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The transparent conductive tantalum doped tin oxide is a potential candidate for applications in concentrated solar power technology, dye-sensitized solar cells and dynamic random access memories [1], [2], [3]. In all these fields, high-temperature stability in air is mandatory to preserve its functionality. In this work we demonstrate the compositional and structural in-air-stability of SnO2:Ta thin films at 650 °C and 800 °C for 12 hours. While the element composition and optical spectra were unchanged and the X-ray diffractograms revealed the conservation of a single-phase rutile-type crystal structure, some strong Raman lines of SnO2:Ta underwent substantial changes upon tempering. Quantum ab initio calculations of pristine and Ta-doped SnO2 with systematically varied point defects indicated that preferentially Sn vacancies and excess O atoms are responsible for these strong and unexpected Raman signatures. These defects are partially healed during high-temperature exposure, but that does not affect the functionality of SnO2:Ta as transparent conductor under these harsh conditions. This study provides a comprehensive understanding of crystal and defect structure of Ta-doped SnO2 prior to and after high temperature treatment in air for the first time and encourages its application in different fields where high-T stability, transparency and conductivity are required. [1] F. Lungwitz et al., Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications, Solar Energy Mater. Solar Cells 199, 84 (2019), doi: 10.1016/j.solmat.2019.03.012 [2] R. Ramarajan, et al. Large-area spray deposited Ta-doped SnO2 thin film electrode for DSSC application, Solar Energy 211, 547-559 (2020), doi:10.1016/j.solener.2020.09.042. [3] C. J. Cho, et al. Ta-Doped SnO2 as a reduction-resistant oxide electrode for DRAM capacitors, Journal of Materials Chemistry C 5, 9405-9411 (2017), doi:10.1039/c7tc03467a Financial support by the EU, grant No. 645725, project FRIENDS2, is gratefully acknowledged.

In situ processing and comprehensive characterization is essential for design and development of materials used and processed at high-temperatures. Here, a new cluster tool for processing and depth-resolved compositional, structural and optical characterization of layered materials with thicknesses ranging from sub-nm to 1 μm at temperatures of -100 to 1000 °C is described [1]. The implemented techniques comprise magnetron sputtering, ion irradiation, Rutherford backscattering spectrometry, Raman spectroscopy and spectroscopic ellipsometry. The combination of techniques enables sample processing by scalable, clean, waste-free, and industry-relevant technologies, quantitative depth-profiling for elements with Z ≥ 6, structural and chemical characterization, sensitivity and nm-precise thickness and optical information for single layers, multilayers and composites. In this study, the cluster tool was used for i) metal-induced crystallization with layer-exchange of a-Si/ Ag layer stacks, and ii) for hightemperature characterization of two types of solar-selective coatings for concentrated solar power (CSP), namely Al Ti (O N )-based single and multilayers [2, 3] and an n-type doped solar-selective transparent conductive oxide [4]. Starting with an a-Si/ Ag bilayer stack, metal-induced silicon crystallization with partial layer exchange occurs at 540 °C. The final stack is approximately described by the sequence crystalline Si (c-Si)/ Ag/ c-Si. All the layers contain minor fractions of the other element. Moreover, the Si volume fraction comprises approximately 10 % of amorphous Si. For the CSP coatings, no compositional and structural changes were found up to a maximum temperature of 840 °C in vacuum. Both types of solar-selective coatings thus represent promising materials for the next generation of CSP technology. [1] R. Wenisch et al., Anal. Chem. 90, 7837-7824 (2018) [2] I. Heras et al., Sol. Energy Mat. Solar Cells, 176, 81-92 (2018) [3] R. Escobar-Galindo et al., Sol. Energy Mat. Solar Cells, 185, 183-191 (2018) [4] F. Lungwitz et al., submitted (2018) Financial support by the EU, grant No. 645725, project FRIENDS , and the HGF via the W3 program (S.G.) is gratefully acknowledged.

