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Since the seminal discovery of conductive polymers four decades ago, organic electronics has grown from an exploratory field to an industry offering novel consumer products. Research has led to the synthesis of new organic molecules and polymers and their applications: organic field-effect transistors (OFETs), organic light-emitting diodes, and organic photovoltaics. The goal for research as well as for industry is producing low-cost, flexible, and, ultimately, sustainable, electronics. Although on the rise, organic electronics faces several challenges: air instability, reliability, and scaling, to name a few. And despite that organic devices and larger systems have been demonstrated, there remains a gap in understanding underlying mechanisms behind light absorption, photoconduction, charge transport and conduction in them. The primary purpose of this thesis is to use a relatively under utilized technique, photocurrent microscopy (PCM), to directly probe charge carriers in pentacene and 6,13-Bis(triisopropylsilylethynyl) (TIPS) pentacene FETs to learn about charge injection and transport. The latter part of the thesis focuses on the use of thiols to modify electrode properties to both increase charge injection efficiency and to provide passivation to low-work function metal electrodes. It is demonstrated for the first time experimentally by directly probing the OFET channel that top-contact geometry OFETs suffer minimally from a charge injection barrier, and that trap filling and altering of trap density-of-states in the channel is directly observable with PCM. PCM was used to investigate grains and grain boundaries in TIPS-pentacene devices. By varying gate bias, it was shown that the PCM maps of grains are not simply a result of varying absorption on the surface of the film; rather, it is an artefact of charge transport between grains and grain boundaries. Through this study, PCM was shown to be a useful, large-area scanning technique, for observing transport in devices with large (on the order of 50 $\mu$m) grains. This is particularly relevant as solution-proccessable films are likely to dominate the flexible electronics industry. The thiol portion of this thesis compares the impact of two distinct thiols on bottom-contact pentacene FETs: perfluorodecanethiol (PFDT) and pentafluorobenzenethiol (PFBT). Using X-ray photoelectron spectroscopy to measure metal oxidation, it was determined that short aromatic thiols are poor choices for low work-function metal passivation. In addition, both passivation and charge injection enhancement can be achieved with long fluorinated alkanethiols. However, there is a trade-off between passivation and on-current. The enhancement of on-current in thiol-treated Cu-electrode pentacene devices is most likely not morphology related, due to the fact that PFDT was found to be in a standing-up orientation on the metal surface. Additionally, it was demonstrated that although highly electronegative atoms such as fluorine can beneficially modify metal work function, too many fluorine atoms in thiols can lead to too high a work function and a large mismatch between the pentacene highest-occupied-molecular-level and metal work function.

Yu, Xiaojiang. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Abstracts in English and Chinese. / ABSTRACT (ENGLISH) --- p.I / ABSTRACT (CHINESE) --- p.III / ACKNOWLEDGEMENTS --- p.IV / TABLE OF CONTENTS --- p.V / Chapter 1. --- Overview of organic thin-film transistors (OTFTs) --- p.1 / Chapter 1.1 --- Introduction to OTFTs --- p.1 / Chapter 1.2 --- Basic mechanism of OTFTs --- p.4 / Chapter 1.3 --- Applications of OTFTs --- p.6 / Chapter 1.3.1 --- Driving of circuits for electronic papers and LCD --- p.6 / Chapter 1.3.2 --- Light-emitting OTFTs --- p.8 / Chapter 1.3.3 --- Sensing --- p.9 / Chapter 1.4 --- Several key issues --- p.9 / Chapter 1.4.1 --- Mobilities of OTFTs --- p.9 / Chapter 1.4.2 --- Performance of bottom-contact OTFTs --- p.10 / Chapter 1.4.3 --- Stability of OTFTs --- p.11 / Chapter 1.4.4 --- Performance of n-type organic semiconductors --- p.13 / Chapter 1.5 --- Why to study metallophthalocyanine-based OTFTs --- p.14 / Chapter 1.6 --- Objective of this thesis --- p.17 / References --- p.17 / Chapter 2. --- Experimental details for fabrication and characterization of OTFTs --- p.22 / Chapter 2.1 --- Purification of organic semiconductors --- p.22 / Chapter 2.2 --- Preparation of the gate dielectrics for OTFTs --- p.24 / Chapter 2.3 --- Deposition of organic thin films and gold source/drain electrodes --- p.26 / Chapter 2.4 --- Process flow for the fabrication of OTFTs --- p.27 / Chapter 2.5 --- Mobility measurement for the organic thin films --- p.28 / Chapter 2.6 --- Characterization of organic thin films --- p.31 / References --- p.31 / Chapter 3. --- Optimizing the growth of VOPc thin films for high-mobility OTFTs --- p.33 / Chapter 3.1 --- Experimental --- p.33 / Chapter 3.2 --- Results and discussion --- p.34 / Chapter 3.2.1 --- Growth of VOPc thin films on Si02 dielectric --- p.34 / Chapter 3.2.2 --- Growth of VOPc thin films on Ta205 and Al203/Si02 dielectrics --- p.41 / Chapter 3.4 --- Conclusion --- p.44 / References --- p.45 / Chapter 4. --- CuPc/CoPc and VOPc/CoPc p-type/p-type heterostructure OTFTs --- p.46 / Chapter 4.1 --- CuPc/CoPc OTFTs in sandwich configuration --- p.47 / Chapter 4.1.1 --- Experimental --- p.47 / Chapter 4.1.2 --- Results and discussion --- p.48 / Chapter 4.1.3 --- Conclusion --- p.57 / Chapter 4.2 --- VOPc/CoPc OTFTs --- p.57 / Chapter 4.2.1 --- Experimental --- p.57 / Chapter 4.2.2 --- Results and discussion --- p.58 / Chapter 4.2.3 --- Conclusion --- p.63 / References --- p.64 / Chapter 5. --- VOPc/F16CuPc p-type/n-type heterostructure OTFTs --- p.66 / Chapter 5.1 --- Unipolar VOPc/F16CuPc OTFTs --- p.67 / Chapter 5.1.1 --- Experimental --- p.67 / Chapter 5.1.2 --- Results and discussion --- p.69 / Chapter 5.1.3 --- Conclusion --- p.73 / Chapter 5.2 --- VOPc/F16CuPc heterostructure for bottom-contact OTFTs --- p.74 / Chapter 5.2.1 --- Experimental --- p.74 / Chapter 5.2.2 --- Results and discussion --- p.74 / Chapter 5.2.3 --- Conclusion --- p.77 / References --- p.77 / Chapter 6. --- Summary and future work --- p.80 / Summary --- p.80 / Future work --- p.81 / References --- p.83 / Appendix A: Capacitance-voltage (C-V) fitting for ITO/organic junction/AI devices --- p.85 / Appendix B: Can electric-filed influence the growth of organic thin films? --- p.89 / Appendix C: Micro-Raman study on organic thin films --- p.93 / Appendix D: Publications which contributed to this thesis --- p.97

