The focus of this thesis involves development of highly transparent, n-channel, accumulation- mode thin-film transistors employing a zinc tin oxide (ZTO) channel layer. ZTO-based transparent thin-film transistors (TTFTs) show improved device performance compared to ZnO-based TTFTs. An estimated peak effective mobility for these devices as high as ~100 cm² V⁻¹sec⁻¹ has been observed, although effective mobilities in the range of 20-50 cm²V⁻¹sec⁻¹ are more common. This performance inconsistency may be due, in part, to the large device dimensions employed in developmental test structures and/or to shadow mask misalignment. Typical drain current on-to-off ratios are > 10⁶. Variation in the post-deposition annealing cycle is found to be an effective means to control the threshold voltage and to improve device performance. Optical characterization of these devices indicates ~84% transparency in the visible spectrum as viewed through the source/drain. Another aspect of this thesis research involves the utilization and extension of quantitative polycrystalline TFT device models with the intention of guiding the design and optimization of future TFTs. In particular, subthreshold conduction is assessed in order to estimate the bulk (and/or grain boundary) and interface trap densities. This leads to a consideration of threshold voltage and channel mobility extraction, as well as establishment of the turn-on voltage, V[subscript turn-on] Finally, a third aspect of this thesis research involves a new radio-frequency (RF) magnetron sputtering system, custom-designed and constructed at OSU by Chris Tasker. Contributions to the development of this tool include assisting in the design and implementation of the computer-controlled interlocks utilized for operation of the tool. The experimental flexibility of this new tool is discussed with respect to its applicability in the design and fabrication of future TTFTS. / Graduation date: 2004

The aim of this thesis is to provide an in-depth investigation of zinc tin oxide, Zn1-xSnxOy or ZTO, grown by atomic layer deposition (ALD) as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. The thesis analyzes how changes in the ALD process influence the material properties of ZTO, and how these in turn affect the performance of CIGS solar cells. It is shown that ZTO grows uniformly and conformably on CIGS and that the interface between ZTO and CIGS is sharp with little or no interdiffusion between the layers. The band gap and conduction band energy level of ZTO are dependent both on the [Sn]/([Zn]+[Sn]) composition and on the deposition temperature. The influence by changes in composition is non-trivial, and the highest band gap and conduction band energy level are obtained at a [Sn]/([Zn]+[Sn]) composition of 0.2 at 120  °C. An increase in optical band gap is observed at decreasing deposition temperatures and is associated with quantum confinement effects caused by a decrease in crystallite size. The ability to change the conduction band energy level of ZTO enables the formation of suitable conduction band offsets between ZTO and CIGS with varying Ga-content. It is found that 15 nm thin ZTO buffer layers are sufficient to fabricate CIGS solar cells with conversion efficiencies up to 18.2 %. The JSC is in general 2 mA/cm2 higher, and the VOC 30 mV lower, for cells with the ZTO buffer layer as compared to cells with the traditional CdS buffer layer. In the end comparable efficiencies are obtained for the two different buffer layers. The gain in JSC for the ZTO buffer layer is associated with lower parasitic absorption in the UV-blue region of the solar spectrum and it is shown that the JSC can be increased further by making changes to the other layers in the traditional CdS/i-ZnO/ZnO:Al window layer structure. The ZTO is highly resistive, and it is found that the shunt preventing i-ZnO layer can be omitted, which further increases the JSC. Moreover, an additional increase in JSC is obtained by replacing the sputtered ZnO:Al front contact with In2O3 deposited by ALD. The large gain in JSC for the ZTO/In2O3 window layer stack compensates for the lower VOC related to the ZTO buffer layer, and it is demonstrated that the ZTO/In2O3 window layer structure yields 0.6 % (absolute) higher conversion efficiency than the CdS/i-ZnO/ZnO:Al window layer structure.

