@misc{JugovacMenteşGenuzioetal., author = {Jugovac, Matteo and Mente{\c{s}}, Tevfik Onur and Genuzio, Francesca and Lachnitt, Jan and Feyer, Vitaliy and Flege, Jan Ingo and Locatelli, Andrea}, title = {Sensitivity to crystal stacking in low-energy electron microscopy}, series = {Applied Surface Science}, volume = {566}, journal = {Applied Surface Science}, issn = {0169-4332}, doi = {10.1016/j.apsusc.2021.150656}, abstract = {In this work we demonstrate the general characteristics of hcp and fcc stacking in low-energy electron reflectivity for transition metal surfaces, by following the restacking during homoepitaxial growth in real-time. For this purpose, the stacking of a model system, single-crystalline Ag islands during layer-by-layer growth at high temperature on O/W(110), is chosen. Multiple scattering calculations are used to model the relation between electron reflectivity and the crystal geometry. The changes in the electron reflectivity are shown to derive from the changes in the stacking sequence of the topmost surface layers. The results allow to distinguish between the hcp and fcc crystalline arrangements at a surface based on typical differences in the reflectivity curves, making the Ag results relevant for a variety of materials with hexagonal surface geometry. In particular, the multiplet structure within the first Bragg peak in the very low electron energy regime is identified with the fcc structure and thus it can be utilized as a fingerprint to determine the stacking sequence.}, language = {en} } @misc{MoralesLeinenFloresetal., author = {Morales, Carlos and Leinen, Dietmar and Flores, Eduardo and Mu{\~n}oz-Cortes, Esmeralda and Leardini, Fabrice and Ares, Jos{\´e} Ram{\´o}n and Flege, Jan Ingo and Soriano, Leonardo and Ferrer, Isabel J. and Sanchez, Carlos}, title = {Imaging the Kirkendall effect in pyrite (FeS2) thin films: Cross-sectional microstructure and chemical features}, series = {Acta Materialia}, volume = {205}, journal = {Acta Materialia}, issn = {1359-6454}, doi = {10.1016/j.actamat.2020.116582}, pages = {12}, abstract = {This investigation provides novel data on the structure and chemical composition of pyrite thin films and new hints concerning their formation mechanism. From TEM-HAADF data, it has been found that the films are composed of two different layers: one is very compact and the other one is quite porous with many voids separating a few groups of grains. This porous layer is always in direct contact with the substrate, and its thickness is quite similar to that of the original Fe film. The average size of pyrite grains is equal in both layers, what suggests that the same process is responsible for their formation. Concentration profiles of sulfur, iron and some impurities (mainly sodium and oxygen from the glass substrate) through both layers are given in this work, and thus chemical inhomogeneities of the films are proved by the obtained stoichiometric ratios (S/Fe). Moreover, Na from sodalime glass substrates mainly accumulates at the pyrite grain boundaries and barely dopes them. The obtained results support the hypothesis that the iron sulfuration process essentially induces the diffusion of iron atoms, what leads to the porous layer formation as a manifestation of the Kirkendall Effect. Therefore, it seems that the same mechanisms that operate in the synthesis of surface hollow structures at the nanoscale are also active in the formation of pyrite thin films ranging from several tens to hundreds of nanometers.}, language = {en} } @misc{EwertBussGenuzioetal., author = {Ewert, Moritz and Buß, Lars and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {Transitions from single-layer MoS2 to bilayer growth: A LEEM study}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, address = {Bad Honnef}, abstract = {Molybdenum disulfide (MoS2) is well-known to change from an indirect to a direct semiconductor as a single layer. We present insights from in-situ low-energy electron microscopy (LEEM) on the extended growth of MoS2 on the Au(111) surface at elevated temperatures of 720°C. Our continuous growth method leads to the formation of micron-sized single-layer MoS2 islands. The single-domain character of these islands is confirmed by employing dark-field imaging and micro-diffraction (LEED). This also reveals the distribution of 90:10 of the two expected MoS2 mirror domains on Au(111). Selected-area angle-resolved photoelectron spectroscopy (ARPES) measurements of these mirror domains underline the threefold symmetry of the two mirror domains and indicate the presence of MoS2 bilayer. Using X-ray photoemission electron microscopy (XPEEM) and intensity-voltage LEEM (I(V))-LEEM we identify the bilayer nucleation areas at nearly full surface coverage and propose a model pathway for their formation.