Tests for conceptual design of 6.6 kV class superconducting fault current limiter with YBCO thin film elements

A superconducting fault current limiter (SFCL) for 6.6 kV and 400 A installed in a cubicle for a distribution network substation was conceptually designed. The SFCL consists of parallel- and series-connected superconducting YBCO elements and a limiting resistor. Before designing the SFCL, some tests were carried out. The width and length of each element used in the tests are 30 mm and 210 mm, respectively. The element consists of YBCO thin film of about 200 nm in thickness on cerium dioxide (CeO2) as a cap-layer on a sapphire substrate by metal-organic deposition with a protective metal coat. In the tests, characteristics of each element, such as over-current, withstand-voltage, and so on, were obtained. From these characteristics, series and parallel connections of the elements, called units, were considered. The characteristics of the units were obtained by tests. From the test results, a single phase prototype SFCL was manufactured and tested. Thus, an SFCL rated at 6.6 kV and 400 A can be designed. © 2009 IEEE.

On optical reflection at heterojunction interface of thin film solar cells

Heterojunction is an important structure for the development of photovoltaic solar cells. In contrast to homojunction structures, heterojunction solar cells have internal crystalline interfaces, which will reflect part of the incident light, and this has not been considered carefully before though many heterostructure solar cells have been commercialized. This paper discusses the internal reflection for various material systems used for the development of heterostructure-based solar cells. It has been found that the most common heterostructure solar cells have internal reflection less than 2%, while some potential heterojunction solar cells such as ITO/GaAs, ITO/InP, Si/Ge, polymer/semiconductors and oxide semiconductors may have internal reflection as high as 20%. Also it is worse to have a window layer with a lower refractive index than the absorption layer for solar cells. Ignoring this strong internal reflection will lead to severe deterioration and reduction of conversion efficiency; therefore measures have to be taken to minimize or prevent this internal reflection. © 2013 Elsevier B.V.

Tantalum-oxide catalysed chemical vapour deposition of single- and multi-walled carbon nanotubes

Tantalum-oxide thin films are shown to catalyse single- and multi-walled carbon nanotube growth by chemical vapour deposition. A low film thickness, the nature of the support material (best results with SiO2) and an atmospheric process gas pressure are of key importance for successful nanotube nucleation. Strong material interactions, such as silicide formation, inhibit nanotube growth. In situ X-ray photoelectron spectroscopy indicates that no catalyst reduction to Ta-metal or Ta-carbide occurs during our nanotube growth conditions and that the catalytically active phase is the Ta-oxide phase. Such a reduction-free oxide catalyst can be technologically advantageous. © 2013 The Royal Society of Chemistry.

Polymer gate dielectrics with self-assembled monolayers for high-mobility organic thin-film transistors based on copper phthalocyanine

Organic thin-film transistors based on polycrystalline copper phthalocyanine (CuPc) were fabricated by using poly(vinyl alcohol) as gate dielectric. After treatment of the gate dielectric using an octadecyltrichlorosilane self-assembled monolayer, a mobility of up to 0.11 cm2/V∈s was achieved, which is comparable to that of single-crystal CuPc devices (0.1-1 cm2/V∈s). The surface morphology was analyzed and the possible reasons for the enhanced mobility are discussed. © 2009 Springer-Verlag.

High-density remote plasma sputtering of high-dielectric-constant amorphous hafnium oxide films