Amorphous oxide semiconductors are of potential interest in the display industry due to their high carrier mobility, transparency at visible wavelengths and excellent operational stability. In this dissertation, n-channel zinc-tin oxide thin-film transistors are fabricated based on a solution-based deposition approach, which allows low fabrication cost and high throughput. The effects of device configuration and process conditions on transistor performance are investigated, and circuit applications including inverters, amplifiers, and ring oscillators are demonstrated. Charge transport in the zinc-tin oxide field-effect transistors is also investigated. A transition from thermally-activated to band-like transport is observed with increasing carrier concentration in high mobility samples, which agrees well with the key predictions of the multiple trap and release model and also Mott’s mobility edge model. In addition, velocity distribution of charge carriers is studied with a time-resolved technique. This provides a more detailed picture of charge transport in field-effect transistors. P-channel organic semiconductor field-effect transistors are also investigated with a view to combine them with n-channel amorphous oxide transistors to create a hybrid organic-inorganic complementary technology. / text

Amorphous Oxide semiconductors have gained large interest in the display industry owing to their high carrier mobilities and low fabrication costs. In this thesis, n-channel solution based zinc-tin oxide (ZTO) thin-film transistors (TFTs) are studied from a circuit design perspective. The study includes an iterative process of circuit design, layout and test procedure of the fabricated devices in the lab. The device models used in circuit simulations are refined following the data fed back from each of these iterations which has enabled more accurate design of complex circuits using ZTO devices. The requirement and development of a physical compact model for performing accurate and predictive circuit simulations has been presented. The use of ZTO devices in low cost, transparent and flexible electronic applications has been investigated through the study of basic circuit blocks such as amplifiers, ring oscillators, inverters and a four stage Operational Amplifier. / text

Amorphous Oxide Semiconductors (AOS) are widely being explored in the field of flexible and transparent electronics. In this thesis, solution processed zinc-tin oxide (ZTO) n-channel TFT based circuits are studied. Inverters, single stage amplifiers and ring oscillators are designed, fabricated and tested. 7-stage ring oscillators with output frequencies up to 106kHz and 5-stage ring oscillators with frequencies up to 75kHz are reported. A stable three stage op-amp with a buffered output is designed for a gain of 39.9dB with a unity gain frequency of 27.7kHz. A 7-stage ring oscillator with output frequency close to 1MHz is simulated and designed. The op-amp and the ring oscillator are ready to be fabricated and tested. An RPI model for a-Si, adapted to fit the ZTO device characteristics, is used for simulation. Development of a new model based on the physics behind charge transport in ZTO devices is explored. An expression for gate bias dependent mobility in ZTO devices is derived. / text

Indium-gallium-zinc oxide (IGZO) and zinc-tin oxide (ZTO) are investigated for thin-film transistor (TFT) applications. Negative bias illumination stress (NBIS) is employed for electrical stability assessment. Unpassivated IGZO and ZTO TFTs suffer from severe NBIS instabilities. Zinc-tin-silicon oxide is found to be an effective passivation layer for IGZO and ZTO TFTs, significantly improving the NBIS stability. NBIS instabilities in unpassivated TFTs are attributed to an NBIS-induced desorption of chemisorbed oxygen from the channel layer top surface, exposing surface oxygen vacancies. A ZTSO layer protects the channel layer top surface from adsorbed gas interactions and also appears to reduce the density of oxygen vacancies. The best device architectures investigated with respect to TFT electrical performance are found to be staggered with aluminum electrodes for unpassivated TFTs and coplanar with ITO electrodes for ZTSO-passivated TFTs. Annealing in wet-O₂ is not found to be effective for improving the performance of IGZO or ZTO TFTs or for reducing the post-deposition annealing temperature. / Graduation date: 2012

