The objective of this thesis is to provide an initial demonstration of the feasibility of constructing highly transparent active electronic devices. Such a demonstration is successfully achieved in the fabrication of ZnO-based thin film transistors (TFTs) exhibiting transparency greater than ~90% in the visible portion of the electromagnetic spectrum and prototypical n-channel, enhancement mode TFT characteristics. Electrical characterization studies of these ZnO-based transparent TFTs and of CuYO��� / ZnO / ITO p-i-n heterojunction diodes serve to elucidate the mechanisms responsible for the behavior of these devices in particular, and of transparent electronic devices in general. Energy band analysis of the degenerate semiconductor / insulator heterojunction yields insight into the phenomenon of charge injection into an insulator, with important implications for the analysis of devices containing heterojunctions of this nature. Finally, a novel technique for simultaneously characterizing carrier injection into an insulator and interface channel formation, the capacitance-(voltage, frequency) [C-(V,f)] technique, is proposed and employed in the characterization of ZnO-based TFT structures. / Graduation date: 2003

Transparent oxide semiconductors (TOSs) are materials that exhibit electrical conduction and optical transparency. The traditional applications of these materials are transparent conducting oxides in flat-panel displays, light-emitting diodes, solar cells, and imaging sensors. Recently, significant research has been driven to extend state-of-the-art applications such as thin-film transistors (TFTs). A new and rapidly developing field is emerging, called transparent electronics. This thesis advances transparent electronics through developing a new technique to fabricate TOSs and demonstrating their applications to active semiconductor devices such as diodes and TFTs. Ion beam assisted evaporation (IBAE) is used to deposit two common TOSs: zinc oxide (ZnO) and indium oxide (In2O3). The detailed material study is carried out through various characterization of their electrical properties, chemical composition, optical properties, crystal structure, intrinsic stress, topology, and morphology, as well as an investigation of thin-film property as a function of the deposition parameters: ion flux and energy, and deposition rate. The study proves that IBAE technique provides the capability for fabricating TOSs with controllable properties. By utilizing the newly developed semiconducting ZnO, p-NiO/i-ZnO/n-ITO and n-ITO/i-ZnO/p-NiO heterostructure photodiodes with a low leakage are proposed and assessed. Analysis of their current-voltage characteristics and current transient behaviour reveals that the dominant source of leakage current stems from the deep defect states in the intrinsic zinc oxide layer, where its dynamic response at low signal levels is limited by the charge trapping. The exploration of the photoconduction mechanism and spectral response confirms that such photodiodes are potentially applicable for ultraviolet (UV) sensors. The comparative study of both device structures provides further insights into the leakage current mechanisms, p-i interface properties, and quantum efficiency. Secondly, with the novel semiconducting In2O3, TFTs are fabricated and evaluated. The device performance is optimized by addressing the source/drain contact issue, lowering the intrinsic channel resistance, and improving the dielectric/channel interface. The best n-channel TFT has a high field-effect mobility of ~30 cm^2/Vs, a high current ON/OFF ratio of ~10^8, and a sub-threshold slope of 2.0 V/decade. More important, high-performance indium oxide TFTs here are integrated with the silicon dioxide and silicon nitride gate dielectrics by conventional plasma-enhanced chemical vapour deposition, which makes indium oxide TFT a competitive alternative for next generation TFTs to meet the technical requirements for flat-panel displays, large area imager arrays, and radio frequency identification tags. The stability study shows that indium oxide TFTs are highly stable with a very small threshold voltage shift under both a long-term constant voltage and long-term current stress. The dynamic behaviour indicates factors that affect the operation speed of such TFTs. A descriptive model is proposed to link the material properties and the processing issues with the device performance to facilitate further research and development of TOS TFTs. The research described in this thesis is one of the first investigations of the fabrication of TOSs by the IBAE and their applications to a variety of thin-film devices, particularly UV sensors and TFTs.

