Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 200-207). Available also in a digital version from Dissertation Abstracts.

Over the past few decades, metal phthalocyanines (MPcs) have been thoroughly investigated as active materials in organic field-effect transistors (OFETs) towards the commercialization of flexible integrated circuits and displays. One of several advantages to MPcs as building blocks for OFETs is the high degree of functionality, from which the choice of metal ion, substituents along with the phthalocyanine framework and axially bound ligands can synergistically tune the physical and self-assembly properties of the material. Recent interest has been directed to the introduction of main-group elements as the central ion of MPcs as an avenue to install both hole and electron transport properties and improve device performance. To prepare materials that are suitable to be employed as the semiconducting active layer in organic field-effect transistors, a family of novel silicon phthalocyanine derivatives was prepared. The synthesis and optoelectronic properties of those new axially disubstituted silicon phthalocyanines are detailed in this work. Axial ligand variation mainly includes alkylsiloxy derivatives. The emphasis of the current thesis, however, is on tailoring the Pc backbone, which includes replacing the four benzene units with pyrazine moieties, extending the degree of conjugation with naphthalene, and introducing substituents on their peripheral positions. Several metal-containing tetra-2,3-pyrazinoporphyrazines are also described, but their applications are limited due to the difficulty of purification. Specifically, Chapter 1 serves as a comprehensive review of main-group phthalocyanines and their use as active materials in organic field-effect transistors. In Chapter 2, silicon tetra-2,3-pyrazinoporphyrazine complexes are explored. The isosteric substitution of CH groups in Pc macrocycle for nitrogen atoms leads to an obvious hypsochromic shift in their main absorption band, and their relatively low energy levels make them promising air-stable n-type organic semiconducting materials for OFETs. The synthesis and characterization of silicon tetra(tert-butyl)-2,3-naphthalocyanine complexes are described in Chapter 3. The extension of π-conjugation leads to obvious bathochromic shifts in the main absorption band. In addition, the introduction of tert-butyl groups on the periphery of the molecule reduces the tendency of the naphthalocyanine molecules to aggregate, thereby increase their solubility. Chapter 4 covers the synthesis and characterization of zinc tetra-2,3-pyrazinoporphyrazine and cobalt tetra-2,3-pyrazinoporphyrazine, whereas more future works are expected. The fifth chapter provides a conclusion to this work, and possible future directions of the research conducted herein.

Reduced graphene oxide (RGO) sheet, a chemically functionalized atomically thin carbon sheet, provides a convenient pathway for producing large quantities of graphene via solution processing. The easy processibility of RGO sheet and its composites offer interesting electronic, chemical and mechanical properties that are currently being explored for advanced electronics and energy based materials. However, a clear understanding of electron transport properties of RGO sheet is lacking which is of great significance for determining its potential application. In this dissertation, I demonstrate fabrication of high-yield solution based graphene field effects transistor (FET) using AC dielectrophoresis (DEP) and investigate the detailed electronic transport properties of the fabricated devices. The majority of the devices show ambipolar FET properties at room temperature. However, the mobility values are found to be lower than pristine graphene due to a large amount of residual defects in RGO sheets. I calculate the density of these defects by analyzing the low temperature (295 to 77K) charge transport data using space charge limited conduction (SCLC) with exponential trap distribution. At very low temperature (down to 4.2 K), I observe Coulomb blockade (CB) and Efros-Shklovskii variable range hopping (ES VRH) conduction in RGO implying that RGO can be considered as a graphene quantum dots (GQD) array, where graphene domains act like QDs while oxidized domains behave like tunnel barriers between QDs. This was further confirmed by studying RGO sheets of varying carbon sp 2 fraction from 55 – 80 % and found that both the localization length and CB can be tuned. From the localization length and using confinement effect, we estimate tunable band gap of RGO sheets with varying carbon sp 2 fraction. I then studied one dimensional RGO nanoribbon iv (RGONR) and found ES VRH and CB models are also applicable to the RGONR. However, in contrast to linear behavior of decrease in threshold voltage (Vt) with increasing temperature (T) in the RGO, sub linear dependence of Vt on T was observed in RGONR due to reduced transport pathways. Finally, I demonstrate synthesis and transport studies of RGO/nanoparticles (CdS and CeO2) composite and show that the properties of RGO can be further tuned by attaching the nanoparticles.

<p> The Metal-Oxide Semiconductor Field-Effect Transistor is first analyzed from an active R-C transmission line view-point. The small signal circuit model and the noise model of the device are then derived and experimental results presented.</p> <p> A Chronologically arranged bibliography concerning MOS devices and associated noise studies is included at the end of this thesis.</p> / Thesis / Master of Engineering (MEngr)

No description available.

