Picosecond pulsed 532 nm laser system for roughening and secondary electron yield reduction of inner surfaces of up to 15 m long tubes.

Laser-induced surface structuring is a promising method to suppress electron mulitpacting in the vacuum pipes of particle accelerators. Electrons are scattered inside the rough surface structure, resulting in a low Secondary Electron Yield (SEY) of the material. However, laser processing of internal pipe surfaces with a large aspect ratio is technologically challenging in terms of laser beam guidance and focusing. We present a 532 nm ultrashort-pulse laser setup to process the inner parts of 15 m long beam vacuum tubes of the Large Hadron Collider (LHC). Picosecond pulses at a repetition rate of 200 kHz are guided through an optical fiber toward an inchworm robot traveling inside the beam pipe. The system was installed, characterized, and tested for reliability. First surface treatments achieved the required scan precision. Cu2O-dominated nano-features were observed when processing at high average laser power (5 W) and slow scanning speed (5 mm s-1) in nitrogen flow, and the maximum SEY of copper was decreased from 2.1 to 0.7.

The Role of Hydrogen Incorporation into Amorphous Carbon Films in the Change of the Secondary Electron Yield.

Over the last few years, there has been increasing interest in the use of amorphous carbon thin films with low secondary electron yield (SEY) to mitigate electron multipacting in particle accelerators and RF devices. Previous works found that the SEY increases with the amount of incorporated hydrogen and correlates with the Tauc gap. In this work, we analyse films produced by magnetron sputtering with different contents of hydrogen and deuterium incorporated via the target poisoning and sputtering of CxDy molecules. XPS was implemented to estimate the phase composition of the films. The maximal SEY was found to decrease linearly with the fraction of the graphitic phase in the films. These results are supported by Raman scattering and UPS measurements. The graphitic phase decreases almost linearly for hydrogen and deuterium concentrations between 12% and 46% (at.), but abruptly decreases when the concentration reaches 53%. This vanishing of the graphitic phase is accompanied by a strong increase of SEY and the Tauc gap. These results suggest that the SEY is not dictated directly by the concentration of H/D, but by the fraction of the graphitic phase in the film. The results are supported by an original model used to calculate the SEY of films consisting of a mixture of graphitic and polymeric phases.

Processing Strategies for High-Performance Schottky Contacts on n-Type Oxide Semiconductors: Insights from In2O3.

Preparation of rectifying Schottky contacts on n-type oxide semiconductors, such as indium oxide (In2O3), is often challenged by the presence of a distinct surface electron accumulation layer. We investigated the material properties and electrical transport characteristics of platinum contact/indium oxide heterojunctions to define routines for the preparation of high-performance Schottky diodes on n-type oxide semiconductors. Combining the evaluation of different Pt deposition methods, such as electron-beam evaporation and (reactive) sputtering in an (O and) Ar atmosphere, with oxygen plasma interface treatments, we identify key parameters to obtain Schottky-type contacts with high electronic barrier height and high rectification ratio. Different photoelectron spectroscopy approaches are compared to characterize the chemical properties of the contact layers and the interface region toward In2O3, to analyze charge transfer and plasma oxidation processes as well as to evaluate the precision and limits of different methodologies to determine heterointerface energy barriers. An oxygen-plasma-induced passivation of the semiconductor surface, which induces electron depletion and generates an intrinsic interface energy barrier, is found to be not sufficient to generate rectifying platinum contacts. The dissolution of the functional interface oxide layer within the Pt film results in an energy barrier of ∼0.5 eV, which is too low for an In2O3 electron concentration of ∼1018 cm-3. A reactive sputter process in an Ar and O atmosphere is required to fabricate rectifying contacts that are composed of platinum oxide (PtOx). Combining oxygen plasma interface oxidation of the semiconductor surface with reactive sputtering of PtOx layers results in the generation of a high Schottky barrier of ∼0.9 eV and a rectification ratio of up to 106. An additional oxygen plasma treatment after contact deposition further reduced the reverse leakage current, likely by eliminating a surface conduction path between the coplanar Ohmic and Schottky contacts. We conclude that processes that allow us to increase the oxygen content in the interface and contact region are essential for fabrication of device-quality-rectifying contacts on various oxide semiconductors.

Microgravimetric and Spectroscopic Analysis of Solid-Electrolyte Interphase Formation in Presence of Additives.

Electrochemical quartz crystal microbalance (EQCM) with damping monitoring is applied for real-time analysis of solid-electrolyte interphase (SEI) formation in diphenyl octyl phosphate (DPOP) and vinylene carbonate (VC) modified electrolytes. Fast SEI formation is observed for the DPOP containing electrolyte, whereas slow growth is detected in VC-modified and reference electrolytes. QCM measurements in a dry state show considerable reduction of the mass quantity for DPOP and reference samples and minor mass decrease for the SEI layer formed in the presence of VC. The results indicate that VC enhances SEI stability, whereas the addition of DPOP or no additive results in incorporation of loosely attached species, leadubg to SEI instability. Resonance frequency damping, Δw, and dissipation factor, D, are used for analyzing mechanical properties of the SEI layers. The apparent increase of Δw and D during SEI formation in presence of DPOP suggests a pronounced viscoelasticity of the layer. QCM results are compared with surface morphology and chemical composition, revealing excellent agreement of the applied characterization approaches.

Spatiotemporal Photopatterning on Polycarbonate Surface through Visible Light Responsive Polymer Bound DASA Compounds.

