In this these, in order to enhance the n-channel organic field-effect transistor (OFET) performance, amino functionalized self-assembled monolayers (A-SAMs) which consist of amino groups, a well-known n-type dopant candidate, were introduced from the top of OFET surfaces and on the gate oxide surfaces. To obtain better understanding for optimization of OFET performances we attempted to elucidate the mechanism of surface doping and surface modification by A-SAMs. Both the surface doping and surface modification of the gate oxide approaches have individual pros and cons. One needs to take into account the surface energy properties of SAMs and the resulting OSC film structure and pick the most suitable method to introduce the SAM material to the OFET (either doping or oxide modification) in order to obtain optimized device performances. Our study strongly suggests that both surface doping and surface modification of the gate oxide with A-SAMs could enhance other semiconductor-based electronic device performances.:Abstract v Chapter 1. Introduction 1 Chapter 2. Theoretical Background 7 2.1. Organic Semiconductors (OSCs) 8 2.1.1. Semiconducting properties of organic molecules 8 2.1.2. Charge Transport Mechanism in OSCs 10 2.2. Organic Field-Effect Transistors (OFETs) 18 2.2.1. Operation Principle 18 2.2.2. Device Geometry of OFETs 20 2.2.3. Contacts (metal/semiconductor junction) in OFETs 21 2.2.4. Dielectric material for OFETs 23 2.2.5. Current-Voltage Characteristics of OFETs 25 2.3. Dominant contributors to OFET Performance 32 2.3.1. Molecular structure and Orientation of OSCs 32 2.3.2. Dielectric/OSC Interface 33 2.3.3. OSC/Contact Interface (Contact resistance) 35 2.3.4. Shallow and deep traps 36 2.4. Strategies to improve OFET performance 37 2.4.1. Introducing dopants to OFETs 37 2.4.2. Modification of Gate Oxide Layer with SAMs 44 Chapter 3. Experimental 51 3.1. Device Fabrication 52 3.1.1. Device type I - Substrate/ODTMS/PTCDI-C8/Au 53 3.1.2. Device type II - Substrate/ODTCS/N2200 (PNDI2OD-2T)/Au 53 3.1.3. Device type III - Substrate/SAMs/PTCDI-C8/Au 54 3.2. Surface doping process 56 3.2.1. Surface dopant – Aminosilanes (A-SAMs) 56 3.2.2. Surface doping method 56 3.3. Characterization 59 3.3.1. Material characterization 59 3.3.2. Surface-wetting characterization - Contact angle measurement 61 3.3.3. Micro-structure characterization - Atomic Force Microscopy (AFM) 62 3.3.4. Surface potential characterization – Kelvin Probe Force Microscopy (KPFM) 63 3.3.5. Molecular Structure Characterization - Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) 64 3.3.6. Electrical Characterization - Current-voltage (I-V) measurement 66 Chapter 4. Result and Discussion 69 4.1. Optimization of OFETs based on PTCDI-C8 and N2200 70 4.1.1. PTCDI-C8 OFETs 70 4.1.2. N2200 OFETs 72 4.1.3. Device measurement condition 75 4.2. Investigation of Surface doping mechanism of Aminosilanes 77 4.2.1. Surface doping effect depending on the dopant processing method 77 4.2.2. Surface doping effect for different types of organic semiconductors 80 4.2.3. Surface doping effect for different types of surface dopants 89 4.2.4. Surface doping effect for different OSC grain sizes 92 4.2.5. Surface doping effect for different OSC film thicknesses 103 4.2.6. Molecular structure of the doped films identified by GIWAXS 106 4.2.7. Stability of the surface doped OFETs 107 4.2.8. Summary 111 4.3. Modification of the gate oxide with various self-assembled monolayers 112 4.3.1. The surface property of SAM-treated substrates 112 4.3.2. The relation between the OSC morphology and the field-effect mobility 115 4.3.3. The origin of the threshold voltage shift 126 4.3.4. Memristive effects in PTCDI-C8 devices on ODTMS 133 4.3.5. Summary 137 4.4. Comparison of the surface doping and the modification of the gate dielectric 138 4.4.1. The reliability factor of OFETs 138 4.4.2. The threshold voltages and field-effect mobility of OFETs 141 4.4.3. Density of Interfacial trap sites and SAM induced mobile carriers 143 4.4.4. Summary 144 Chapter 5. Conclusion 145 Bibliography 148 List of Figures 158 List of Tables 166 List of Equations 167 Acknowledgment 168 Erklärung zur Eröffnung des Promotionsverfahrens 169

