemblies. Chapter four highlights new design for solid porous CO2 reduction catalysts realized by introducing cobalt phthalocyanine into NU-1000. Importantly, we interpreted the catalytic activity from the aspect of charge transport efficiency, by comparing with catalysts constituted by NU-1000 and different molecular catalysts. To harvest the photo-generated electrons, an external electric field can be applied on MOF films deposited on transparent electrodes under photoexcitation, for which sufficient electrical conductivity is a must. Therefore, in chapter five, a new semiconducting coordination polymer framework was developed by employing a novel carbodithioate group for the linkage with nickel(II) that extends in three dimensions, which shows enhanced, electrical conductivity (i.e. 10-6 – 10-7 S cm-1) in contrast to traditional carboxylate-based MOFs due to a more delocalized electronic feature of the carbodithioate-nickel cluster. More importantly, its unique electronic properties, especially a long-lived charge-separation state captured by transient-absorption technique, could alleviate the compromise between electrical conductivity and charge separation (resulted from bandgap) of light-harvesting material. We then extend this binding group to chromium(III), as introduced in chapter 6, leading to a paramagnetic 3D coordination polymer with metallic conductivity as opposed to its nickel counterpart.

<p> The interaction of light with semiconductors provides essential insight into their electronic and photonic properties. Excitons, excited electron-hole pairs, determine the optical response of nanomaterials and act as nanoscale energy carriers, making excitonic materials excellent candidates for optoelectronic, photovoltaic, and quantum devices. Unique phenomena can be brought about by using excitonic materials as building blocks in designing new systems and taking advantage of excitons’ dimensionality. For example, growing quantum dots into highly ordered arrays enhances exciton transport due to the strong dipolar coupling between excitons. Alternatively, forming vertical heterostructures between monolayer transition metal dichalcogenides introduces moiré superlattices, which localize the excitons introducing nonlinear interactions that be exploited for quantum information processing. Understanding these complex excitonic systems requires experimental tools capable of high spatial and temporal resolutions.</p> <p><br></p> <p>This thesis aims to contribute to understanding the complex excitons and charges formed at nanoscale interfaces with ultrafast techniques. In the discussed work, we take advantage of the 100s of fs time resolution and 10s of nm spatial precision to visualize exciton migration and dynamics associated with complex excitonic systems. First, we introduce the optical techniques needed to help us understand the fundamental photophysics of the studied systems (Chapter 2). Next, we provide an example of how we can use these methods to understand exciton coherence in perovskite quantum dot solids exhibiting superradiance (Chapter 3) and enhanced exciton transport (Chapter 4) due to low disorder and strong dipolar coupling. We also characterize and explore the behavior of highly excited excitons, Rydberg states, in transition metal dichalcogenides (Chapter 5). Then, we examine the properties of heterostructures formed between two monolayers of transition metal dichalcogenides exhibiting moiré superlattices and investigate the nonlinear exciton-exciton interactions modulated by the moiré potentials (Chapter 6). We also explore charge carrier behavior at interfaces of two different excitonic materials in molybdenum disulfide-single-wall carbon nanotube heterojunctions containing one- and two-dimensional excitons (Chapter 7). Finally, we visualize and quantify charge carrier migration across an alloyed cadmium sulfide and cadmium selenide lateral heterojunction (Chapter 8). We hope to give the reader a better understanding of these complex systems and open up new possibilities for their efficient use through the results presented in this thesis. </p>

The main motivation in this thesis is to compare the conformation and charge transport in conducting polymers and carbon nanotubes (CNTs) and to investigate those physical properties in their combined form of nanocomposites. It is known that both conducting polymers and carbon nanotubes are intrinsically 1-dimensional systems which consist of delocalized π-electrons. However, the main difference between these is the fact that flexibility of conducting polymers can be varied depending on the extent of conjugation while CNTs are rigid. Hence a comparison of electronic properties as correlated to their morphology has been carried out and their individual role in nanocomposites is further studied. The thesis consists of 6 chapters and appendix. Chapter 1 consists of brief introduction of general properties of both conducting polymers, CNTs and their nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used for the work. Chapter 3 elaborates on the conformational / structural studies on the systems. Chapter 4 focuses on the transport measurements to study the electronic properties of the samples. Chapter 5 reveals the magnetic properties of these systems which can be applied in technological devices. And chapter 6 gives the conclusion and future directions of the work being done. Chapter 1: Nanocomposites represent a guest-host matrix consisting of easily processible functionalized conjugated polymer as host, incorporating carbon nanotubes as fillers with versatile electronic and magnetic properties, which provide a wide range of technological applications. The conformation, charge dynamics as well as magnetic properties of these conducting polymers and carbon nanotubes, and various aspects of transport mechanism and spin dynamics present in the nanocomposite matrix are studied