}, language = {en} } @misc{JanowitzMahmoodinezhadNaumannetal., author = {Janowitz, Christoph and Mahmoodinezhad, Ali and Naumann, Franziska and Plate, Paul and Henkel, Karsten and Flege, Jan Ingo}, title = {XPS and UPS investigation of an ALD prepared Al2O3/ZnO heterojunction}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The band alignment of two large band gap oxides was studied by a combination of XPS and UPS using consecutive sputter steps to unravel the electronic structure and elemental composition of each layer and the interface region. An Al2O3/ZnO heterointerface (10 nm Al2O3 on 59 nm ZnO) was grown on top of a Si single crystal substrate by consecutive thermal and plasma-assisted atomic layer deposition (ALD) respectively. The valence band maximum of Al2O3 was found to be 1.1 eV below that of ZnO, the conduction band minimum 2.3 eV above, resulting in a type-I staggered heterojunction. A reduction of ZnO to elemental Zn in the interface region was detected by the apparent shoulder of the Zn 2p and 2s core levels and by the Zn LMM Auger. This suggests an ALD interface formation mechanism different from previous models identified for other heterointerfaces.}, language = {en} } @misc{MahmoodinezhadMoralesNaumannetal., author = {Mahmoodinezhad, Ali and Morales, Carlos and Naumann, Franziska and Plate, Paul and Meyer, Robert and Janowitz, Christoph and Henkel, Karsten and Kot, Małgorzata and Flege, Jan Ingo}, title = {Low-temperature atomic layer deposition of indium oxide thin films using trimethylindium and oxygen plasma}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {Indium oxide thin films were deposited on Si (100) by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium (TMIn) and oxygen plasma (O2) in a low-temperature range of 80 to 200 °C. The In2O3 layers were characterized by in-situ spectroscopic ellipsometry (SE), ex-situ X-ray photoelectron spectroscopy (XPS) and electrical measurements. The SE data show a growth rate of 0.56 {\AA}/cycle within the ALD window (100 to 150 °C) with a thickness inhomogeneity of ≤1.2\%. In addition, the highest refractive index is 2.07 (at 632.8 nm) for the layer grown at 150 °C, and the films exhibit indirect and direct band gaps of 2.8±0.1 eV and 3.3±0.2 eV, respectively. XPS characterization indicates no carbon incorporation and a temperature-dependent off-stoichiometry of the layers. The chemical analysis of the In 3d and O 1s core levels confirms the formation of In-O bonds and suggests the additional presence of hydroxyl groups and defects. With increasing temperature, the contribution of OH groups and defects decreases whereas that of In-O bonds increases. Notably, higher growth temperatures result in an indium rich phase within the layers.}, language = {en} } @misc{MoralesMahmoodinezhadSchubertetal., author = {Morales, Carlos and Mahmoodinezhad, Ali and Schubert, Andreas Markus and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {Functional ultra-thin oxide films deposited by atomic layer deposition on structured substrates}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {In the last decades, atomic layer deposition (ALD) has gained prominence in the materials and surface science communities owing to its high potential for integration as a scalable process in microelectronics. ALD's largest strengths are its well-controlled layer-by-layer deposition and growth conformity on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric thin films, resulting in modified materials properties that may even preferentially be used in certain applications. To study these issues, we have developed an in-situ ALD reactor attached to an X-ray photoelectron spectroscopy (XPS) system, capable of switching between both pump and flow-type operation. This novel tool allows to cover the entire range of compounds and recipes used in ALD, thus clarifying the role of such defects at different deposition stages, growth conditions and film/substrate interfaces. To exemplify these sorts of studies, we show the deposition of Al2O3 5-10 nm films on nanostructured Si, and their use as substrates for functional CeOx ALD deposits.}, language = {en} } @misc{PozarowskaMoralesFlege, author = {Pozarowska, Emilia and Morales, Carlos and Flege, Jan Ingo}, title = {Growth of samarium thin films and subsequent oxidation on polycrystalline copper}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The growth of samaria thin films on copper sheets has been chemically studied by in situ X-ray photoelectron spectroscopy (XPS). The early stages of growth (0.1-14 ML) were studied by consecutive evaporations of Sm by chemical vapor deposition followed by XPS measurements. Subsequently, samaria films of different thicknesses, namely 0.1, 1, and 14 ML, were oxidized at room temperature (RT). The evolution of the sample morphology was determined through inelastic peak shape analysis (IPSA) using the QUASES software as an indirect method to study the relationship between Sm oxidation state and its surface arrangement. Our results show that samarium grows as 2D islands up to 2ML, which is followed by 3D growth. Chemical analysis indicates that at low coverages (<0.5ML) Sm is already oxidized, leading to the appearance of Sm3+ as the only oxidation state. The increase in the intensity of the O1s peak with time and the absence of spectral changes in the Cu2p and LMM Auger (substrate) indicate that the transformation is mainly due to adventitious oxidation of the layer. With further deposition at RT the metallic state Sm0 appears at higher coverages, which is readily postoxidized by subsequent exposure to molecular oxygen, leading to complete oxidation. No intermediate oxidation states (Sm2+) were observed, in contrast to the reported prevalence of Sm2+ on single crystal surfaces during the early stages of growth.