Transparent oxide semiconductors (TOSs) are materials that exhibit electrical conduction and optical transparency. The traditional applications of these materials are transparent conducting oxides in flat-panel displays, light-emitting diodes, solar cells, and imaging sensors. Recently, significant research has been driven to extend state-of-the-art applications such as thin-film transistors (TFTs). A new and rapidly developing field is emerging, called transparent electronics. This thesis advances transparent electronics through developing a new technique to fabricate TOSs and demonstrating their applications to active semiconductor devices such as diodes and TFTs. Ion beam assisted evaporation (IBAE) is used to deposit two common TOSs: zinc oxide (ZnO) and indium oxide (In2O3). The detailed material study is carried out through various characterization of their electrical properties, chemical composition, optical properties, crystal structure, intrinsic stress, topology, and morphology, as well as an investigation of thin-film property as a function of the deposition parameters: ion flux and energy, and deposition rate. The study proves that IBAE technique provides the capability for fabricating TOSs with controllable properties. By utilizing the newly developed semiconducting ZnO, p-NiO/i-ZnO/n-ITO and n-ITO/i-ZnO/p-NiO heterostructure photodiodes with a low leakage are proposed and assessed. Analysis of their current-voltage characteristics and current transient behaviour reveals that the dominant source of leakage current stems from the deep defect states in the intrinsic zinc oxide layer, where its dynamic response at low signal levels is limited by the charge trapping. The exploration of the photoconduction mechanism and spectral response confirms that such photodiodes are potentially applicable for ultraviolet (UV) sensors. The comparative study of both device structures provides further insights into the leakage current mechanisms, p-i interface properties, and quantum efficiency. Secondly, with the novel semiconducting In2O3, TFTs are fabricated and evaluated. The device performance is optimized by addressing the source/drain contact issue, lowering the intrinsic channel resistance, and improving the dielectric/channel interface. The best n-channel TFT has a high field-effect mobility of ~30 cm^2/Vs, a high current ON/OFF ratio of ~10^8, and a sub-threshold slope of 2.0 V/decade. More important, high-performance indium oxide TFTs here are integrated with the silicon dioxide and silicon nitride gate dielectrics by conventional plasma-enhanced chemical vapour deposition, which makes indium oxide TFT a competitive alternative for next generation TFTs to meet the technical requirements for flat-panel displays, large area imager arrays, and radio frequency identification tags. The stability study shows that indium oxide TFTs are highly stable with a very small threshold voltage shift under both a long-term constant voltage and long-term current stress. The dynamic behaviour indicates factors that affect the operation speed of such TFTs. A descriptive model is proposed to link the material properties and the processing issues with the device performance to facilitate further research and development of TOS TFTs. The research described in this thesis is one of the first investigations of the fabrication of TOSs by the IBAE and their applications to a variety of thin-film devices, particularly UV sensors and TFTs.

Thesis (Ph. D.)--University of Alabama at Birmingham, 2007. / Title from PDF t.p. (viewed Feb. 3, 2010). Additional advisors: Derrick R. Dean, Sergey B. Mirov, Sergey Vyazovkin, Mary Ellen Zvanut. Includes bibliographical references (p. 122-145).

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 139-143). Also available on the World Wide Web.