Besides interesting applications in drug delivery, photoresponsive molecules have great potential to serve as an efficient basis for postfunctionalization photopatterning of polymer surfaces. To the best of our knowledge, only UV light sources have been exploited as a photoinducer for creating patterned templates with or without hydrogels. In this work, we present a practically facile method for grafting visible light responsive donor-acceptor stenhouse adducts (DASAs) on amino-functionalized polycarbonate surfaces. DASA grafted surfaces have shown excellent lithographic performance using visible light. The functionalized surfaces exhibit significant changes of their physical properties after being illuminated with visible light. By using suitable masks, well-defined patterns can be replicated with high precision and resolution. Since the DASA ligand synthesis and surface functionalization is not cumbersome, this method may serve as a facile protocol for obtaining photopatterned polymer surfaces for various applications.

Morphology, Crystal Structure and Charge Transport in Donor-Acceptor Block Copolymer Thin Films.

We studied structure and charge transport properties of thin films of donor-acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.

Modification of the active layer/PEDOT:PSS interface by solvent additives resulting in improvement of the performance of organic solar cells.

The influence of various polar solvent additives with different dipole moments has been investigated since the performance of a photovoltaic device comprising a donor-acceptor copolymer (benzothiadiazole-fluorene-diketopyrrolopyrrole (BTD-F-DKPP)) and phenyl-C60-butyric acid methyl ester (PCBM) was notably increased. A common approach for controlling bulk heterojunction morphology and thereby improving the solar cell performance involves the use of solvent additives exhibiting boiling points higher than that of the surrounding solvent in order to allow the fullerene to aggregate during the host solvent evaporation and film solidification. In contrast to that, we report the application of polar solvent additives with widely varied dipole moments, where intentionally no dependence on their boiling points was applied. We found that an appropriate amount of the additive can improve all solar cell parameters. This beneficial effect could be largely attributed to a modification of the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-active layer interface within the device layer stack, which was successfully reproduced for polymer solar cells based on the commonly used PCDTBT (poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)]) copolymer.

Thermal functionalization of GaN surfaces with 1-alkenes.

A thermally induced functionalization process for gallium nitride surfaces with 1-alkenes is introduced. The resulting functionalization layers are characterized with atomic force microscopy and X-ray photoelectron spectroscopy and compared to reference samples without and with a photochemically generated functionalization layer. The resulting layers show very promising characteristics as functionalization for GaN based biosensors. On the basis of the experimental results, important characteristics of the functionalization layers are estimated and a possible chemical reaction scheme is proposed.

Preparation and characterization of poly(L-histidine)/poly(L-glutamic acid) multilayer on silicon with nanometer-sized surface structures.

The specific design and modification of surfaces is of great interest, especially for functional surfaces and medical applications. In order to obtain films on a surface, the layer-by-layer deposition of polyelectrolytes represents a well-established methodology. The alternating deposition of poly(L-histidine) and poly(L-glutamic acid) results in a defined, continuous surface coating that was thoroughly characterized using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, ellipsometry, X-ray reflectometry, atomic force microscopy, scanning electron microscopy, contact angle, and electrokinetic measurements. Surface charge, film growth, and final thickness were measured and cross-validated. Additionally, the chemical composition and distribution of polyelectrolytes in the layerstack were determined. Finally, the optical parameters were specified and the surface topography was visualized by several methods. These characterizations revealed a coating with embedded spheroids forming from the bottom layers. This rough surface formed by (PLH/PGA)(8) was highly reproducible and might provide unique features for the design of tailored surfaces.

Changes of the near-surface chemical composition of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide room temperature ionic liquid under the influence of irradiation.

Radiation induced degradation effects are studied for a model ionic liquid (IL)--[EMIm]Tf(2)N--in order to distinguish in which way the results of X-ray based material analysis methods can be falsified by the radiation supplied by typical X-ray sources itself. Photoelectron spectroscopy is commonly used for determining the electronic structure of ionic liquids. Degradation effects, which often occur e.g. in organic materials during X-ray or electron irradiation, are potentially critical for the interpretation of data obtained from ionic liquids. The changes of the chemical composition as well as the radiation-induced desorption of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]Tf(2)N) fragments are analysed by X-ray photoelectron spectroscopy (XPS) as well as quadrupole mass spectroscopy (QMS) upon exposure to monochromated or non-monochromated AlKα X-rays from typical laboratory sources. During the irradiation of [EMIm]Tf(2)N, an increasing carbon concentration is observed in both cases and especially the [Tf(2)N](-) ion is strongly altered. This observation is supported by the results from the QMS analysis which revealed a variety of different IL fragments that are desorbed during X-ray irradiation. It is shown that the decomposition rate is directly linked to the photon flux on the sample and hence has to be considered when planning an XPS experiment. However, for typical experiments on this particular IL the measurements suggest that the changes are on a larger time scale as typically required for spectra acquisition, in particular if monochromated X-ray sources are used.

Effect of annealing on the properties of indium-tin-oxynitride films as ohmic contacts for gan-based optoelectronic devices.

Indium-tin-oxynitride (ITON) films have been fabricated by rf sputtering from an indium-tin-oxide target in nitrogen plasma. The influence of postdeposition annealing up to 800 degrees C is analyzed by electrical, optical, and surface characterization of the films in comparison to indium-tin-oxide (ITO) films fabricated in argon plasma. High-temperature annealing resulted in ITO(N) films with similar carrier concentrations. However, the resistivity and optical transmittance of the ITON films were higher than those of the ITO films. Photoelectron spectroscopy revealed that nitrogen is incorporated into the ITON structure in an unbound state as well as through the formation of metal-nitrogen and oxynitride bonds that decorate oxygen vacancies. When the core level electron spectra of ITO and ITON films are compared, a correlation between carrier concentration and the incorporated nitrogen is found. Changes in ITON electrical properties are mainly induced by the release of nitrogen at temperatures above 550 degrees C. In this context, ohmic contact behavior was achieved for ITON on p-type GaN after annealing at 600 degrees C, while no ohmic contact could be realized using ITO.