One of the major challenges in Concentrating Solar Power (CSP) implies an increase of the working temperature of the solar receiver. In particular, current central tower systems operate at maximum temperatures of 550 ºC mainly due to the severe degradation that the state of the art absorber paints (i.e. Pyromark®) suffer at higher temperatures. In previous works [1,2] aluminum titanium oxynitrides AlyTi1-y(OxN1-x) were shown to be excellent candidate materials for solar selective coatings (SSC). These results confirmed that the designed SSCs based on materials withstand breakdown at 600 ºC in air after 900 hours of thermal cycling. In this paper we discuss the high temperature (up to 700ºC) stability in air of a solar absorber coating based on AlyTi1-y(OxN1-x) deposited by cathodic vacuum arc (CVA) at higher working pressure (P = 2.1 Pa) than those discussed in [1] and [2]. The composition, morphology and microstructure of the films were characterized by ion beam analysis, scanning and transmission electron microscopy and X-ray diffraction. The optical properties were determined by ellipsometry and spectrophotometry (UV-Vis-NIR, FTIR). The microstructural and morphological characterization shows the formation of a solid solution of AlTiN crystalline nanoparticles embedded in an amorphous Al2(O, N)3 matrix. This particular microstructure results in a coating with a high absorption coefficient within the whole wavelength range of interest (0,3 to 25 um) as modeled by spectroscopic ellipsometry. Hence, this single layer absorber shows a solar absorptance, α, of 92% and an emissivity, εRT, of 70%. The addition of an antireflective Al2O3 layer and post deposition thermal treatments improved the optical properties of the absorber to better values (α=96% and εRT=60%) than those of Pyromark®. The thermal stability in air of the absorber was firstly analyzed by cyclic heating tests, showing no degradation after 300h of cycles in air at 700ºC. Subsequently, the samples were tested in a solar furnace at 650 °C and 800 ºC for 12 hours at environmental conditions. Therefore, this absorber coating can be a feasible alternative to absorber paints for next generation of concentrated solar power plants operating at high temperature. [1] I. Heras et al., Sol. Energy Mat. Solar Cells, 176, 81-92 (2018) [2] R. Escobar-Galindo et al., Sol. Energy Mat. Solar Cells, 185, 183-191 (2018)

In concentrated solar power plants, the receiver tubes are one of the key components to increase the solar-thermal conversion efficiency. Absorber materials of those tubes have to exhibit high-temperature and air stability, high optical absorption in the solar region and low thermal emittance. In state of the art central tower plants black paints (i.e. Pyromark 2500) are used as absorber material. However, the high emissivity of those paints leads to high radiative energy losses. Moreover, these paints undergo a temporal degradation and performance loss during the lifetime of the plant. Here, an alternative concept for high-temperature stable solar-selective coatings is presented. It consists of a transparent conductive oxide (TCO) deposited as solar-selective transmitter on a black body absorber. For this purpose, SnO2:Ta thin films were reactively sputtered on fused quartz substrates. Their vacuum and in-air stability up to 800°C were studied by in situ Rutherford backscattering spectrometry (RBS), Raman spectroscopy and spectroscopic ellipsometry (SE). The correlation between structural, optical and electric transport properties was analyzed by RBS, SE, UV-Vis spectrometry, and Hall effect measurements. Solar selective properties are demonstrated as proof of concept for the TCO deposited on a silicon substrate. Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature absorbers in solar selective coatings due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. The composition, morphology, phase structure and microstructure of the films were characterized by elastic recoil detection (ERD), scanning and transmission electron microscopy and X-ray diffraction. An fcc phase structure is found in a broad compositional range of AlyTi1-y(OxN1-x). Simultaneously, sample microstructure and morphology undergo systematic changes from a columnar growth to the development of a heterogeneous structure with spherical nanoparticle inclusions when the oxygen concentration is increased. The optical properties were determined by spectroscopic ellipsometry and UV–Vis–NIR and FTIR spectrophotometry. A comprehensive analysis of the film properties allowed an accurate modelling of the optical constants of the AlyTi1-y(OxN1-x) in the whole wavelength range of solar interest (from 190 nm to 25 µm). It points to a transition from metallic to dielectric behaviour with increasing oxygen content. Consequently, it is demonstrated that the optical properties of these AlyTi1-y(OxN1-x) deposited films can be controlled in a wide range from metallic to dielectric character by adjusting the oxygen concentration, opening a huge range of possibilities for the design of solar selective coatings (SSC) based on this material. Complete SSC, including a TiN layer as IR reflector, were designed by applying optical simulations, obtaining excellent optical selective properties of α=94.0% and εRT = 4.8%.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature absorbers in solar-selective coatings (SSC) due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. A comprehensive analysis of the sample microstructure and morphology allowed an accurate modelling of the optical constants in the whole wavelength range of solar interest (190 nm - 25 μm). The optical properties of these films can be controlled from metallic to dielectric character by adjusting the oxygen content. Complete multilayer SSC, including a TiN layer as IR reflector, were designed by applying optical simulations, obtaining excellent optical selective properties (α = 94.0% and εRT = 4.8%). The design concepts were validated by an excellent agreement between simulated and experimental stacking order, composition and optical properties. The durability of two multilayers was studied under conditions simulating realistic operation of central receiver power plants. Both SSC stacks were stable in single stage tests of 12 h at 650°C in air. During cyclic tests, the coarser microstructure of multilayer 1, was found to be more resistant against oxidation than multilayer 2 constituted of four oxynitride layers with a graded oxygen content. Multilayer 1 fulfilled the performance criterion of PC ≤ 5% for 300 symmetric, 3 h long cycles at 600°C in air confirming that the designed SSCs are exciting candidate material for concentrated solar power applications at high temperature. Financial support by the EU, grant No. 645725, project FRIENDS22, is gratefully acknowledged.

