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Thesis (M.S.)--Oregon State University, 2006. / Printout. Includes bibliographical references (leaves 54-56). Also available on the World Wide Web.

This thesis focuses on the use of surface modifiers as tools for probing and/or controlling interfaces. Surface modification of transparent conducting oxides (TCOs) with organic and organometallic modifiers can be used as a tool for mediating interfacial energetics as well as probing the kinetics of charge-transfer at the metal oxide/organic interface. The synthetic tunability of these modifiers allows us to design molecules based on various parameters, which include the nature of the binding, spacer, and terminal groups. Based on this framework, several modifiers were synthesized and used to investigate surface energy tuning as well as charge injection kinetics as a function of molecular structure. More specifically, we use XPS/UPS to examine the evolution of the chemical structure and frontier orbital levels of the TCO/organic interface as a function of the chosen surface modifier. In addition, we investigate the impact that various molecular binding groups have on mediating the kinetics of charge-transfer. In the last section of this body of work we examine the development of dielectric nanocomposite films for capacitor applications. More specifically, we examine the use of phosphonic acid modifiers to functionalize barium titanate nanoparticles in order to provide miscibility with a suitable polymer host. The effect of various modifiers on the dielectric properties not nanocomposite thin films was examined.

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

Die vorliegenden Arbeit befasst sich mit der Herstellung und Charakterisierung von transparenten Metall-Halbleiter- Feldeffekttransistoren. Dazu werden im ersten Kapitel transparente gleichrichtende Kontakte, basierend auf dem Konzept von Metalloxidkontakten, hergestellt und im Hinblick auf chemische Zusammensetzung des Kontaktmaterials, Barriereninhomogenität und Kompatibilität mit amorphen Halbleitern untersucht. Außerdem wird die Anwendbarkeit der Kontakte als UV-Sensor studiert. Im zweiten Kapitel werden transparente leitfähige Oxide vorgestellt und insbesondere deren optische und elektrische Eigenschaften in Abhängigkeit von den Herstellungsbedingungen studiert. Das dritte Kapitel beinhaltet Untersuchungen zu transparenten Feldeffektransistoren, die auf den im ersten Kapitel untersuchten transparenten gleichrichtenden Kontakten basieren (TMESFETs). Insbesondere die elektrischen Stabilität der Bauelemente hinsichtlich Beleuchtung, erhöhten Temperaturen und Spannungsstress wird untersucht. Auch die Langzeitstabilität, Reproduzierbarkeit und der Effekt gepulster Spannungen wird betrachtet. Weiterhin wird die Verwendung amorpher Halbleiter im Kanal und damit auch die Herstellung flexibler Transistoren auf Folie demonstriert. Zuletzt werden die TMESFETs integriert und als Inverterschaltkreise aufgebaut und untersucht. Außerdem wird die Eignung der Transistoren zur Messung von Aktionspotentialen von Nervenzellen studiert.

Thesis (Ph. D.)--Oregon State University, 2010. / Printout. Includes bibliographical references. Also available on the World Wide Web.

xix, 142 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Transparent amorphous oxide semiconductors are a relatively new class of materials which show significant promise for electronic device applications. The electron mobility in these materials is at least ten times greater than that of the current dominant material for thin-film transistors: amorphous silicon. The density of states within the gap of a semiconductor largely determines the characteristics of a device fabricated from it. Thus, a fundamental understanding of the electronic structure within the mobility gap of amorphous oxides is crucial to fully developing technologies based around them. Amorphous zinc tin oxide (ZTO) and indium gallium zinc oxide (IGZO) were investigated in order to determine this sub-gap structure. Junction-capacitance based methods including admittance spectroscopy and drive level capacitance profiling (DLCP) were used to find the free carrier and deep defect densities. Defects located near insulator-semiconductor interfaces were commonly observed and strongly depended on fabrication conditions. Transient photocapacitance spectroscopy (TPC) indicated broad valence band-tails for both the ZTO and IGZO samples, characterized by Urbach energies of 110±20 meV. These large band-tail widths imply that significant structural disorder exists in the atomic lattice of these materials. While such broad band-tails generally correlate with poor electronic transport properties, the density of states near the conduction band is more important for devices such as transistors. The TPC spectra also revealed an optically active defect located at the insulator-semiconductor junction. Space-charge-limited current (SCLC) measurements were attempted in order to deduce the density of states near the conduction band. While the SCLC results were promising, their interpretation was too ambiguous to obtain a detailed picture of the electronic state distribution. Another technique, modulated photocurrent spectroscopy (MPC), was then employed for this purpose. Using this method narrow conduction band-tails were determined for the ZTO samples with Urbach energies near 10 meV. Thus, by combining the results of the DLCP, TPC and MPC measurements, a quite complete picture of the density of states within the mobility gap of these amorphous oxides has emerged. The relationship of this state distribution to transistor performance is discussed as well as to the future development of device applications of these materials. / Committee in charge: Stephen Kevan, Chairperson, Physics; J David Cohen, Member, Physics; David Strom, Member, Physics; Jens Noeckel, Member, Physics; David Johnson, Outside Member, Chemistry