and presented in a consistent framework. Chapter 2: The multiwall carbon nanotubes (MWNTs) are grown by thermal chemical vapor deposition (CVD). The MWNTs are dispersed in solution of conducting polymers by ultrasonication and then the suspension is cast on glass substrate and slowly dried by moderate heating. Once dried completely, the free-standing films of thickness 15-25 μm are peeled off the substrate for measurements. The MWNTs, above a certain concentration, form an interconnected network in the 3-dimensional polymer matrix, following percolation mechanism. The disorder is brought into the system mainly by bundling of tubes and bundle intersections. The morphology and conformation of the samples are studied by SEM, TEM and small angle X-ray scattering (SAXS) techniques. Chapter 3: Small angle X-ray scattering (SAXS) studies in polymeric systems are carried out to probe local nanoscale morphology at various length scales to show the correlation among conformation and assembly of chains. Small angle X-ray scattering (SAXS) studies are carried out in poly [2-methoxy5-(2’–ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) solution of varying conjugation lengths as well as different solvents. By increasing the extent of πconjugation from 30 to 100 %, the persistence length increases by a factor of three. Moreover, a pronounced second peak in the pair distribution function is observed in fully conjugated chain, at larger length scales which indicates that the chain segments tend to self-assemble as the conjugation along the chain increases. The chain assembly and aggregation are further studied for suspensions of MWNTs in polyethylene dioxythiophene-polystyrene (PEDOT-PSS) with aqueous medium and DMSO (dimethyl sulphoxide). The SAXS profile of MWNT dispersion in aqueous PEDOT-PSS clearly show rigid-rod feature of the individual nanotubes evident by the q-1 behavior at short ranges. The crossover from q-1 to q-2 in the longer range further suggest that the suspension consists of individual nanotubes, nanotubes bundles and aggregates that give rise to a 3-dimensonal meshwork of intersecting tubes and ropes. For the MWNT dispersion in PEDOT-PSS with DMSO, however, such q-1 behavior is absent; which evidently shows that the rods are not isolated in the solution and are rather agglomerated. How these conformations affect the electrical and magnetic properties of these systems are studied further in Chapter 4. Chapter 4: Transport mechanism in single wall carbon nanotubes (SWNT), MWNT pellets, and nanocomposite films of MWNT and PEDOT-PSS is studied. The positive and negative magnetoresistance (MR) data in various SWNT samples are analyzed by taking into account the electron-electron interaction (EEI) contribution, in addition to the weak localization (WL) regime. The contribution from EEI to the total MR is confirmed from the universal scaling of MC relation showing that EEI plays a significant role at higher fields and lower temperatures. Intrinsic parameters like inelastic scattering length extracted for barely metallic sample follows the T-3/4 dependence due to inelastic electron-electron scattering in the dirty limit. Conductivity and magnetoresistance (MR) measurements on nanocomposite films with varying MWNT content (0.03 - 3 %) are performed at a field range 0-11 Tesla, and temperature range 1.3–300 K. The temperature dependence of resistance above 4 K suggests a Coulomb-gap variable range hopping (CG-VRH) transport in the network. Alhough solely negative MR (~ 5-6 %) is observed for pristine MWNT pellets; the nanocomposite films show a combination of large negative MR (~ 80 %) at T < 4 K, and a comparatively weaker positive MR (~ 30 %) for T > 4 K. This suggest that there are two mechanism interplaying and dominant at different temperature regimes which can be explained by the mechanism of transport of the charge carriers of MWNT intervened by that of the polymer matrix. In conclusion how the individual properties of conducting polymer and carbon nanotubes contribute to the unique electronic and conformational properties in their nanocomposites is framed in this investigation. Chapter 5: Magnetic properties of the pristine MWNTs as well as metal nanowires of nickel, nickel-iron (NiFe), nickel-iron-cobalt (NiFeCo) encapsulated in the MWNTs are studied using superconducting quantum interference device (SQUID) magnetometer. A typical example of Ni nanowires encapsulated in MWNT (Ni-MWNT) is taken and the results are compared to other forms of nickel (bulk, nanorod cluster, nanowire) to see the effect of size, shape and environment on the magnetic kproperties. The saturation magnetization and coercivity for Ni-MWNTs are 1.0 emu/gm and 230 Oe. The temperature dependence of magnetization indicates superparamagnetic which is supported by the field-cooled and zero-field-cooled plots determining a blocking temperature ~ 300 K. These altered magnetic properties of Ni-MWNTs are mainly due to the contribution from carbon nanotube encapsulation. Both the shape and environment enhance the total magnetic anisotropy of encapsulated nanowires at least by a factor of four. The encapsulation of metal nanowires in MWNTs tunes the magnetic properties of the system widely, e.g. from diamagnetic (pristine MWNTs) to paramagnetic (Ni-MWNT) to ferromagnetic (NiFe-MWNT) and a combination of para and ferro (NiFeCo-MWNT). Chapter 6: The conclusions of the different works presented in the thesis are coherently summarized in this chapter. Thoughts for future directions are also summed up. Appendix A: Spin dynamics in conducting polymer PEDOT-PSS in its pristine, processed with DMSO and nanocomposite form (with carbon nanotubes) is studied using solid state nuclear magnetic resonance (NMR). Plots of proton spin lattice relaxation times vs. temperature at a fixed frequency 23.4 MHz are compared to study the effect of the external agents on the polymer dynamics.