Resistive random access memory (RRAM) is a non-volatile memory technology based on resistive switching in a dielectric or semiconductor sandwiched between two different metals. Also known as memristors, these devices are potential candidates for a next-generation replacement for flash memory. In this thesis, bipolar resistive switching is reported for the first time in solution-deposited zinc-tin-oxide (ZTO). The impact of the compliance current on device operation, including the SET and RESET voltages, pre-SET, RESET and post-RESET currents, the resistance ratio between the low and high resistance states, retention, and the endurance, is investigated for an isolated Al dot/ZTO/Ir blanket device and for Al/ZTO/Pt crossbar RRAM devices. A gradual forming process is devised to improve device stability and performance. It is found that the device performance depends critically on the compliance current density that is used to limit the breakdown conduction during the SET operation. In addition, it was found that the conduction and switching mechanisms are consistent with the filament model of formation and rupture of conductive filaments. / Graduation date: 2012

The primary objective of this thesis is to develop a process for fabricating integrated circuits based on thin-film transistors (TFTs) using zinc tin oxide (ZTO) as the channel layer. ZTO, in contrast to indium- or gallium-based amorphous oxide semiconductors (AOS), is perceived to be a more commercially viable AOS choice due to its low cost and ability to be deposited via DC reactive sputtering. In the absence of an acceptable ZTO wet etch process, a plasma-etching process using Ar/CH₄ is developed for both 1:1 and 2:1 ZTO compositions. An Ar/CH₄ plasma etch process is also designed for indium gallium oxide (IGO), indium gallium zinc oxide (IGZO), and indium tin oxide (ITO). Ar/CH₄ dry etches have excellent selectivity with respect to SiO₂, providing a route for obtaining patterned ZTO channels. A critical asset of ZTO process integration involves removing polymer deposits after ZTO etching without active layer damage. A ZTO process is developed for the fabrication of integrated circuits which use ZTO channel enhancement-mode TFTs. Such ZTO TFTs exhibit incremental and average mobilities of 23 and 18 cm²V⁻¹s⁻¹, respectively, turn-on voltages approximately 0 to 1.5 V and subthreshold swings below 0.5 V/dec when annealed in air at 400 °C for 1 hour. Several types of ZTO TFT circuits are realized for the first time. Despite large parasitic capacitances due to large gate-source and gate-drain overlaps, AC/DC rectifiers are fabricated and found to operate in the MHz range. Thus, they are usable for RFID and other equivalent-speed applications. Finally, a ZTO process for simultaneously fabricating both enhancement-mode and depletion-mode TFTs on a single substrate using a single target and anneal step is developed. This dual-channel process is used to build a high-gain two-transistor enhancement/depletion inverter. At a rail voltage of 10 V, this inverter has a gain of 10.6 V/V, the highest yet reported for an AOS-based inverter. This E/D inverter is an important new functional block which will enable the realization of more complex digital logic circuits. / Graduation date: 2009

