107 research outputs found
Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties
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
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
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
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
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Secondary electron yield engineering of copper surfaces by 532 nm ultrashort laser pulses
Nanostructured surfaces exhibit outstanding properties and enable manifold industrial applications. In this study the laser surface processing of polycrystalline, flat copper surfaces by 532 nm picosecond laser irradiation for secondary electron yield (SEY) reduction is reported. The laser beam was scanned in parallel lines across the sample surface in order to modify large surface areas. Morphology and SEY are characterized in dependence of the process parameters to derive correlations and mechanisms of the laser-based SEY engineering process. The nano- and microstructure morphology of the laser-modified surface was characterized by scanning electron microscopy and the secondary electron yield was measured. In general, an SEY reduction with increasing accumulated laser fluence was found. In particular, at low scanning speed (1 mm/s - 10 mm/s) and “high” laser power (~ 1 W) compact nanostructures with a very low SEY maximum of 0.7 are formed
The two-dimensional electron gas of the In2O3 surface: Enhanced thermopower, electrical transport properties, and its reduction by adsorbates or compensating acceptor doping
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
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
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
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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