L'absence d'oxydes semi-conducteurs de type p de haute performance retarde le développement de d’électronique transparente et du photovoltaïque à base d’oxydes. Dans le groupe des composés semi-conducteurs, les oxydes à base de cuivre présentent des caractéristiques électriques, optiques et de fabrication prometteuses qui établissent cette famille de matériaux comme bien adaptés aux applications semi-conductrices de type p. Dans ce travail, nous nous concentrons sur la croissance de films minces d’une part de Cu2O dopée par des cations et d’autre part de CuCrO2, visant à améliorer leurs propriétés optiques et électriques. De plus, nous avons mis en œuvre ces films d'oxyde dans des dispositifs de jonction pn tels que des cellules solaires et des photodétecteurs UV.Dans le travail sur Cu2O, nous avons réalisé l'incorporation de magnésium jusqu'à 17% dans des films minces par dépôt chimique en phase vapeur assisté par aérosol, entraînant des changements de morphologie. La résistivité électrique a été réduite jusqu’à des valeurs de 6,6 ohm.cm, en raison de l'augmentation de la densité de porteur de-charges jusqu'à 10^18 cm-3. L'incorporation du magnésium a en outre eu un impact sur la stabilité de la phase Cu2O. En effet la transformation du Cu2O en CuO en conditions oxydantes est considérablement retardée par la présence de Mg dans les films, en raison de l'inhibition de la formation d’un type particulier de lacune de cuivre (split vacancy). L'intégration dans les jonctions pn a été réalisée avec succès en utilisant uniquement des voies de dépôt chimique en phase vapeur, en combinaison avec le ZnO de type n. Néanmoins, l'application de Cu2O dopé au Mg dans les cellules solaires présente un effet photovoltaïc très faible, loin des meilleures valeurs de l’état de l’art.Dans le travail sur CuCrO2, nous démontrons la première fabrication d'hétérostructures de nanofils en configuration cœur/coquille ZnO/CuCrO2 utilisant des techniques de dépôt chimique adaptées pour des grandes surface, à faible coût, facilement implémentées à des températures modérées et leur intégration dans des photodétecteurs UV auto-alimentés. Une coquille conforme de CuCrO2 avec la phase de delafossite et avec une uniformité élevée a été élaborée par un dépôt chimique en phase vapeur assisté par aérosol sur un réseau de nanofils ZnO alignés verticalement, obtenu par dépôt par bain chimique. Les hétérostructures ZnO/CuCrO2 coeur-coquille présentent un comportement rectificatif significatif, avec un ratio de rectification maximal de 5500 à ± 1V, ce qui est bien meilleur que les dispositifs 2D similaires rapportés dans la littérature, ainsi qu'une absorption élevée supérieure à 85% dans la région UV. Lorsqu'ils sont appliqués en tant que photodétecteurs UV auto-alimentés, les hétérojonctions optimisées présentent une réponse maximale de 187 μA / W sous une polarisation nulle à 374 nm ainsi qu'une sélectivité élevée avec un ratio de rejet entre l’UV-et le visible (374-550 nm) de 68 sous irradiance de 100 mW/cm2. / The lack of a successful p-type semiconductor oxides delays the future implementation of transparent electronics and oxide-based photovoltaic devices. In the group semiconducting compounds, copper-based oxides present promising electrical, optical and manufacturing features that establish this family of materials suitable for p-type semiconductor applications. In this work, we focused on the growth of cation doped Cu2O and intrinsic CuCrO2 thin films, aiming for enhancements of their optical and electrical response. Furthermore, we implemented these oxide films into pn junction devices, such as solar cells and UV photodetectors.In the work on Cu2O, we achieved the incorporation of magnesium up to 17% in thin films by aerosol-assisted chemical vapor deposition, resulting in morphology changes. Electrical resistivity was reduced down to values as low as 6.6 ohm.cm, due to the increase of charge-carrier density up to 10^18 cm-3. The incorporation of magnesium had additionally an impact on the stability of the Cu2O phase. The transformation of Cu2O into CuO under oxidizing conditions is significantly postponed by the presence of Mg in the films, due to the inhibition of copper split vacancies formation. The integration into pn junctions was successfully achieved using only chemical vapor deposition routes, in combination with n-type ZnO. Nevertheless, the application of Mg-doped Cu2O in solar cells present a meager photovoltaic performance, far from the state-of-the-art reports.In the work on CuCrO2, we demonstrate the first fabrication of ZnO/CuCrO2 core-shell nanowire heterostructures using low-cost, surface scalable, easily implemented chemical deposition techniques at moderate temperatures, and their integration into self-powered UV photodetectors. A conformal CuCrO2 shell with the delafossite phase and with high uniformity is formed by aerosol-assisted chemical vapor deposition over an array of vertically aligned ZnO nanowires grown by chemical bath deposition. The ZnO/CuCrO2 core-shell nanowire heterostructures present a significant rectifying behavior, with a maximum rectification ratio of 5500 at ±1V, which is much better than similar 2D devices, as well as a high absorption above 85% in the UV region. When applied as self-powered UV photodetectors, the optimized heterojunctions exhibit a maximum responsivity of 187 µA/W under zero bias at 374 nm as well as a high selectivity with a UV-to-visible (374-550 nm) rejection ratio of 68 under an irradiance of 100 mW/cm2.

