The dielectric polarization processes, conduction mechanisms and space charge effects occurring in tantalum / tantalum pentoxide / metal devices are investigated. The dielectric properties are analyzed on the basis of an ionic relaxation process with a nearly flat distribution of activation energies. This distribution leads to step response polarization currents following an inverse time law. The effect of an injected electronic space charge on the response of the device due to the removal of a step voltage is analyzed and results are given demonstrating this effect. The devices used exhibit a rectification behaviour. For tantalum positive the currents follow a Schottky law and for tantalum negative, the bulk Poole-Frenkel law. Hysteresis effects are observed as well as the effects of a space charge on the Schottky law currents. On one sample, sufficiently high fields causes an increase in the conductance by a factor of 10³ to 10⁵. For this deformed sample, no hysteresis or rectification is observed and the currents follow a Schottky law for both polarities. The validity of the ionic relaxation model is discussed in light of the observed dielectric losses at low temperatures. The experimental results indicate that an electronic rather than an ionic process could be responsible for the dielectric losses. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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by Man Wah-Kit. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 177-180). / LIST OF FIGURES / LIST OF TABLES / abstract --- p.1 / Chapter 1. --- introduction --- p.3 / Chapter 2. --- fabrication process / Chapter 2-1 --- INTRODUCTION --- p.7 / Chapter 2-2 --- PROCESS DEVELOPMENT --- p.8 / Chapter 2-3 --- FABRICATION PROCEDURES FOR TIN FILMS --- p.10 / Chapter 2-4 --- FABRICATION PROCEDURES FOR TIN OXIDE FILMS --- p.14 / Chapter 2-5 --- FABRICATION THEORY --- p.21 / Chapter 2-6 --- OXYGEN ION IMPLANTATION OF TIN FILMS --- p.24 / Chapter 3. --- structural characterization / Chapter 3-1 --- INTRODUCTION --- p.30 / Chapter 3-2 --- MICROSTRUCTURE / Chapter 3-2-1 --- SOME RELATED THEORIES OF GRAIN GROWTH / Chapter (1) --- Classical Theory of Grain Growth --- p.30 / Chapter (2) --- Hillock Growth --- p.31 / Chapter (3) --- Dislocation Creep Theory --- p.33 / Chapter (4) --- Biaxial Stress in Thin Films --- p.35 / Chapter (5) --- Surface Cluster Growth --- p.37 / Chapter 3-3 --- EXPERIMENTATION AND RESULTS / Chapter 3-3-1 --- MICROSTRUCTURAL ANALYSIS UNDER OPTICAL MICROSCOPE --- p.39 / Chapter 3-3-2 --- THE STRESS AND HILLOCK HEIGHT ANALYSIS OF TIN OXIDE FILMS --- p.48 / Chapter 3-3-3 --- MICROSTRUCTURAL ANALYSIS BY MEANS OF ATOMIC FORCE MICROSCOPE (AFM) --- p.52 / Chapter 3-3-4 --- MICROSTRUCTURAL ANALYSIS BY X-RAY DIFFRACTION --- p.69 / Chapter 3-3-5 --- SURFACE ANALYSIS BY MEANS OF X-RAY PHOTOELECTRON SPECTROSCOPY / Chapter (1) --- Introduction --- p.73 / Chapter (2) --- Basic Theory --- p.73 / Chapter (3) --- Experimentation And Results --- p.75 / Chapter 3-3-6 --- SURFACE STUDY OF ION IMPLANTED TIN OXIDE FILMS / Chapter (1) --- Experimental Results --- p.82 / Chapter 3-4 --- DISCUSSION / Chapter 3-4-1 --- QUALITATIVE ANALYSIS OF MICROSTRUCTURE WITH THE OPTICAL MICROSCOPE --- p.88 / Chapter 3-4-2 --- QUALITATIVE ANALYSIS OF MICROSTRUCTURE WITH SEM AND AFM / Chapter (1) --- Grain Growth of Tin Oxide Films --- p.89 / Chapter (2) --- Dependence of Grain Size on Deposition Rate --- p.91 / Chapter (3) --- Dependence of Grain Size on Film Thickness --- p.92 / Chapter (4) --- Dependence of Grain Size on Substrate Temperature --- p.92 / Chapter (5) --- Origin of Hillock Growth of Tin Oxide Films --- p.93 / Chapter 3-4-3 --- FILM COMPOSITIONAL ANALYSIS WITH X-RAY DIFFRACTION --- p.95 / Chapter 3-4-4 --- SURFACE ANALYSIS WITH X-RAY PHOTOELECTRON SPECTROSCOPY …… --- p.95 / Chapter 3-4-5 --- SURFACE ANALYSIS OF OXYGEN IMPLANTED TIN FILMS --- p.96 / Chapter 4. --- OPTICAL CHARACTERIZATION / Chapter 4-1 --- INTRODUCTION --- p.98 / Chapter 4-2 --- THEORY / Chapter (1) --- Free Electron Model --- p.99 / Chapter (2) --- Effect of Film Thickness --- p.100 / Chapter (3) --- Effect of Oxygen Contents --- p.101 / Chapter (4) --- Electron-Lattice Interaction and Bandgap Studies --- p.102 / Chapter 4-3 --- EXPERIMENTATION AND RESULTS --- p.105 / Chapter 4-4 --- DISCUSSION / Chapter 4-4-1 --- BANDGAP STUDIES FOR TIN OXIDE FILMS WITH DIFFERENT DEPOSITION CONDITIONS / Chapter (1) --- Variation of Film Thickness --- p.122 / Chapter (2) --- Film Appearance --- p.123 / Chapter (3) --- Variation of Substrate Temperature --- p.123 / Chapter (4) --- Variation of Oxidation Conditions --- p.123 / Chapter 5. --- ELECTRICAL CHARACTERIZATION / Chapter 