Cette thèse a exploré comment, en mélangeant des matériaux avec des propriétés électriques différentes, il est possible de fabriquer des transistors avec des performances accrues. Des transistors organiques à effet de champ basés sur un oligothiophène (DH4T) ont été fabriqués et optimisés jusqu’à ce que les mobilités mesurées fussent supérieures à celles observées dans des films évaporés. Ces résultats ont été obtenus par le contrôle précis des interfaces et de la vitesse d’évaporation. Des polymères de type p ont été mélangés à des polymères de type n. Chaque solution obtenue a été utilisée pour la fabrication de transistors ambipolaires. Les transistors ont été caractérisés et il a été possible de fabriquer des transistors avec des mobilités équilibrées pour chaque paire de polymères. Des transistors à effet de champ basés sur un mélange de P3HT et d’une molécule photochromique (DAE-Me) ont été fabriqués. Le courant a été mesuré pendant et entre les irradiations et il a été démontré qu’une mémoire non-volatile à multiple niveaux peut être fabriquée / This thesis explored how, by blending of materials with different electrical characteristics, it is possible to fabricate transistors with new or improved performances. First, organic field-effect transistors based on a single oligothiophene, DH4T, were fabricated and optimized until the measured mobility was superior to that observed in vacuum deposited films. This was achieved through careful tuning of the interfaces using self-assembled monolayers and by strong control of the solvent- evaporation rate. P-type polymers were blended with an n-type polymer. Each resulting solution was used for the fabrication of ambipolar field-effect transistors. These devices were characterized and it was found that for each pair of p- and n-type polymers, a transistor with balanced mobilities and high Ion/Ioff could be fabricated. Finally field-effect transistors based on a blend of P3HT and a photoswitchable diarylethene (DAE-Me) were fabricated. The current was measured during and between irradiations and it was demonstrated that a non-volatile multilevel memory could be fabricated.

This thesis consists of three main parts, starting with the use of improved nitridation processes to grow acceptable quality gate oxides on silicon carbide (SiC)[1]–[7], to the comprehensive investigation of basic electron-hole generation process in 4H SiC-based metal–oxide–semiconductor (MOS) capacitors [8], [9], and concluding with the experimental demonstration and analysis of nonvolatile characteristics of 4H SiC-based memory devices [10]–[15]. In the first part of the thesis, two improved versions of nitridation techniques have been introduced to alleviate oxide-growth rate and toxicity problems. Using a combination of nitridation and oxidation processes, a sandwich technique (nitridation–oxidation–nitridation) has been proposed and verified to solve the lengthy and expensive oxide-growing process in direct nitric oxide (NO) gas [1]. The nitrogen source from the toxic-NO gas has been replaced by using a nontoxic nitrous oxide (N2O) gas. The best combination of process parameters in this gas is oxide-growing temperature at 1300oC with 10% N2O [2], [3]. The quality of nitrided gate oxides obtained by this technique is lower than the sandwich technique [6], [13]. Using 4H SiC-based MOS with nitrided gate oxides grown by either of the abovementioned nitridation techniques, the fundamentals of electron-hole generation have been investigated using high-temperature capacitance–transient measurements. The contributions of carrier generation, occurring at room temperature, in the bulk and at the SiC–SiO2 interface are evaluated and compared using a newly developed method [8], [9]. The effective bulk-generation rates are approximately equal for both types of nitrided oxides, whereas the effective surface-generation rates have been shown to exhibit very strong dependencies on the methods of producing the nitrided gate oxide. Based on analysis, the prevailing generation component in a SiC-based MOS capacitor with nitrided gate oxide is at SiC–SiO2 interface located below the gate. Utilizing the understanding of electron-hole generation in SiC, the nonvolatile characteristics of memory device fabricated on SiC have been explored. The potential of developing a SiC-based one-transistor one-capacitor (1T/1C) nonvolatile-dynamic memory (NDM) has been analyzed using SiC-based MOS capacitors as storage elements or test structures. Three possible leakage mechanisms have been evaluated [10]–[16]: (1) leakage via MOS capacitor dielectric, (2) leakage due to electron-hole generation in a depleted MOS capacitor, and (3) junction leakage due to generation current occurred at a reverse-biased pn junction surrounding the drain region of a select metal–oxide– semiconductor field–effect–transistor (MOSFET). Among them, leakage through capacitor oxide remains an important factor that could affect the nonvolatile property in the proposed device, whereas others leakage mechanisms are insignificant. Based on the overall results, the potential of developing a SiC-based 1T/1C NDM is encouraging.