Hafnium oxide (HfOx) is a high dielectric constant (k) oxide which has been identified as being suitable for use as the gate dielectric in thin film transistors (TFTs). Amorphous materials are preferred for a gate dielectric, but it has been an ongoing challenge to produce amorphous HfOx while maintaining a high dielectric constant. A technique called high target utilization sputtering (HiTUS) is demonstrated to be capable of depositing high-k amorphous HfOx thin films at room temperature. The plasma is generated in a remote chamber, allowing higher rate deposition of films with minimal ion damage. Compared to a conventional sputtering system, the HiTUS technique allows finer control of the thin film microstructure. Using a conventional reactive rf magnetron sputtering technique, monoclinic nanocrystalline HfOx thin films have been deposited at a rate of ∼1.6nmmin-1 at room temperature, with a resistivity of 1013Ωcm, a breakdown strength of 3.5MVcm-1 and a dielectric constant of ∼18.2. By comparison, using the HiTUS process, amorphous HfOx (x=2.1) thin films which appear to have a cubic-like short-range order have been deposited at a high deposition rate of ∼25nmmin-1 with a high resistivity of 1014Ωcm, a breakdown strength of 3MVcm-1 and a high dielectric constant of ∼30. Two key conditions must be satisfied in the HiTUS system for high-k HfOx to be produced. Firstly, the correct oxygen flow rate is required for a given sputtering rate from the metallic target. Secondly, there must be an absence of energetic oxygen ion bombardment to maintain an amorphous microstructure and a high flux of medium energy species emitted from the metallic sputtering target to induce a cubic-like short range order. This HfOx is very attractive as a dielectric material for large-area electronic applications on flexible substrates. A remote plasma sputtering process (high target utilization sputtering, HiTUS) has been used to deposit amorphous hafnium oxide with a very high dielectric constant (∼30). X-ray diffraction shows that this material has a microstructure in which the atoms have a cubic-like short-range order, whereas radio frequency (rf) magnetron sputtering produced a monoclinic polycrystalline microstructure. This is correlated to the difference in the energetics of remote plasma and rf magnetron sputtering processes. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Thin film coil design considerations for wireless power transfer in flat panel display

Wireless power transfer is experimentally demonstrated by transmission between an AC power transmitter and receiver, both realised using thin film technology. The transmitter and receiver thin film coils are chosen to be identical in order to promote resonant coupling. Planar spiral coils are used because of the ease of fabrication and to reduce the metal layer thickness. The energy transfer efficiency as a function of transfer distance is analysed along with a comparison between the theoretical and the experimental results. © 2012 Materials Research Society.

Subwavelength plasmonics for graded-index optics on a chip.

Planar plasmonic devices are becoming attractive for myriad applications, owing to their potential compatibility with standard microelectronics technology and the capability for densely integrating a large variety of plasmonic devices on a chip. Mitigating the challenges of using plasmonics in on-chip configurations requires precise control over the properties of plasmonic modes, in particular their shape and size. Here we achieve this goal by demonstrating a planar plasmonic graded-index lens focusing surface plasmons propagating along the device. The plasmonic mode is manipulated by carving subwavelength features into a dielectric layer positioned on top of a uniform metal film, allowing the local effective index of the plasmonic mode to be controlled using a single binary lithographic step. Focusing and divergence of surface plasmons is demonstrated experimentally. The demonstrated approach can be used for manipulating the propagation of surface plasmons, e.g., for beam steering, splitting, cloaking, mode matching, and beam shaping applications.

High performance non-volatile ferroelectric copolymer memory based on a ZnO nanowire transistor fabricated on a transparent substrate

A high performance ferroelectric non-volatile memory device based on a top-gate ZnO nanowire (NW) transistor fabricated on a glass substrate is demonstrated. The ZnO NW channel was spin-coated with a poly (vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) layer acting as a top-gate dielectric without buffer layer. Electrical conductance modulation and memory hysteresis are achieved by a gate electric field induced reversible electrical polarization switching of the P(VDF-TrFE) thin film. Furthermore, the fabricated device exhibits a memory window of ∼16.5 V, a high drain current on/off ratio of ∼105, a gate leakage current below ∼300 pA, and excellent retention characteristics for over 104 s. © 2014 AIP Publishing LLC.

Optical investigations of the effect of solvent and thermal annealing on the optoelectronic properties of Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) films

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most promising conducting polymers that can be used as transparent electrode or as buffer layer for organic electronic devices. However, when used as an electrode, its conductivity has to be optimized either by the addition of solvents or by post-deposition processing. In this work, we investigate the effect of the addition of the polar solvent dimethylsulfoxide (DMSO) to an aqueous PEDOT:PSS solution on its optical and electrical properties by the implementation of the Drude model for the analysis of the measured pseudo-dielectric function by Spectroscopic Ellipsometry from the near infrared to the visible-far ultraviolet spectral range. The results show that the addition of DMSO increases significantly the film conductivity, which reaches a maximum value at an optimum DMSO concentration as it has confirmed by experimentally measured conductivity values. The post-deposition thermal annealing has been found to have a smaller effect on the film conductivity. © 2013 Elsevier B.V.

