107 research outputs found

    Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties

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    The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of H2 and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab-initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100) surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga-N surface dimer buckling. The signatures of the Ga-H and N-H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean GaN(1-100) surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion

    Surface characterization of indium compounds as functional layers for (opto)electronic and sensoric applications

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    Neue Entwicklungen im Bereich der Dünnschichtsynthese eröffnen eine Vielzahl neuer Anwendungen von Indiumverbindungen im Bereich der Sensorik und (Opto)-Elektronik. Vor diesem Hintergrund wurden spezielle Aspekte der Eigenschaften von Indiumnitrid (InN), Indiumoxid (In2O3) und Indiumzinnoxinitrid (ITON) untersucht. Zukünftige Hochfrequenztransistoren auf der Basis von InN können die technologischen Möglichkeiten erweitern, während für dünne In2O3 Filme ein hohes Potenzial zur Verwendung in günstigen, integrierbaren Ozonsensoren vorausgesagt wird. Zusätzlich kann für optoelektronische Applikationen durch Stickstoffeinbau in Sn-dotierte Indiumoxidschichten die optische Transparenz im UV-Bereich erweitert werden. Ein Grundsatz für die Implementierung der Materialien ist die detaillierte Kenntnis der Einflüsse der Zusammensetzung, sowie der strukturellen und elektronischen Eigenschaften auf wichtige Mechanismen der Funktionsweise und Stabilität von Bauelementen. In diesem Zusammenhang wurden zum Verständnis wichtiger Materialparameter die Einflüsse von Prozessparametern auf die Oberflächeneigenschaften dünner Filme untersucht, sowie Wechselwirkungen mit reaktiven Molekülen analysiert. Dünne InN Schichten wurde mittels plasmainduzierter Molekularstrahlepitaxie abgeschieden, während weitere Untersuchungen an durch metallorganische Gasphasenabscheidung aufgebrachten In2O3 Filmen sowie durch Magnetronsputtern hergestellten ITON Schichten durchgeführt wurden. Zur Analyse wurden Methoden der Elektronenspektroskopie (XPS, UPS, AES, (HR)EELS), der Elektronenbeugung (RHEED) sowie Rastersondenverfahren (AFM) verwendet. Durch in-situ Analyse von InN(0001) Schichten konnten erstmals Korrelationen zwischen Oberflächenrekonstruktionen und der Existenz von Elektronenzuständen innerhalb der Bandlücke nachgewiesen, sowie Einflüsse der Oxidation durch O2 untersucht werden. Zusätzlich wurde der Wechselwirkungsmechanismus zwischen Ozon und defektreichen In2O3-x Schichten analysiert und Rückschlüsse auf das Prinzip der reversiblen O3-induzierten Oxidation und UV-induzierten Reduktion gezogen, welche auf der Adsorption/Desorption von O- Ionen und gleichzeitig stattfindendem Ladungstransfer basiert. Der durch Aufsputtern in N2 eingebrachte Stickstoff, liegt in ITON in verschiedenen chemischen Bindungen vor und verändert die optischen und elektrischen Eigenschaften, ist aber thermisch nicht stabil und desorbiert oberhalb von 550°C, einhergehend mit der gleichzeitigen Oberflächensegregation von Sn. Diese Arbeit demonstriert den Nutzen der Kombination von Schichtwachstum und Oberflächenanalytik, um fundamentale Erkenntnisse für den Einsatz in Halbleiterbauelementen zu gewinnen.Abstract: New developments in thin film synthesis using different deposition methods open the pathway to a variety of new applications of indium compounds in sensoric and (opto)electronic devices. Under this perspective, special aspects of the material properties of indium nitride (InN), indium oxide (In2O3) and indium tin oxynitride (ITON) have been investigated. Future transistors based on InN are promising candidates to extend the technological capabilities of high frequency applications, while a huge potential of thin In2O3 films is predicted for the implementation in cost-efficient integrated ozone sensors. Furthermore, the incorporation of nitrogen in Sn-doped indium oxide films leads to an expansion of the optical transparency in the UV region, which is of interest for optoelectronic applications. An important aspect for technological implementation is to thoroughly understand the influence of chemical composition as well as structural and electronic surface properties on important mechanisms that impact device operation and stability. In this context, this thesis investigates the influences of synthesis and processing parameters on the surface properties of thin films as well as their interaction with reactive molecules. Thin InN layers were grown using plasma assisted molecular beam epitaxy. Further analyses were performed on In2O3 films prepared by metal organic chemical vapour deposition and ITON layers deposited by rf-magnetron sputtering. For the characterization and comparison of surface properties, electron spectroscopy methods (XPS, UPS, AES, (HR)EELS), electron diffraction (RHEED) together with scanning probe microscopy (AFM) were employed and supported by solid state analyses. Due to the performed in-situ characterization of InN(0001) layers, it was possible for the first time to experimentally verify the correlation between surface reconstructions with the existence of occupied electron states inside the band gap as well as to study the interaction of clean surfaces with O2. In addition, the interaction mechanism between ozone and defect-rich In2O3-x surfaces was investigated in order to obtain insight into the principle of reversible O3-induced oxidation and UV-induced reduction, which is based on the adsorption/desorption of O- ions and a simultaneous charge transfer. Nitrogen that is incorporated into ITON by sputtering in N2 is present in different chemical states and modifies the optical and electrical film properties. However, annealing this material above 550°C results in desorption of nitrogen accompanied by a segregation of Sn towards the surface. This thesis demonstrates the utility of combining growth, surface and solid state analysis for providing fundamental knowledge for application in semiconductor devices