At present the field of molecular electronics - also known as molecular semiconductors, organic semiconductors, plastic electronics or organic electronics - is dominated by organic materials, both polymeric and molecular, with much less attention being focused on transition metal based complexes despite the advantages they can offer. Such advantages include tuneable frontier orbitals through the ligand/metal interaction and the ability to generate stable paramagnetic species. Devices containing radical materials are particularly interesting in order to examine the interplay between conduction and spin - an effect which is not yet properly understood but can give rise to exotic behaviour. A series of homoleptic, bis-ligand Ni(II) and Cu(II) complexes were prepared using three structurally related phenolic oxime ligands, 2-hydroxy-5-t-octylacetophenone oxime (t-OctsaoH), 2-hydroxy-5-n-propylacetophenone oxime (n-PrsaoH) and 2- hydroxyacetophenone oxime (HsaoH). The complexes were characterised by single-crystal X-ray diffraction, cyclic voltammetry, UV/Vis spectroscopy, field-effect-transistor measurements, DFT/TD-DFT calculations and in the case of the paramagnetic species, EPR and magnetic susceptibility. Variation of the substituent on the ligand from t-octyl to n-propyl to H enabled electronic isolation of the complexes in the crystal structures of M(t-OctsaoH)2, which contrasted with π-stacking interactions observed in the crystal packing of M(n-PrsaoH)2 and of M(HsaoH) (M = Ni, Cu). This was further evidenced by comparing the antiferromagnetic interactions observed in samples of Cu(n-PrsaoH)2 and Cu(HsaoH)2 with the ideal paramagnetic behaviour for Cu(t-OctsaoH)2 down to 1.8 K. Despite isostructural single crystal structures for M(n-PrsaoH)2, thin-film X-ray diffraction and SEM revealed different morphologies depending on the metal and the deposition method employed. However, the complexes of M(n-PrsaoH)2 and M(HsaoH) failed to demonstrate significant charge transport in an FET device despite displaying the ability to form π- stacking structures. A series of planar Ni(II), Cu(II) and Co(II) dibenzotetraaza[14]annulenes (dbtaa) and dinapthotetraaza[14]annulenes (dntaa) were synthesised and studied crystallographically, optically, electrochemically and magnetically. Thin films of each of these complexes have been prepared by vacuum deposition to evaluate the field-effect transistor (FET) performance as well as the morphology and crystallinity of the film formed. Single crystal data revealed that Ni(dbtaa) and Cu(dbtaa) are isomorphous to each other, with Co(dbtaa) displaying a different crystallographic packing. The electrochemistry and UV/Vis absorption studies indicate the materials are redox active and highly coloured, with molar extinction coefficients as large as 80,000 M-1cm-1 in the visible region. The paramagnetic Cu(II) and Co(II) complexes display weak 1-dimensional antiferromagnetic interactions and were fit to the Bonner-Fisher chain model. The data revealed that the Co(II) species possesses much stronger magnetic exchange interactions compared with the Cu(II) complex. Each of the materials formed polycrystalline films when vacuum deposited and all showed ptype field-effect transistor behaviour, with modest charge carrier mobilities in the range of 10-5 to 10-9 cm2 V-1 s-1 . SEM imaging of the substrates indicates that the central metal ion, and its sublimation temperature, has a crucial role in defining the morphology of the resulting film. Structurally related Cu(II) and Ni(II) dithiadiazoletetraaza[14]annulene (dttaa) macrocycles were synthesised and studied in the context of their thin film electrochemical, conducting and morphological properties. Both the Ni(II) and Cu(II) complexes were found to be volatile under reduced pressure, which allowed crystals of both materials to be grown and the single crystal structures solved. Interestingly, the crystal packing of these heterocyclic macrocycles varies depending on whether the central metal ion is Cu(II) or Ni(II), which is in contrast to the analogous dibenzotetrazaannulenes complexes. Soluble Ni(II) analogues containing benzoyl groups on the meso- positions of the macrocycle (dttaaBzOR) were also prepared and contrasted with the insoluble Ni(dttaa) complexes in terms of their solution optical and electrochemical properties. Thin film electrochemical studies of Cu(dttaa) and Ni(dttaa) showed chemically reversible oxidative processes but on scanning to reductive potentials the films disintegrated almost immediately as the bulky counter tetrabutylammonium cation entered the thin film. FET studies undertaken on polycrystalline films of both complexes, using various device configurations and surface treatments, failed to realise any gate effect. Thin film XRD measurements indicate that films of both complexes formed by vacuum deposition are crystalline and contain a mixture of molecular alignments, with molecules aligning “edge on” and “face down” to the substrate. SEM imaging failed to effectively resolve the morphology of the films implying the sizes of the crystallites are small, which may help to explain the lack of FET effect. A series of bis-ligand diimine Ni, Cu and Pd complexes have been synthesised from the ligand 4,5-bis(dodecyloxy)benzene-1,2-diamine (dbdaH2). The same ligand was also used to prepare a series of soluble Cu(II) and Ni(II) tetraaza[14]annulene macrocycles. All the bis-ligand diimine complexes were found to suffer from instability in air due to the ease at which the complexes are oxidised. The Ni complex, Ni(dbda)2, was found to display a NIR transition in the region of 971 to 1024 nm depending on the polarity of the solvent that the molecule is dissolved in. Solution electrochemistry studies of Ni(dbda)2 reaffirmed the facile nature of the first oxidative process, with the HOMO energy calculated at -4 eV by hybrid-DFT. This compound failed to yield semiconducting behaviour in an FET device despite the use of surface treatments aimed at promoting suitable molecular alignment across the conducting channel.