Im ersten Teil der vorliegenden Arbeit werden die physikalischen Eigenschaften, insbesondere die elektrische Leitfähigkeit, von Zink-Zinn-Oxid Dünnschichten sowie darauf basierenden Schottky-Dioden in Abhängigkeit von der Kationenkomposition bestimmt. Zur Herstellung dieser Dünnschichten wurde ein Verfahren genutzt, welches die Herstellung von kontinuierlichen Kompositiongradienten im Rahmen eines gepulsten Laserabscheidungsprozesses bei Raumtemperatur ermöglicht. Erster Schwerpunkt der Diskussion ist die Abhängigkeit elektrischer Eigenschaften der Dünnschichten sowie die Diodeneigenschaften vom Kationenverhältnis. Des Weiteren wird die Langzeitstabilität der Schottky-Dioden und der Einfluss der Sauerstoffzufuhr während der Kontaktherstellung auf die Eigenschaften der Schottky-Dioden herausgestellt. DieErgebnissetiefenaufgelösterRöntgenphotoelektronenspektroskopiewerden diskutiert und ein Mechanismus, welcher zu einer Verbesserung der Schottky-Dioden über die Zeit führt, wird vorgestellt. Die Erkenntnisse über die optimale Kationenkomposition und den Einfluss des Sauerstoffs auf die Eigenschaften von Schottky-Dioden wurden genutzt, um Metall-Halbleiter-Feldeffekttransistoren herzustellen, welche im zweiten Teil der vorliegenden Arbeit beschrieben werden. In einem ersten Schritt wurden hierfür die Abscheidebedingungen in der Sputterkammer optimiert und eine neue Abscheiderezeptur für die Herstellung von Feldeffekttransistoren eingeführt. Auch hier finden alle Abscheidungen bei Raumtemperatur statt. Die Abscheidung mittels Sputtern wurde gewählt, da diese Abscheidemethode größere industrielle Relevanz als die gepulste Laserabscheidung hat. Metall-Halbleiter-Feldeffekttransistoren mit zwei verschiedenen Gate-Typen werden vorgestellt und jeweils der Einfluss der Kanalschichtdicke auf die Transistoreigenschaften untersucht. Der Einfluss des durch die Herstellung erzeugten Sauerstoffreservoirs in dem Schottky-Gate Kontakt auf die Eigenschaften der Feldeffekttransistoren wird ebenso gezeigt wie der Einfluss eines thermischen Ausheizprozesses auf die Schaltgeschwindigkeit der Feldeffekttransistoren. Außerdem werden einfache Inverter, welche auf zwei gleichartigen Feldeffekttransistoren basieren, vorgestellt. Ebenfalls werden SchottkyDioden Feldeffekttransistoren Logik basierte Inverter vorgestellt und charakterisiert. AbschließendwerdenRingoszillatoren,aufgebautausmehrereninReihegeschaltetenSchottkyDiodenFeldeffekttransistorenLogikbasiertenInverternvorgestellt. DerEinflussderKanalschichtdicke und der Gate-Geometrie auf die Oszillationsfrequenz wird diskutiert.:Contents 1 Introduction 2 Theoretical Descriptions 2.1 The Amorphous Semiconductor Zinc Tin Oxide 2.2 Schottky Barrier Diodes 2.3 Field-Effect Transistors 2.4 Inverter 2.5 Inverter Chain and Ring Oscillator 3 Methods 3.1 Growth and Structuring Techniques 3.1.1 Pulsed Laser Deposition 3.1.2 Sputtering Deposition 3.1.3 Photolithography 3.2 Characterization Techniques 3.2.1 Hall Effect Measurements 3.2.2 XRD and XRR Measurements 3.2.3 Static and Dynamic Current-Voltage Measurements 3.2.4 Further Characterization Techniques 4 Physical Properties of Amorphous Zinc Tin Oxide 4.1 Characterization of Pulsed Laser Deposited Zinc Tin Oxide Thin Films Having a Continuous Composition Spread 4.2 Properties of Schottky Barrier Diodes in Dependence on the Cation Composition 4.3 Long Term Stability of Schottky Barrier Diodes 4.4 ImportantRoleofOxygenfortheFormationofHighlyRectifyingContacts 4.5 Processes Governing the Long Term Stability 5 Demonstration and Characterization of Zinc Tin Oxide Based Devices 5.1 Implementation of a New Sputtering Recipe 5.1.1 CharacterizationandElectricalOptimizationoftheZincTinOxide Thin Films .1.2 Optimization of the Gate Contact 5.2 Devices with PtOx/Pt Gate Contact 5.2.1 Variation of the Channel Thickness 5.2.2 Influence of the Oxygen Reservoir on the Performance and Long Term Stability of Devices 5.2.3 Tuning of the Electron Mobility 5.2.4 Frequency Dependent Switching of Transistors 5.3 Devices with i-ZTO/PtOx/Pt Gate Contact 5.3.1 Transistors with Varying Channel Thickness 5.3.2 Simple Inverter 5.3.3 SDFL Inverter 5.3.4 Inverter Chain 5.3.5 Ring Oscillators 5.4 Comparison to Literature 6 Summary and Outlook Abbreviations List of Symbols Bibliography List of Own and Contributed Articles Appendix / In the first part of the present work the physical properties, especially the electrical properties, of zinc tin oxide thin films as well as Schottky diodes based thereon are determined as a function of the cation composition. For film growth, a room temperature pulsed laser deposition process was used, which allows the realization of a continuous