Die vorliegende Arbeit ist eine umfassende Studie über das Wachstum mittels Molekularstrahlepitaxie (MBE) und die gemessenen Seebeck Koeffizienten und Lochtransport Eigenschaften von p‑Typ Oxiden, eine Materialklasse welche die optische Transparenz und die einstellbare Leitfähigkeit verbindet. Insbesondere, Nickeloxid (NiO) und Zinnmonoxid (SnO) wurden mittels plasmaunterstützter MBE unter Einsatz von einer Metall‑Effusionszelle und einem Sauerstoffplasma gewachsen. Für das NiO Wachstum wurden vor allem die Wachstumsgrenzen bei hohen Temperaturen festgelegt, welche von der Substratstabilität im Falle von Magnesiumoxid und Galliumnitrid abhängen. Es wird die Möglichkeit der Qualitätsbewertung mittels Ramanspektroskopie für Natriumchlorid-Strukturen gezeigt. Untersuchung der NiO Dotierung durch Oberflächen-Akzeptoren und der damit verbundenen Oberflächen‑Loch‑Anreicherungsschicht offenbart eine neue Dotierungsmöglichkeit für p‑leitende Oxide im Allgemeinen. Die metastabile Phase des SnO wird mittels PAMBE unter Verwendung bekannter Wachstumskinetik von Zinndioxid und verschiedener in‑situ Methoden stabilisiert, die anwendungsrelevante thermische Stabilität wird untersucht. Anschließende ex‑situ Charakterisierungen durch XRD und Ramanspektroskopie identifizieren das kleine Wachstumsfenster für das epitaktische Wachstum von SnO. Elektrische Messungen bestätigen die p‑Typ Ladungsträger mit vielversprechenden Löcherbeweglichkeiten welche auch für Hall Messungen zugänglich sind. Temperaturabhängige Hall Messungen zeigen einen bandähnlichen Transport welcher auf eine hohe Qualität der gewachsenen Schichten hindeutet. Die Funktionalität der gewachsenen Schichten wird durch verschiedene Anwendungen nachgewiesen. Zum Beispiel werden pn‑Heteroübergänge wurden durch das heteroepitaktische Wachstum der SnO Schichten auf einem Galliumoxid-Substrat erlangt. Die ersten bisher berichteten SnO-basierten pn‑Übergänge mit einem Idealitätsfaktor unter zwei wurden erreicht. / This thesis presents a comprehensive study on the growth by molecular beam epitaxy (MBE) and the measured Seebeck coefficients and hole transport properties of p‑type oxides, a material class which combines transparency and tunable conductivity. Specifically, Nickel oxide (NiO) and tin monoxide (SnO) were grown by plasma‑assisted MBE using a metal effusion cell and an oxygen plasma. For NiO growth, the focus lies on high temperature growth limits which were determined by the substrate stability of magnesium oxide and gallium nitride. Quality evaluation by Raman spectroscopy for rock‑salt crystal structures is demonstrated. Investigations of NiO doping by surface acceptors and the related surface hole accumulation layer reveal a new doping possibility for p‑type oxides in general. The meta‑stable SnO is stabilized by PAMBE utilizing known growth kinetics of tin dioxide and various in‑situ methods, its application-relevant thermal stability is investigated. Following ex‑situ characterizations by XRD and Raman spectroscopy identify secondary phases and a small growth window for the epitaxial growth of SnO. Electrical measurements confirm the p‑type carriers with promising hole mobilities accessible to Hall measurements. Temperature dependent Hall measurements show band‑like transport indicating a high quality of the grown layers. The functionality of the grown layers is proven by various applications. For example, pn‑heterojunctions were achieved by heteroepitaxial growth of the SnO layers on gallium oxide substrates. The first reported SnO based pn‑junction with an ideality factor below two is accomplished.