To better understand the physical and electronic properties of donor and acceptor-based structures used in organic electronic applications, a variety of oligothiophene and naphthalene diimide-based small conjugated molecules were designed, synthesized, and characterized. The materials were initially synthesized using oxidative copper-chloride coupling reactions, palladium-catalyzed amination reactions, Friedal-Crafts acylations, Negishi coupling reactions, and Stille coupling reactions. Once isolated, the physical properties of the compounds were characterized through a combination of X-ray crystal structure, thermogravimetric analysis, differential scanning calorimetry, UV-vis. absorption spectroscopy, cyclic voltammetry, and differential pulse voltammetry, along with comparison to quantum-chemical calculations. In some cases, the radical cations or radical anions were generated by chemical oxidation and analyzed by vis-NIR spectroscopy. Furthermore, the electronic properties of the materials were investigated through incorporation as solution-processed active layers in organic field-effect transistors. Multiple examples exhibited hole- and / or electron-transport properties with electron mobility values of up to 1.5 cm²V⁻¹s⁻¹, which is among the highest yet reported for an n-channel OFET based on a solution-processed small molecule.

GaN-based HEMTs are among the most promising candidates for high-power and high-frequency applications; a niche for millimeter-wave technologies. Nitride materials indeed outperform other mainstream III-V materials (InP or GaAs) because of several properties, including wider bandgaps, high peak and saturation velocities, large breakdown voltages, together with good thermal conductivities. Nonetheless, the state-of-the-art of nitrides is not yet industrially mature to exploit the entire millimeter-wave range. A way to push further performance is to develop innovative designs, notably by exploring novel materials. The purpose of this research was therefore to investigate the use of boron-containing III-nitrides in high electron mobility transistors (HEMTs). The study was first conducted theoretically, through solving the Schrodinger-Poisson equation. Key parameters and relevant equations were derived to implement BGaN materials in our simulations. A GaN/ultrathin-BGaN/GaN heterojunction was showed to provide an electrostatic barrier to electrons and to improve the confinement of the two-dimensional electron gas. GaN back-barrier layers happen to limit leakage in the GaN buffer thanks to two effects: (i) a polarization-induced band discontinuity and (ii) a resistive barrier originating from excellent insulation properties of BGaN. The study was then, experimentally, several growth campaigns were carried out that led to the fabrication of devices. First, we confirmed the key characteristics of BGaN materials by electrical and optical measurements. Second, we demonstrated the evidence of a significant enhancement of performance of standard AlGaN/GaN structures by the introduction of a BGaN layer in the buffer layer. Compared to conventional AlGaN/GaN HEMTs, structures grown with BGaN back-barriers showed a significant improvement of static performances, transport properties, and trapping effects involving a limited current collapse in dynamic regime.

Metal-oxide-semiconductor field-effect transistors (MOSFETs) are major devices inintegrated circuit, extensively used in various electronic products. In order to improve the electrical characteristics, scaling channel width and length, using high-£e gate dielectric insulator, and strained silicon may be utilized to increase the driving current and circuit speed. Nevertheless, the scaling of the channel width and length must overcome the limitation of the photolithographytechnology and cost. Once the method is employed, the MOSFETs will face a serious short-channel effect and gate leakage current. In the aspect of high-£e gate dielectric insulator, there still have problems, containing the trap states, phonon scattering, dipole-induced threshold voltage variation, needed to be solved. This dissertation focuses on the properties of MOSFETs experienced an external-mechanical strain, where the channel will be strained. Hence, the mobility, driving current, and circuit speed will increase. Our research can be divided into three topics: fabricating process-induced strained Si, external mechanical stress-induced strained Si, and the properties of strained Si MOSFETs at different temperatures. Except the electrical measurement, we also used the ISE-TCAD to simulate the electrical characteristic of MOSFETs under stress. Firstly, we apply the stress on n-MOSFETs by utilizing the nitride-capping layer. Once the lattice is strained, the mobility will increase, hence resulting in the operating speed. Secondly, the electrical characteristics under external stress is explored by introduced the external mechanical stress along the channel length of nMOSFETs. In addition to the fabricating process-induced strain, the fabricating process condition will also influence the device characteristics. As a result, we propose a new strain technology for our following research. Thirdly, the device performance of strained Si under different temperatures is investigated. Finally, we discuss the gate leakage current in strained Si depending on the ultra-thin gate oxide layer.