Multi-walled carbon nanotube conductivity enhancement and band gap widening via rapid pulsed thermal annealing

Herein we report on the transport characteristics of rapid pulsed vacuum-arc thermally annealed, individual and network multi-walled carbon nanotubes. Substantially reduced defect densities (by at least an order of magnitude), measured by micro-Raman spectroscopy, and were achieved by partial reconstruction of the bamboo-type defects during thermal pulsing compared with more traditional single-pulse thermal annealing. Rapid pulsed annealed processed networks and individual multi-walled nanotubes showed a consistent increase in conductivity (of over a factor of five at room temperature), attributed to the reduced number density of resistive axial interfaces and, in the case of network samples, the possible formation of structural bonds between crossed nanotubes. Compared to the highly defective as-grown nanotubes, the pulsed annealed samples exhibited reduced temperature sensitivity in their transport characteristics signifying the dominance of scattering events from structural defects. Transport measurements in the annealed multi-walled nanotubes deviated from linear Ohmic, typically metallic, behavior to an increasingly semiconducting-like behavior attributed to thermally induced axial strains. Rapid pulsed annealed networks had an estimated band gap of 11.26 meV (as-grown; 6.17 meV), and this observed band gap enhancement was inherently more pronounced for individual nanotubes compared with the networks most likely attributed to mechanical pinning effect of the probing electrodes which possibly amplifies the strain induced band gap. In all instances the estimated room temperature band gaps increased by a factor of two. The gating performance of back-gated thin-film transistor structures verified that the observed weak semiconductivity (p-type) inferred from the transport characteristic at room temperature. © 2014 Copyright Taylor & Francis Group, LLC.

Amorphous silicon thin film transistor biosensing system

Electronic systems are a very good platform for sensing biological signals for fast point-of-care diagnostics or threat detection. One of the solutions is the lab-on-a-chip integrated circuit (IC), which is low cost and high reliability, offering the possibility for label-free detection. In recent years, similar integrated biosensors based on the conventional complementary metal oxide semiconductor (CMOS) technology have been reported. However, post-fabrication processes are essential for all classes of CMOS biochips, requiring biocompatible electrode deposition and circuit encapsulation. In this work, we present an amorphous silicon (a-Si) thin film transistor (TFT) array based sensing approach, which greatly simplifies the fabrication procedures and even decreases the cost of the biosensor. The device contains several identical sensor pixels with amplifiers to boost the sensitivity. Ring oscillator and logic circuits are also integrated to achieve different measurement methodologies, including electro-analytical methods such as amperometric and cyclic voltammetric modes. The system also supports different operational modes. For example, depending on the required detection arrangement, a sample droplet could be placed on the sensing pads or the device could be immersed into the sample solution for real time in-situ measurement. The entire system is designed and fabricated using a low temperature TFT process that is compatible to plastic substrates. No additional processing is required prior to biological measurement. A Cr/Au double layer is used for the biological-electronic interface. The success of the TFT-based system used in this work will open new avenues for flexible label-free or low-cost disposable biosensors. © 2013 Materials Research Society.

Carbon nanotube forests growth using catalysts from atomic layer deposition

We have grown carbon nanotubes using Fe and Ni catalyst films deposited by atomic layer deposition. Both metals lead to catalytically active nanoparticles for growing vertically aligned nanotube forests or carbon fibres, depending on the growth conditions and whether the substrate is alumina or silica. The resulting nanotubes have narrow diameter and wall number distributions that are as narrow as those grown from sputtered catalysts. The state of the catalyst is studied by in-situ and ex-situ X-ray photoemission spectroscopy. We demonstrate multi-directional nanotube growth on a porous alumina foam coated with Fe prepared by atomic layer deposition. This deposition technique can be useful for nanotube applications in microelectronics, filter technology, and energy storage. © 2014 AIP Publishing LLC.