Due to their low-temperature processing capability and ionic bonding configuration, amorphous oxide semiconductors (AOS) are well suited for applications within future mechanically flexible electronics. Over the past couple of years, amorphous zinc tin oxide (ZTO) has been proposed as indiumand gallium-free and thus more sustainable alternative to the widely deployed indium gallium zinc oxide (IGZO). The present study specifically focuses on the strain-dependence of elastic and electrical properties of amorphous zinc tin oxide thin-films sputtered at room temperature. Corresponding MESFETs have been compared regarding their operation stability under mechanical bending for radii ranging from 5 to 2 mm. Force-spectroscopic measurements yield a plastic deformation of ZTO as soon as the bending-induced strain exceeds 0.83%. However, the electrical properties of ZTO determined by Hall effect measurements at room temperature are demonstrated to be unaffected by residual compressive and tensile strain up to 1.24 %. Even for the maximum investigated tensile strain of 1.26 %, the MESFETs exhibit a reasonably consistent performance in terms of current on/off ratios between six and seven orders of magnitude, a subthreshold swing around 350 mV/dec and a field-effect mobility as high as 7.5 cm2V−1s−1. Upon gradually subjecting the transistors to higher tensile strain, the channel conductivity steadily improves and consequently, the field-effect mobility increases by nearly 80% while bending the devices around a radius of 2 mm. Further, a reversible threshold voltage shift of about −150 mV with increasing strain is observable. Overall, amorphous ZTO provides reasonably stable electrical properties and device performance for bending-induced tensile strain up to at least 1.26% and thus represent a promising material of choice considering novel bendable and transparent electronics.

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) constitute the central theme of this thesis. Within this theme, three primary areas of focus are pursued. The first focus is the realization of a transparent three-stage ring oscillator with buffered output and an output frequency in the megahertz range. This leads to the possibility of transparent radio frequency applications, such as transparent RFID tags. At the time of its fabrication, this ring oscillator was the fastest oxide electronics ring oscillator reported, with an output frequency of 2.16 MHz, and a time delay per stage of 77 ns. The second focus is to ascertain whether a three-terminal device (i.e., a TFT) is an appropriate structure for conducting space-charge-limited-current (SCLC) measurements. It is found that it is not appropriate to use a diode-tied or gate-biased TFT configuration for conducting a SCLC assessment since square-law theory shows that transistor action alone gives rise to I proportional to V² characteristics, which can easily be mistakenly attributed to a SCLC mechanism. Instead, a floating gate TFT configuration is recommended for accomplishing SCLC assessment of AOS channel layers. The final focus of this work is to describe an assessment procedure appropriate for determining if a dielectric is suitable for use as a TFT gate insulator. This is accomplished by examining the shape of a MIM capacitor's log(J)-ξ curve, where J is the measured current density and ξ is the applied electric field. An appropriate dielectric for use as a TFT gate insulator will have a log(J)-ξ curve that expresses a clear breakover knee, indicating a high-field conduction mechanism dominated by Fowler-Nordheim tunneling. Such a dielectric produces a TFT with a minimal gate leakage which does not track with the drain current in a log(I[subscript D])-V[subscript GS] transfer curve. An inappropriate dielectric for use as a TFT gate insulator will have a log(J)-ξ curve that does not express a clear breakover knee, indicating that the dominate conduction mechanism is defect driven (i.e., pin-hole like shunt paths) and, therefore, the dielectric is leaky. It is shown that experimental log(J)-ξ leakage curves can be accurately simulated using Ohmic, space-charge-limited current (SCLC), and Fowler-Nordheim tunneling conduction mechanisms. / Graduation date: 2010