In this work Scanning Kelvin Probe Microscopy (SKPM) was used to characterize surface states and device surface charging in nitride materials. Samples grown by Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD) and Hydride Vapor Phase Epitaxy (HVPE) typically show a high surface band bending of about 1 eV. In an n-type sample with 3X1017 cm-3 carrier concentration, 1 eV upward band bending corresponds to 1.7X1012 cm-2 trapped charge density in the surface states. Under continuous ultraviolet (UV) illumination up to 0.6 eV surface photo voltage effect could be observed in some samples, which further indicates that surface band bending is very likely larger than 0.6 eV, i.e. close to 1 eV. Reactive Ion Etching (RIE)damage was observed to increase surface band bending by about 0.4 eV where as surface treatments in organic solvents and inorganic acids did not affect surface band bending significantly. These results indicate presence of high density of surface states in devices fabricated in nitride materials. Surface potential measurements immediately after turning off a reverse bias to the Schottky contact of a GaN Schottky diode as well as an AlGaN/GaN Hetero-junction Field Effect Transistor (HFET) show an increase of band bending near the Schottky contact edge. For an applied reverse bias of 4 V, about 0.5 eV increase of band bending was observed. This increase of band bending was caused by tunneling of electrons from the Schottky contact and their subsequent capture by surface states near the contact edge. In case of the HFET, the increase of band bending for a bias that caused no current flow through the device was similar to a bias that did. This showed that hot electron injection from the channel did not play a significant role in increasing surface band bending. The accumulated charge near the gate edge of a HFET can deplete the channel, which would cause the drain current to decrease. The total times of accumulation and dissipation of excess surface charge near the gate edge of the HFET were comparable to the time scales of drain current transients of current collapse and recovery. From this observation we attributed current collapse phenomena to charge accumulation near the edge of the reverse biased gate contact of a HFET.

Engineers know well from an early point in their training the trials and tribulations of having to make design tradeoffs in order to optimize one performance parameter for another. Discovering tradeoff conditions that result in the elimination of a loss associated with the enhancement of some other parameter (an improvement over a typical tradeoff), therefore, ushers in a new paradigm of design in which the constraints which are typical of the task at hand are alleviated. We call such a design paradigm “tuning” as opposed to “trading off”, and this is the central theme of this work. We investigate two types of microwave electronic devices, namely GaN-based heterostructure field effect transistors (HFETs) and tunable ferroelectric-ferrite-based microwave phase shifters. The “tuning” associated with these types of devices arises from the notion of an optimal 2DEG density, capable of achieving higher performance in terms of electron velocity and enhanced reliability in the case of the HFET, and the coupling of ferroelectric and ferrite materials in tunable microwave phase shifters, capable of achieving high differential phase shifts while at the same time mitigating the losses associated with impedance mismatching which typically arise when the phase is tuned. Promises and problems associated with HFET devices based on the intriguing InAlN/GaN material system will be described. We focus on the fundamental problem associated with the induction of the large density of carriers at the interface, namely the disintegration of an excess of longitudinal optical phonons (hot phonons) in the channel. We use microwave measurements in conjunction with stress tests to evidence the existence of an optimal 2DEG density wherein the hot phonon effect can be “tuned,” which allows for enhanced high frequency performance as well as device reliability. Next, we focus on the design, fabrication, and measurement of tunable phase shifters consisting of thin films of BaxSr1-xTiO3 (BST), which has the advantage of having high dielectric tunability as well as relatively low microwave loss. We discuss the design, fabrication, and measurement of a simple coplanar waveguide (CPW) type of phase shifter as well as a more complicated “hybrid” phase shifter consisting of a ferrite (YIG) in addition to BST. The use of such a bilayer allows one to “tune” the impedance of the phase shifters independently of the phase velocity through careful selection of the DC biasing magnetic fields, or alternatively through the use of an additional piezoelectric layer, bonded to YIG whose permeability can then be tuned through magnetostriction.