One of the aim of this study is to prepare conductive polymer nanocomposites of polypropylene to obtain better mechanical and electrical properties. Composite materials based on conductive fillers dispersed within insulating thermoplastic matrices have wide range of application. For this purpose, conductive polymer nanocomposites of polypropylene with nano dimentional conductive fillers like carbon black, carbon nanotube and fullerene were prepared. Their mechanical, electrical and thermal properties were investigated. Polypropylene (PP)/carbon black (CB) composites at different compositions were prepared via melt blending of PP with CB. The effect of CB content on mechanical and electrical properties was studied. Test samples were prepared by injection molding and compression molding techniques. Also, the effect of processing type on mechanical and electrical properties was investigated. Composites become semiconductive with the addition of 2 wt% CB. Polypropylene (PP) / Carbon Nanotube (CNT) and Polypropylene / Fullerene composites were prepared by melt mixing. CNT&rsquo / s and fullerenes were surface functionalized with HNO3 : H2SO4 before composite preparation. The CNT and fullerene content in the composites were varied as 0.5, 1.0, 2.0 and 3.0 % by weight. For the composites which contain surface modified CNT and fullerene four different compatibilizers were used. These were selected as TritonX-100, Poly(ethylene-block-polyethylene glycol), Maleic anhydride grafted Polypropylene and Cetramium Bromide. The effect of surface functionalization and different compatibilizer on mechanical, thermal and electrical properties were investigated. Best value of these properties were observed for the composites which were prepared with maleic anhydride grafted polypropylene and cetramium bromide. Another aim of this study is to built and characterize transistors which have polyethylene as dielectric layers. While doing this, polyethylene layer was deposited on gate electrode using vacuum evaporation system. Fullerene , Pentacene ve Indigo were used as semiconductor layer. Transistors work with low voltage and high on/off ratio were built with Aluminum oxide - PE and PE dielectrics.

Conjugated polymers and oligomers are great materials for use in the next generation devices namely organic field effect transistors, light emitting diodes and polymeric solar cells. Apart from having the potential for developing power-efficient, flexible, robust and inexpensive devices, conjugated polymers can also be tuned by molecular design to optimize device characteristics. One key problem for the full commercial exploitation of conjugated polymers is that the charge carrier mobility of the state-of-the-art polymer semiconductors is much lower than required for many applications. The performance of the devices is strongly dependent on the molecular structure and supermolecular assembly of the conjugated polymer chains. This thesis covers our attempts to design molecular structure to control and improve the solid state properties of conjugated polymers. The relative placement of side chains along the backbone has a great influence on the solid state ordering of conjugated polymers. Poly(2,5-disubstituted-1,4-phenylene ethynylene)s (PPE)s, an important class of conjugated polymers, are generally synthesized by Pd-catalyzed coupling polymerizations of appropriately substituted diiodo and diethynyl benzenes (i.e., A-A and B-B type monomers). In asymmetrically substituted PPEs, this results in an irregular substitution pattern of the side chains along the polymer backbone. We report a new synthetic approach to prepare regioregular unsymmetrically substituted PPEs by polymerization of 4-iodophenylacetylenes (i.e., A-B type monomer). We provide a detailed discussion of various approaches to the synthesis of PPEs with different regioregularities and provide a description of the differences between regioregular and regiorandom analogs. The effect of regioregularity becomes even more important when the two side chains are very dissimilar or amphiphilic. We explore the effect of relative placement hydrophobic (dodecyloxy) / hydrophilic (tri(ethylene glycol) and hydrophobic (dodecyloxy)/fluorophilic (fluoroalkyl) side chains along the poly(1,4-phenylene ethynylene) backbone. We found that the regioregular substitution of the polymer backbone provides a structure in which the side chains segregate to afford a Janus-type structure. The regioregular polymer chains pack more densely in a monolayer at the air-water interface, and pack into a bilayer in the solid state to form a highly crystalline material. Pentacenes are very important organic molecules for use as semiconductor in oFETs due to their low band gap and high field effect mobility. One approach to reduce the bandgap of a polymeric system and improve performance is to include low bandgap small molecules into the conjugated backbone. A new copolymer system consisting of pentacene and terthiophene was developed and its optical and electronic properties along with its stability were evaluated. We report the use of ultrasonication of P3HT as a novel operationally-simple process to significantly improve the field effect mobility of P3HT-based FETs, thereby potentially eliminating the need for dielectric surface modifications or further processing of the device. Investigation of the sonicated polymer samples by number of characterization techniques indicates that ultrasonication leads to aggregation and ordering of the P3HT chains resulting in increase in the mobility.