    Optimization of the secondary electron yield of laser-structured copper surfaces at room and cryogenic temperature

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    Electron cloud (e-cloud) mitigation is an essential requirement for proton circular accelerators in order to guarantee beam stability at a high intensity and limit the heat load on cryogenic sections. Laser-engineered surface structuring is considered a credible process to reduce the secondary electron yield (SEY) of the surfaces facing the beam, thus suppressing the e-cloud phenomenon within the high luminosity upgrade of the LHC collider at CERN (HL-LHC). In this study, the SEY of Cu samples with different oxidation states, obtained either through laser treatment in air or in different gas atmospheres or via thermal annealing, has been measured at room and cryogenic temperatures and correlated with the surface composition measured by x-ray photoelectron spectroscopy. It was observed that samples treated in nitrogen display the lowest and more stable SEY values, correlated with the lower surface oxidation. In addition, the surface oxide layer of air-treated samples charges upon electron exposure at a low temperature, leading to fluctuations in the SEY

    Role of surface microgeometries on electron escape probability and secondary electron yield of metal surfaces

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    The influence of microgeometries on the Secondary Electron Yield (SEY) of surfaces is investigated. Laser written structures of different aspect ratio (height to width) on a copper surface tuned the SEY of the surface and reduced its value to less than unity. The aspect ratio of microstructures was methodically controlled by varying the laser parameters. The results obtained corroborate a recent theoretical model of SEY reduction as a function of the aspect ratio of microstructures. Nanostructures - which are formed inside the microstructures during the interaction with the laser beam - provided further reduction in SEY comparable to that obtained in the simulation of structures which were coated with an absorptive layer suppressing secondary electron emission

    The two-dimensional electron gas of the In2O3 surface: Enhanced thermopower, electrical transport properties, and its reduction by adsorbates or compensating acceptor doping