composition gradient within one sample. First focus of the discussion is the dependence of electrical properties of thin films as well as diode properties on the cation ratio. Furthermore, the long-term stability of the Schottky diodes and the influence of the oxygen supply during contact fabrication on the properties of the Schottky diodes are highlighted. The results of depth-resolved Xray photoelectron spectroscopy measurements are discussed and a mechanism leading to an improvement of the Schottky diodes over time is elucidated. The findings on the optimal cation composition and the influence of oxygen on the properties of Schottky diodes were used to produce metal-semiconductor field-effect transistors, which are described in the second part of this thesis. In a first step, the deposition conditions in the sputter chamber were optimized and a new deposition recipe for the fabrication of field effect transistors was developed. Here, too, all depositions take place at room temperature. Sputter deposition was chosen because this deposition method has greater industrial relevance than pulsed laser deposition. Metal-semiconductor field-effect-transistors with two different gate types are presented and the influence of the channel layer thickness on the transistor properties is investigated. The influence of the oxygen reservoir in the Schottky gate contact on the properties of the field-effect-transistors is shown as well as the influence of a thermal annealing process on the switching speed of the field-effect-transistors. In addition, simple inverters based on two identical field-effect-transistors are demonstrated. Also Schottky diode field-effect-transistor logic based inverters are presented and characterized. Finally, ring oscillators consisting of several series-connected Schottky diode field-effecttransistor logic based inverters are presented. The influence of channel layer thickness and gate geometry on the oscillation frequency is discussed.:Contents 1 Introduction 2 Theoretical Descriptions 2.1 The Amorphous Semiconductor Zinc Tin Oxide 2.2 Schottky Barrier Diodes 2.3 Field-Effect Transistors 2.4 Inverter 2.5 Inverter Chain and Ring Oscillator 3 Methods 3.1 Growth and Structuring Techniques 3.1.1 Pulsed Laser Deposition 3.1.2 Sputtering Deposition 3.1.3 Photolithography 3.2 Characterization Techniques 3.2.1 Hall Effect Measurements 3.2.2 XRD and XRR Measurements 3.2.3 Static and Dynamic Current-Voltage Measurements 3.2.4 Further Characterization Techniques 4 Physical Properties of Amorphous Zinc Tin Oxide 4.1 Characterization of Pulsed Laser Deposited Zinc Tin Oxide Thin Films Having a Continuous Composition Spread 4.2 Properties of Schottky Barrier Diodes in Dependence on the Cation Composition 4.3 Long Term Stability of Schottky Barrier Diodes 4.4 ImportantRoleofOxygenfortheFormationofHighlyRectifyingContacts 4.5 Processes Governing the Long Term Stability 5 Demonstration and Characterization of Zinc Tin Oxide Based Devices 5.1 Implementation of a New Sputtering Recipe 5.1.1 CharacterizationandElectricalOptimizationoftheZincTinOxide Thin Films .1.2 Optimization of the Gate Contact 5.2 Devices with PtOx/Pt Gate Contact 5.2.1 Variation of the Channel Thickness 5.2.2 Influence of the Oxygen Reservoir on the Performance and Long Term Stability of Devices 5.2.3 Tuning of the Electron Mobility 5.2.4 Frequency Dependent Switching of Transistors 5.3 Devices with i-ZTO/PtOx/Pt Gate Contact 5.3.1 Transistors with Varying Channel Thickness 5.3.2 Simple Inverter 5.3.3 SDFL Inverter 5.3.4 Inverter Chain 5.3.5 Ring Oscillators 5.4 Comparison to Literature 6 Summary and Outlook Abbreviations List of Symbols Bibliography List of Own and Contributed Articles Appendix