Conjugated polymers comprise some of the most promising materials for new technologies such as organic field effect transistors, solar light harvesting technology and sensing devices. In spite of tremendous research initiatives in materials chemistry, the potential to optimize device performance and develop new technologies is remarkable. Understanding relationships between the structure of conjugated polymers and their electronic properties is critical to improving device performance. The design and synthesis of new materials which self-organize into ordered nanostructures creates opportunities to establish relationships between electronic properties and morphology or molecular packing. This thesis details our progress in the development of synthetic routes which provide access to new classes of conjugated polymers that contain dissimilar side chains that segregate or dissimilar conjugated blocks which phase separate, and summarizes our initial attempts to characterize these materials. Poly(1,4-phenylene ethynylene)s (PPEs) have been used in a variety of organic electronic applications, most notably as fluorescent sensors. Using traditional synthetic methods, asymmetrically disubstituted PPEs have irregular placement of side chains on the conjugated backbone. Herein, we establish the first synthetic route to an asymmetrically substituted regioregular PPEs. The initial PPEs in this study have different lengths of alkoxy side chains, and both regioregular and regiorandom analogs are synthesized and characterized for comparison. The design of amphiphilic structures provides additional opportunities for side chains to influence the molecular packing and electronic properties of conjugated polymers. A new class of regioregular, amphiphilic PPEs has been prepared bearing alkoxy and semifluoroalkoxy side chains, which have a tendency to phase separate. Fully conjugated block copolymers can provide access to interesting new morphologies as a result of phase separation of the conjugated blocks. In particular, donor-acceptor block copolymers that phase separate into electron rich and electron poor domains may be advantageous in organic electronic devices such as bulk heterojunction solar cells, of which the performance relies on precise control of the interface between electron donating and accepting materials. The availability of donor-acceptor block copolymers is limited, largely due to the challenges associated with synthesizing these materials. In this thesis, two new synthetic routes to donor-acceptor block copolymers are established. These methods both utilize the catalyst transfer condensation polymerization, which proceeds by a chain growth mechanism. The first example entails the synthesis of a monofunctionalized, telechelic poly(3-alkylthiophene) which can be coupled to electron accepting polymers in a subsequent reaction. The other method describes the first example of a one-pot synthesis of a donor-acceptor diblock copolymer. The methods of synthesis are described, and characterization of the block copolymers is reported.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

In this thesis, we investigate spin-dependent transport through ferromagnet-contacted single-walled carbon nanotubes (SWCNTs), in which charge transport shows the Fabry-Perot (FP) interference effect, the Kondo effect and the Coulomb blockade effect at low temperatures. Hysteric magnetoresistance (MR) is observed in all three transport regimes, which can be controlled by both the external magnetic field and the gate voltage. The MR in the FP interference regime can be well understood by a model considering the intrinsic electronic structure of SWCNTs and the quantum interference effect. In the strongly interacting Kondo regime, the Kondo effect is not suppressed by the presence of nearby ferromagnetism. Several observed MR features including the non-splitted zero-bias Kondo peak and positive MR switching can be explained by the strong Kondo effect and weak ferromagnetism in the leads. In the Coulomb blockade regime, several effects that can be associated with the magneto-Coulomb effect have been observed, and isolated spin accumulation and transport through the SWCNT quantum dot have been realized by a four-probe non-local measurements. We also studied charge transport behavior through organic semiconductor pentacene thin film transistors (OTFTs) in the limit of single- or a few molecular layers of pentacene films. The charge transport in these devices can be well explained by the multiple trapping and release model. The structural disorders induced by the physical and chemical causes, such as grain boundaries, interactions with gate insulator, metal contacts and ambient conditions can be responsible for the localized trap states in the ultrathin layer OTFTs, which are further confirmed by the electric force microscopy (EFM) measurements. / text