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    In2O3 is an n-type transparent semiconducting oxide possessing a surface electron accumulation layer (SEAL) like several other relevant semiconductors, such as InAs, InN, SnO2, and ZnO. Even though the SEAL is within the core of the application of In2O3 in conductometric gas sensors, a consistent set of transport properties of this two-dimensional electron gas (2DEG) is missing in the present literature. To this end, we investigate high quality single-crystalline as well as textured doped and undoped In2O3(111) films grown by plasma-assisted molecular beam epitaxy to extract transport properties of the SEAL by means of Hall effect measurements at room temperature while controlling the oxygen adsorbate coverage via illumination. The resulting sheet electron concentration and mobility of the SEAL are 1.5E13 cm^-2 and 150 cm^2/Vs, respectively, both of which get strongly reduced by oxygen-related surface adsorbates from the ambient air. Our transport measurements further demonstrate a systematic reduction of the SEAL by doping In2O3 with the deep compensating bulk acceptors Ni or Mg. This finding is supported by X-ray photoelectron spectroscopy measurements of the surface band bending and SEAL electron emission. Quantitative analyses of these XPS results using self-consistent, coupled Schroedinger-Poisson calculations indicate the simultaneous formation of compensating bulk donor defects (likely oxygen vacancies) which almost completely compensate the bulk acceptors. Finally, an enhancement of the thermopower by reduced dimensionality is demonstrated in In2O3: Seebeck coefficient measurements of the surface 2DEG with partially reduced sheet electron concentrations between 3E12 and 7E12 cm^-2 (corresponding average volume electron concentration between 1E19 and 2E19 cm^-3 indicate a value enhanced by 80% compared to that of bulk Sn-doped In2O3 with comparable volume electron concentration.Comment: Main article: 11 pages, 7 figures Supplement: 4 pages, 2 figures To be submitted in Physical Review

    Effect of dislocations on electrical and electron transport properties of InN thin films. I. Strain relief and formation of a dislocation network

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    The strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H-InN. Edge-type threading dislocations and dislocations of mixed character have been found to be dominating defects in the wurtzite InN layers. It has been shown that the threading dislocation density decreases exponentially during the film growth due to recombination and, hence, annihilation of dislocations, reaching 109 cm−2 for 2200 nm thick InN films.Unión Europea NMP4-CT2003-505614Unión Europea NMP4-CT-2004-500101Comisión Interministerial de Ciencia y Tecnología MAT2004-01234 Españ

    RF Characterisation of Laser Treated Copper Surfaces for the Mitigation of Electron Cloud in Accelerators

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    In accelerator beam chambers and RF waveguides, electron cloud and multipacting can be mitigated effectively by reducing the secondary electron yield (SEY). In recent years, it has been established that laser-engineered surface structuring is a very efficient method to create a copper surface with a SEY maximum close to or even below unity. Different laser pulse durations, from nanoseconds to picoseconds, can be used to change surface morphology. Conversely, the characteristics that minimise the SEY, such as the moderately deep grooves and the redeposited nanoparticles, might have unfavourable consequences, including increased RF surface resistance. In this study, we describe the techniques used to measure the surface resistance of laser-treated copper samples using an enhanced dielectric resonator with 12 cm diameter sample sizes operating in the GHz range. The quantification basis lies in a non-contact measurement of the high-frequency losses, focusing on understanding the variation of surface resistance levels depending on the specifics of the treatment and possible post-treatment cleaning procedures.</p

    Electrical conductivity and gas-sensing properties of Mg-doped and undoped single-crystalline In2O3 thin films: Bulk vs. surface

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    This study aims to provide a better fundamental understanding of the gas-sensing mechanism of In2O3-based conductometric gas sensors. In contrast to typically used polycrystalline films, we study single crystalline In2O3 thin films grown by molecular beam epitaxy (MBE) as a model system with reduced complexity. Electrical conductance of these films essentially consists of two parallel contributions: the bulk of the film and the surface electron accumulation layer (SEAL). Both these contributions are varied to understand their effect on the sensor response. Conductance changes induced by UV illumination in air, which forces desorption of oxygen adatoms on the surface, give a measure of the sensor response and show that the sensor effect is only due to the SEAL contribution to overall conductance. Therefore, a strong sensitivity increase can be expected by reducing or eliminating the bulk conductivity in single crystalline films or the intra-grain conductivity in polycrystalline films. Gas-response measurements in ozone atmosphere test this approach for the real application
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