5-1 --- INTRODUCTION --- p.126 / Chapter 5-2 --- RELATED THEORY / Chapter 5-2-1 --- CURRENT-VOLTAGE (I-V) CHARACTERISTICS --- p.127 / Chapter 5-2-2 --- CAPACITANCE-VOLTAGE (C-V) CHARACTERISTICS --- p.131 / Chapter 5-2-3 --- RELATION OF ELECTRICAL TO STRUCTURAL PROPERTIES / Chapter (A) --- Effects of Deposition Conditions --- p.133 / Chapter (B) --- Effects of Grain Boundaries --- p.133 / Chapter (C) --- Effects of Ionic Impurities --- p.134 / Chapter (D) --- Effects of The Interface Properties --- p.134 / Chapter 5-2-4 --- MEASURING TECHNIQUES / Chapter (A) --- I-V Measurment of Tin Oxide on a Silicon Substrate --- p.136 / Chapter (B) --- C-V Measurement of Tin Oxide Films on Silicon Substrates --- p.137 / Chapter (C) --- Electrical Measurement of Tin Oxide Films on a Quartz Substrate --- p.137 / Chapter 5-3 --- EXPERIMENTATION --- p.138 / Chapter 5-4 --- RESULTS --- p.141 / Chapter 5-5 --- DISCUSSION / Chapter 5-5-1 --- Analysis of the Conduction Mechanism for Sn02/Si n-p Heterojunctions --- p.161 / Chapter 5-5-2 --- Analysis of the Conduction Mechanism for Sn02/Si n-n Heterojunctions --- p.162 / Chapter 5-5-3 --- Effect on the Conduction Mechanisms of Film Thickness --- p.164 / Chapter 5-5-4 --- Effect on the Conduction Mechanisms of Oxidation Time --- p.166 / Chapter 5-5-5 --- Interfacial Properties of SnOx/Si Heterojunctions --- p.166 / Chapter 5-5-6 --- Electrical Properties of SnOx Films on Quartz / Chapter (1) --- Dependence of Film Conductivity on Measuring Temperatures --- p.168 / Chapter (2) --- Dependence of Film Conductivity on Oxidation Time --- p.168 / Chapter (3) --- Dependence of Film Conductivity on Oxidation Temperature --- p.169 / Chapter (4) --- Invariance of Film Conductivity at Some Certain Measuring Temperatures --- p.170 / Chapter (5) --- Activation Energy of Sn02 Films on Quartz --- p.170 / Chapter 6. --- CONCLUSIONS --- p.172 / Chapter 7. --- FUTURE WORKS --- p.175 / Chapter 8. --- REFERENCES --- p.177 / Chapter 9. --- APPENDICES / Chapter 9-1 --- APPENDIX A List of photos --- p.181 / Chapter 9-2 --- APPENDIX B (1) ED AX results for some selected regions on samples with hillocks --- p.182 / Chapter (2) --- Relations between mean surface roughness and oxidation conditions --- p.185 / Chapter (3) --- XPS original data and typical XPS spectra for vacuum- evaporated SnO2 thin film --- p.186 / Chapter 9-3 --- "APPENDIX C Variations of optical parameters, refractive index n and extinction coefficient k in visible region with different oxidation conditions" --- p.189 / Chapter 9-4 --- APPENDIX D Electrical results for Sn02/Si heterojunction s --- p.191 / Chapter 9-5 --- APPENDIX E Calculations of band diagram for Sn02/Si heterojunctions --- p.194 / Chapter 9-6 --- APPENDIX F Resistivity versus impurity concentration for silicon at 300K --- p.196

Solution processed circuits are expected to be the main components to achieve low cost, large area, flexible electronics. However, the commercialization of solution processed flexible electronics face several challenges. The passive component such as capacitors are limited in frequency range and operating voltage. The active component such as transistors suffer from low mobility ultimately leading to limited current-carrying capacity. Just as in traditional silicon technology, the fabrication process and material choices significantly impact the performance of the fabricated devices. My thesis focuses on the optimization of the performance of printed capacitors and transistors through investigation of several aspects of the device structure and fabrication process. The first part of this work focuses on the optimization of printed nanoparticle/polymer composite capacitors. Thin film metal oxide nanoparticle/polymer composites have enormous potential to achieve printable high-k dielectrics. The combination of high-k ceramic nanoparticle and polymer enables room temperature deposition of high dielectric constant film without the need of high temperature sintering process. The polymer matrix host fills the packing voids left behind by the nanoparticles resulting to higher effective dielectric permittivity as a system and suppresses surface states leading to reduced dielectric loss. Such composite systems have been employed in a number of flexible electronic applications such as the dielectrics in capacitors and thin film transistors. One of the most important properties of thin film capacitors is the breakdown field. In a typical capacitor system, the breakdown process leads to catastrophic failure that destroys the capacitor; however, in a nanoparticle/polymer composite system with self-healing property, the point of breakdown is not well-defined. The