}, language = {en} } @misc{AngrickHenriksenMutzkeetal., author = {Angrick, Christoph and Henriksen, Annika and Mutzke, Nicole and Reimann, Andre and Ewert, Moritz and Buß, Lars and Falta, Jens and Flege, Jan Ingo and Donath, Markus}, title = {Spin-polarized VLEED from single-layer MoS2/Au(111): Investigation of spin-orbit-induced effects}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The influence of spin-orbit interaction on low-energy electron reflection from single-layer MoS2 on Au(111) was studied by VLEED (very-low-energy electron diffraction) [1,2]. The spin-dependent electron reflection was investigated for a wide range of electron incidence angles and kinetic energies. Since the adlayer coverage is about 30\%, we studied the Au(111) substrate and a MoS2 bulk sample for comparison. This approach enabled us to distinguish between adlayer and substrate signals. For MoS2/Au(111), we detected a spin asymmetry of the reflected intensities, which shows a characteristic feature with alternating sign in the energy region of a VLEED fine structure [1]. The Au(111) substrate, in contrast, shows qualitatively different spin-asymmetry features, partially with reversed sign compared with MoS2/Au(111). The results of bulk MoS2 confirm that the characteristic feature in the single-layer data originates from MoS2. The influence of the adlayer-substrate interaction on the results will be discussed. [1] Burgbacher et al., Phys. Rev. B 87, 195411 (2013) [2] Angrick et al., J. Phys.: Condens. Matter 33, 115001 (2020)}, language = {en} } @misc{FischerFlegeFoersteretal., author = {Fischer, Simon and Flege, Jan Ingo and Foerster, Michael and Aballe, Lucia and Falta, Jens and Krisponeit, Jon-Olaf}, title = {Strongly Strained VO2 Thin Film Growth}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The semiconductor-metal transition temperature of VO2 strongly shifts as a result of strain applied along the rutile c axis, making it interesting for various switching applications, as smart coatings and as sensors. In the past, this has been demonstrated, for instance, through the application of epitaxial strain on TiO2 substrates. We extend this tailoring approach by utilizing the much larger lattice mismatch of 8.78 \% occurring in the VO2/RuO2 system for orientations where the c axis lies in-plane. Depositing vanadium oxide by atomic oxygen-supported reactive MBE on an oxidized Ru(0001) template, we have grown VO2 thin films on single domain RuO2 islands with distinct orientations. Locally resolved electron spectroscopy was used to ascertain the correct stoichiometry of the grown VO2 films on all template island types. Low energy electron diffraction reveals the VO2 films to grow indeed fully strained on RuO2(110) but fully relaxed on RuO2(100). Hence, the presented template allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to regions of massive tensile strain.}, language = {en} } @misc{PleinesBussMenteşetal., author = {Pleines, Linus and Buß, Lars and Mente{\c{s}}, Tevfik Onur and Genuzio, Francesca and Locatelli, Andrea and Flege, Jan Ingo and Falta, Jens}, title = {High temperature reduction and reoxidation of cerium oxide on Cu(111)}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {Cerium oxide is of great interest due to its technological importance in various electronic, optical and catalytic applications. The inverse model catalyst cerium oxide on Cu(111) shows a high activity for the production of methanol. Oxygen vacancies, e.g. in form of reduced ceria, are necessary for the production of methanol from CO2 and H2. The reduction of ceria may be achieved by exposure to H2 at elevated temperatures. We studied the interaction of H2 and CO2 with cerium oxide islands on a Cu(111) substrate with low-energy electron microscopy (LEEM) and x-ray absorption spectroscopy (XAS). From earlier studies, the orientation of the cerium oxide is known to be decisive for the catalytic activity. In our experiments, the impact of both orientations are directly compared via growth of (100) and (111) cerium oxide islands side by side. At temperatures around 550 °C, exposure to H2 leads to partial reduction and exposure to CO2 leads to complete reoxidation of the cerium oxide. The (100) and (111) orientations show different reduction and reoxidation behaviors.}, language = {en} }