Emerging transparent semiconducting oxide (TSO) materials have achieved their initial commercial success in the display industry. Due to the advanced electrical performance, TSOs have been adopted either to improve the performance of traditional displays or to demonstrate the novel transparent and flexible displays. However, due to the lack of feasible p-type TSOs, the applications of TSOs is limited to unipolar (n-type TSOs) based devices. Compared with the prosperous n-type TSOs, the performance of p-type counterparts is lag behind. However, after years of discovery, several p-type TSOs are confirmed with promising performance, for example, tin monoxide (SnO). By using p-type SnO, excellent transistor field-effect mobility of 6.7 cm2 V-1 s-1 has been achieved. Motivated by this encouraging performance, this dissertation is devoted to further evaluate the feasibility of integrating p-type SnO in p-n junctions and complementary metal oxide semiconductor (CMOS) devices. CMOS inverters are fabricated using p-type SnO and in-situ formed n-type tin dioxide (SnO2). The semiconductors are simultaneously sputtered, which simplifies the process of CMOS inverters. The in-situ formation of SnO2 phase is achieved by selectively sputtering additional capping layer, which serves as oxygen source and helps to balance the process temperature for both types of semiconductors. Oxides based p-n junctions are demonstrated between p-type SnO and n-type SnO2 by magnetron sputtering method. Diode operating ideality factor of 3.4 and rectification ratio of 103 are achieved. A large temperature induced knee voltage shift of 20 mV oC-1 is observed, and explained by the large band gap and shallow states in SnO, which allows minor adjustment of band structure in response to the temperature change. Finally, p-type SnO is used to demonstrating the hybrid van der Waals heterojunctions (vdWHs) with two-dimensional molybdenum disulfide (2D MoS2) by mechanical exfoliation. The hybrid vdWHs show excellent rectifying performance. Due to the ultra-thin nature of MoS2, the operation of hybrid vdWHs is gate-tunable, and we further discover such gate-tunability depends on the layer number of MoS2, i.e., the screening effect. The detailed study in such hybrid vdWHs provides valuable information for understanding the switching performance of junctions contain 2D materials.

Transparent oxide electronics have emerged as promising materials to shape the future of electronics. While several n-type oxides have been already studied and demonstrated feasibility to be used as active materials in thin film transistors, high performance p-type oxides have remained elusive. This dissertation is devoted to the study of transparent p-type oxide semiconductor tin monoxide and its use in the fabrication of field effect devices. A complete study on the deposition of tin monoxide thin films by direct current reactive magnetron sputtering is performed. Carrier density, carrier mobility and conductivity are studied over a set of deposition conditions where p-type conduction is observed. Density functional theory simulations are performed in order to elucidate the effect of native defects on carrier mobility. The findings on the electrical properties of SnO thin films are then translated to the fabrication of thin films transistors. The low processing temperature of tin monoxide thin films below 200 oC is shown advantageous for the fabrication of fully transparent and flexible thin film transistors. After careful device engineering, including post deposition annealing temperature, gate dielectric material, semiconductor thickness and source and drain electrodes material, thin film transistors with record device performance are demonstrated, achieving a field effect mobility >6.7 cm2V-1s-1. Device performance is further improved to reach a field effect mobility of 10.8 cm2V-1s-1 in SnO nanowire field effect transistors fabricated from the sputtered SnO thin films and patterned by electron beam lithography. Downscaling device dimension to nano scale is shown beneficial for SnO field effect devices not only by achieving a higher hole mobility but enhancing the overall device performance including better threshold voltage, subthreshold swing and lower number of interfacial defects. Use of p-type semiconductors in nonvolatile memory applications is then demonstrated by the fabrication of hybrid ferroelectric field effect transistors composed of organic ferroelectric layer polyvinylidene fluoride trifluoroethylene and inorganic p-type semiconductor tin monoxide. Both rigid and flexible devices are demonstrated, showing the advantages of low temperature oxides over polymer semiconductors by achieving much better performance, such as order of magnitude higher hole mobility.

Die vorliegenden Arbeit befasst sich mit der Herstellung und Charakterisierung von transparenten Metall-Halbleiter- Feldeffekttransistoren. Dazu werden im ersten Kapitel transparente gleichrichtende Kontakte, basierend auf dem Konzept von Metalloxidkontakten, hergestellt und im Hinblick auf chemische Zusammensetzung des Kontaktmaterials, Barriereninhomogenität und Kompatibilität mit amorphen Halbleitern untersucht. Außerdem wird die Anwendbarkeit der Kontakte als UV-Sensor studiert. Im zweiten Kapitel werden transparente leitfähige Oxide vorgestellt und insbesondere deren optische und elektrische Eigenschaften in Abhängigkeit von den Herstellungsbedingungen studiert. Das dritte Kapitel beinhaltet Untersuchungen zu transparenten Feldeffektransistoren, die auf den im ersten Kapitel untersuchten transparenten gleichrichtenden Kontakten basieren (TMESFETs). Insbesondere die elektrischen Stabilität der Bauelemente hinsichtlich Beleuchtung, erhöhten Temperaturen und Spannungsstress wird untersucht. Auch die Langzeitstabilität, Reproduzierbarkeit und der Effekt gepulster Spannungen wird betrachtet. Weiterhin wird die Verwendung amorpher Halbleiter im Kanal und damit auch die Herstellung flexibler Transistoren auf Folie demonstriert. Zuletzt werden die TMESFETs integriert und als Inverterschaltkreise aufgebaut und untersucht. Außerdem wird die Eignung der Transistoren zur Messung von Aktionspotentialen von Nervenzellen studiert.