The present thesis addresses a sustainable approach to mechanically flexible and transparent electronic devices based on the amorphous oxide semiconductor zinc tin oxide (ZTO) as abundant and low-cost alternative to already industrially established materials such as amorphous indium gallium zinc oxide. ZTO thin films are deposited by radio frequency long-throw magnetron sputtering at room temperature to generally enable the implementation of common photolithography processes and further facilitate patterning of digital circuit elements on thermally unstable organic substrates. Starting with the most basic device building blocks of integrated circuitry, various types of field-effect transistors are fabricated by implementation of amorphous ZTO as active channel material. Metal-semiconductor field-effect transistors and pn heterodiode based junctions field-effect transistors as well as conventional metal-insulatorsemiconductor field-effect transistors are then compared regarding their electrical performance and long-term stability over a couple of months. A decisive step toward the successful interconnection of fundamental digital circuit elements, such as previously demonstrated simple inverters, is to ensure sufficient output level compatibility between the signals of associated logic components. Accordingly, the Schottky diode field-effect transistor logic approach is adapted for amorphous ZTO based devices in order to facilitate cascading of multiple inverters consisting of unipolar devices. Field-effect transistor properties as well as the circuit design have been continuously improved to enhance the overall performance in terms of functionality and low-voltage operation. Corresponding logic inverters are finally integrated in ring oscillator circuits to gain insights into the dynamic properties of digital circuit building blocks based on amorphous ZTO. Ultimately, ZTO has been fabricated on mechanically flexible polyimide substrates to determine the elastic and electrical properties of amorphous ZTO thin films in dependence on external tensile and compressive stress induced by mechanical bending. Further, associated flexible metal-semiconductor field-effect transistor are investigated regarding their performance stability under tensile strain. / Die vorliegende Arbeit umfasst die Herstellung und Charakterisierung aktiver elektrischer Bauelemente und integrierter Schaltkreise auf Basis des amorphen Oxidhalbleiters Zink-Zinnoxid (ZTO). Als vielversprechende nachhaltige und kostengünstigere Alternative zu dem bereits industriell etablierten Halbleiter Indium-Gallium-Zinkoxid wird insbesondere die Eignung von ZTO in optisch transparenter sowie mechanisch flexibler Elektronik untersucht. Um entsprechend Kompatibilität mit thermisch instabilen organischen Substraten sowie herkömmlichen Fotolithografieverfahren zu gewährleisten, beschränkt sich die Züchtung von ZTO-Dünnfilmen mittels Hochfrequenz-Magnetron-Distanzkathodenzerstäubung ausschließlich auf Herstellungsprozesse bei Raumtemperatur. Zunächst wird auf die Umsetzung verschiedener Feldeffekttransistor-Typen auf Basis amorphen ZTOs eingegangen, welche elektrisch charakterisiert und schließlich vor dem Hintergrund der Anwendung in integrierten Schaltkreisen vergleichend gegenübergestellt werden. Neben konventionellen Metall-Isolator-Halbleiterstrukturen wird vor allem näher auf Metall-Halbleiter-Feldeffekttransistoren sowie Sperrschicht-Feldeffekttransistoren auf der Grundlage von pn-Heteroübergängen eingegangen, da diese hauptsächlich in Bereichen hoher geforderter Schaltfrequenzen zum Einsatz kommen. Da integrierte Schaltkreise auf Basis unipolarer Feldeffekttransistoren eines Ladungsträgertyps inkonsistente Signaleingangs- sowie -ausgangspegel aufweisen, wird die Schottky- Dioden-Transistorlogik adaptiert, um entsprechend die Verknüpfung mehrerer Logikgatter auf Basis amorphen ZTOs zu gewährleisten. Durch geeignete Signalrückkopplung werden komplexere Schaltungen wie Ringoszillatoren realisiert, welche anhand von Laufzeitanalysen Aufschluss über die Schaltgeschwindigkeit ZTO basierter Feldeffekttransistoren geben. Abschließend werden amorphe ZTO-Dünnfilme auf flexiblen Polyimid-Substraten hergestellt und bezüglich der elastischen sowie elektrischen Eigenschaften in Abhängigkeit von exzessivem mechanischen Stress untersucht. Darüber hinaus werden flexible Metall-Halbleiter-Feldeffekttransistoren hinsichtlich ihrer Funktionalität und Stabilität gegenüber durch Biegeprozesse induzierte Verspannungen elektrisch charakterisiert.