acase@tulane.edu / Ever since the successful isolation of graphene, plenty of researches have been pursued to study fundamental physics in low-dimensional van der Waals materials, referred to as materials with the existence of out of plane vdW force. Not only graphene but also many other novel vdW materials start to emerge and play important roles in quantum physics. Due to the highly preserved crystal quality of the nanostructures achieved by micromechanical exfoliation, a variety of new phenomenon have been discovered in these novel materials. This dissertation focuses on the discovery and electronic properties study of new vdW materials both in 2D and 1D systems. Semiconducting transition metal dichalcogenides with layered structure have been viewed as the promising channel materials for field-effect transistors (FETs) in modern electronics. To characterize the performance, we have fabricated FETs based on multilayer WS2 thin crystals. By using gold as the contact metal and varying the thickness of the crystal, high-performance FETs with on/off ratio of 108 and mobility up to 234 cm2V-1s-1 at room temperature have been realized. The high performance is associated with the minimized Schottky barrier and a shallow impurity level below the conduction band. Elementary substance and binary compound crystals have limited members belong to 2D or 1D family. Thus, expanding the research to ternary compound materials is necessary. In this regard, we focused on a novel ternary compound 2D material Nb3SiTe6 and studied its magneto-transport. We have discovered that by using such a high crystalline 2D metal, we could study the inelastic electron-phonon (e-ph) interactions involved with reducing dimensions. From 3D bulk to 2D films with a rigid substrate, the weak antilocalization (WAL) signature is gradually enhanced according to our magnetoresistance (MR) measurements. Systematic studies of the temperature dependence of the dephasing rate in the crystal with various thicknesses suggest the suppression of electron-phonon interaction due to quantum confinement of the phonon spectrum. Our work shows great consistency with the long-standing predicted theory. We have successfully expanded the mechanical exfoliation method to 1D material group. As demonstrated by semiconducting quasi-1D materials, Ta2Pd3Se8 (TPdS) and Ta2Pt3Se8 (TPtS), the external force can efficiently break the weak vdW interactions between ribbons. In our work, we have produced ultrathin 1D TPdS and TPtS nanowires, and fabricated 1D FETs showing p-type and n-type transistor behavior respectively. Moreover, we have successfully built the functional logic NOT gate using these two different 1D FETs. / 1 / Xue Liu

The dependence of the substrate resistance, R[subscript sub], for MOS transistor RF modeling on transistor biasing and layout is studied from device simulations and measurements. Though R[subscript sub] is found to be bias dependent, the error incurred by assuming a constant value equal to the DC resistance is not significant. A scalable model for R[subscript sub] of multiple gate fingers is developed. This model is simple to extract and gives good agreement for the output admittance of a MOSFET. The model is validated by measurements on DC test structures fabricated in a TSMC 0.35 ��m CMOS process. The dependence of Rb on transistor dimensions and the location of substrate contacts with respect to device active area is also presented. A low noise amplifier (LNA) is designed and fabricated in the 0.35 ��m TSMC process to show the effect of R[subscript sub] on the performance of a LNA. / Graduation date: 2003

Complementary delta-doped AlGaAs/GaAs Heterojunction Field Effect Transistor (CHFET) devices and circuits were fabricated using MBE and a 2�� non-planar gate recess process. Several schemes were used in an attempt to improve the performance of the p-channel HFETs. These included delta-doping, carbon-doping and dipole-doping. Circuits and individual n- and p- channel devices were fabricated on a stacked delta-doped complementary structure. The circuits failed to perform due to complications with adjusting the threshold voltage. However, Individual devices were successfully characterized, p-channel devices with extrinsic transconductances up to 14 mS/mm, n-channel devices with extrinsic transconductances up to 120 mS/mm and a unity power gain bandwidth of 5.5 GHz. / Graduation date: 1995

Graduation date: 1994

Graduation date: 1994

Thesis (Ph.D.)--Stanford University, 2006. / Adviser: Krishna C. Saraswat. Includes bibliographical references (p. 160-177)