breakdown of the dielectric or electrodes in the system limits the leakage observed. It is possible, however, to define a voltage/field tolerance. Field tolerance is defined as the highest practical field at which the device stays operational with low failure rate by qualifying the devices with defined leakage current density. In my work, the optimization of the field tolerance of (Ba,Sr)TiO₃ (BST)/parylene-C composite capacitors is achieved by studying the influence of the electromigration parameter on leakage and field strength through the inherit asymmetrical structure of the fabricated capacitors. One approach to creating these composites is to use a spin-coated nanoparticle film together with vapor deposited polymers, which can yield high performance, but also forms a structurally asymmetric device. The performance of a nanoparticle BST/parylene-C composite capacitor is compared to that of a nanoparticle BST capacitor without the polymer layer under both directions of bias. The composite device shows a five orders of magnitude improvement in the leakage current under positive bias of the bottom electrode relative to the pure-particle device, and four orders of magnitude improvement when the top electrode is positively biased. The voltage tolerance of the device is also improved, and it is asymmetric (44 V vs. 28 V in bottom and top positive bias, respectively). This study demonstrates the advantage of this class of composite device construction, but also shows that proper application of the device bias in this type of asymmetrical system can yield an additional benefit. The dependence of the field tolerance of nanoparticle/polymer composite capacitors on the electromigration parameter of the electrodes is investigated using the symmetrical dielectric system. The breakdown is suppressed by selecting the polarity used in nanoparticle (Ba,Sr)TiO₃/parylene-C composite film-based capacitors. Metals including gold, silver, copper, chromium, and aluminum with comparable surface conditions were examined as the electrodes. The asymmetric silver, aluminum, gold, copper, and chromium electrode devices show a 64 %, 29 %, 28 %, 17 %, 33 %, improvement in the effective maximum operating field, respectively, when comparing bias polarity. The field at which filament formation is observed shows a clear dependence on the electromigration properties of the electrode material and demonstrates that use of electromigration resistant metal electrodes offers an additional route to improving the performance of capacitors using this nanoparticle/polymer composite architecture. The second part of my thesis focuses on the novel pneumatic printing process that enables manipulation of the crystal growth of the organic semiconductors to achieve oriented crystal with high mobility. Small molecule organic semiconductors are attracting immense attention as the active material for the large-area flexible electronics due to their solution processability, mechanical flexibility, and potential for high performance. However, the ability to rapidly pattern and deposit multiple materials and control the thin-film morphology are significant challenges facing industrial scale production. A novel and simple pneumatic nozzle printing approach is developed to control the crystallization of organic thin-films and deposit multiple materials with wide range of viscosity including on the same substrate. Pneumatic printing uses capillary action between the nozzle and substrate combined with control of air pressure to dispense the solution from a dispense tip with a reservoir. Orientation and size of the crystals is controlled by tuning the printing direction, speed, and the temperature of the substrate. The main advantages of pneumatic printing technique are 1) simple setup and process, 2) multi-material layered deposition applicable to wide range of solution viscosity, 3) control over crystal growth. The manipulation of crystal growth will be discussed in the next chapter. This method for performance optimization and patterning has great potential for advancing printed electronics. The dependence of the mobility of printed thin film 6,13-bis(triisopropylsilylethynyl) pentacene [TIPS-pentacene] and C8-BTBT on printing conditions is investigated, and the result indicates that the formation of well-ordered crystals occurs at an optimal head translation speed. A maximum mobility of 0.75 cm²/(Vs) is achieved with 0.3 mm/s printing speed and 1.3 cm²/(Vs) with 0.3 mm/s printing speed at 50C for TIPS-pentacene and C8-BTBT respectively. In summary, pneumatic printing technique can be an attractive route to industrial scale large area flexible electronics fabrication.