Cette thèse démontre le potentiel d'utilisation du graphène pour la fabrication de transistors à effet de champ à couche mince. Celui-ci est préparé par exfoliation en phase liquide et co-déposé avec un polymère semiconducteur du type n. Cette stratégie montre que le graphène améliore le comportement ambipolaire du polymère et plus particulièrement le transport des trous ce qui renforce l'application des matériaux composites au graphène dans les circuits logiques.Par la même approche de mélange, de nouveaux nanorubans de graphène dispersés en solution, ont été utilisés pour améliorer la performance des dispositifs basés sur un polymère amorphe de type p. Ces nanorubans forment une voie de percolation pour les charges améliorant ainsi la performance des dispositifs dans l'obscurité ainsi que sous illumination. Finalement, les dispositifs photosensibles multifonctionnels ont été examinés par l'introduction de molécules photochromiques avec différents substituants au sein des films semi-conducteurs à base de polymère ou de molécules de petite taille qui ont été trouvés influer la photocommutation. / This thesis demonstrates that graphene produced by liquid-phase exfoliation can be co-deposited with a polymerie semiconductor for the fabrication of thin film field-effect transistors. The introduction of graphene to the n-type polymeric matrix enhances not only the electrical characteristics of the devices, but also the ambipolar behavior and the hole transport in particular. This provides a prospective pathway for the application of graphene composites for logic circuits.The same approach of blending was adopted to enhance the electrical characteristics of an amorphous p-type polymer semiconductor by addition of an unprecedented solution processable ultra-narrow graphene nanoribbon. GNRs form percolation pathway for the charges resulting in enhanced deviee performance in daras weil as under illumination therefore paving the way for applications in (opto)electronics.Finally, multifunctional photoresponsive devices were examined by introducing photochromic molecules exposing different substituents into small molecule or polymeric semiconductor films that were found to affect the photoswitching behavior.

This Ph.D. thesis focuses on the high-frequency electrical capabilities of the carbon-nanotube, field-effect transistor (CNFET). The thesis can be categorized into three stages, leading up to an assessment of the RF capabilities of realistic array-based CNFETs. In the first stage, the high-frequency and time-dependent behavior of ballistic CNFETs is examined by numerically solving the time-dependent Boltzmann transport equation (BTE) self-consistently with the Poisson equation. The RF admittance matrix, which contains the transistor’s y-parameters, is extracted. At frequencies below the transistor’s unity-current-gain frequency fT, the y-parameters are shown to agree with those predicted from a quasi-static equivalent circuit, provided that the partitioning factor for the device charge is properly extracted. It is also shown that a resonance behavior exists in the transistor’s y-parameters. In the second stage, non-quasi-static effects in ballistic CNFETs are examined by analytically developing a transmission-line model from the BTE and Poisson equation. This model includes nonclassical transistor elements, such as the "quantum capacitance" and "kinetic inductance," and it is shown to represent the intrinsic (contact-independent) transistor’s behavior at high frequencies, including a correct prediction of the resonances in the y-parameters. Moreover, it is shown that the kinetic inductance can be represented using lumped elements in the transistor’s small-signal equivalent circuit, and it is demonstrated that the resulting circuit is capable of modeling intrinsic CNFET behavior to frequencies beyond fT. In the last stage, by building upon the first two stages, a comprehensive study is performed to assess the RF performance potential of array-based CNFETs. First, phonon scattering is incorporated into the time-dependent BTE to study the impacts of collisions on different aspects of intrinsic CNFET operation, including the intrinsic fT and the small-signal equivalent circuit. These results are then further extended by adding the effects of extrinsic (contact-dependent) parasitics and then examining the behavior of key RF figures of merit, such as the extrinsic fT, the attainable power gain, and the unity-power-gain frequency. The results are compared to those of state-of-the-art high-frequency transistors and to the next generation of RF CMOS, and they provide an indication of the potential advantages of array-based CNFETs for RF applications. / Micro-Electro-Mechanical Systems (MEMS) and Nanosystems