Metal-induced crystallization (MIC) with and without layer exchange (LE) is a method to decrease the crystallization temperature of amorphous group 14 elements by up to several hundred degrees. In situ experiments are expected to provide new insights into thin film evolution and elementary process steps of MIC and to improve existing models of this type of phase transformation. In this contribution in situ Rutherford backscattering spectrometry (RBS), Raman spectroscopy and spectroscopic ellipsometry studies were performed during annealing of amorphous carbon/nickel (a-C/Ni) layer stacks at temperatures up to 750°C. LE was observed independently of the initial stacking sequence, while transformation rate and temperature differ significantly. The positions of the G, D and 2D Raman lines as well as the I(D)/I(G) ratio changed during the LE process. These were assigned in agreement with the Three-Stage-Model [1], confirming the transformation of a-C to nc-graphite. In situ RBS measurements demonstrated an opposite shift of the C- and Ni- related backscattering energies, proving that LE and graphitization occur simultaneously. [1] Ferrari et al., Phys. Rev. B 61 (2000) 14095

The development of solar-selective CSP receiver coatings with high-temperature and environmental stability requires new concepts of design and in operando monitoring. Solar receiver tubes are a key component of solar thermal power plants. The increase of their operation temperature from today’s maximum of 550°C to about 800°C could increase the CSP efficiency by approximately 15 to 20% and improve the competiveness of this technology compared to other ones of carbon-free electricity generation. Potential alternatives to fast degrading state-of-the-art pigment paint receiver tube coatings are based on refractive metal carbides, nitrides, and oxides because of their high thermal stability and oxidation resistance. New types of solar-selective coatings were studied in situ at temperatures of up to 830°C by Rutherford backscattering spectrometry, Raman spectroscopy, and spectroscopic ellipsometry within a cluster tool. High-temperature stability in high-vacuum is demonstrated for carbon-metal- and oxynitride-absorber-based multilayers as well as for a solar-selective transmitter based on a transparent conductive oxide. Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

In solar-thermal power plants the receiver tubes are one of the key components for increasing the efficiency of concentrated solar power (CSP) technology. Absorber materials of those tubes have to exhibit high-temperature and air stability, high optical absorption in the solar region and low thermal emittance. Temperatures of up to 400 °C and up to 550 °C are reached in currently operated parabolic trough and central receiver plants, respectively. The CSP efficiency could be increased by 15 to 20% applying operation temperatures of around 800 °C. In state of the art central tower plants black paints are used as absorber material. Due to limited stability they have to be periodically replaced. Furthermore the high emissivity of those paints leads to high radiative energy losses. Most of the R&D approaches for high-temperature solar receiver materials are focused on complex multilayer coatings. Here, an alternative concept for high-temperature stable solar-selective coatings is presented. It consists of a transparent conductive oxide (TCO) deposited as solar-selective transmitter on a black body absorber. The latter is responsible for high absorption in the solar spectral range (300 nm – 2000 nm), the former ensures low emissivity in the infrared range (> 2000 nm) and oxidation resistance. The concept is easily implementable and combines significant improvements of CSP technology performance and cost competiveness. This study is focused on the solar-selective transmitter component of the concept. For this purpose, SnO2:Ta thin films are reactively magnetron co-sputtered from tantalum doped and undoped tin targets at high temperatures on fused quartz, silicon and carbon substrates. Their optical properties are tailored to meet the specific requirements of a solar-selective transmitter coating. The correlation between structural, optical, and electrical properties is analyzed by Raman spectroscopy, spectroscopic ellipsometry (SE) and Rutherford backscattering spectrometry (RBS). All the techniques are applied in situ at high- temperatures in a cluster tool. In order to simulate real operating conditions, cyclic heating tests and heating in reactive atmospheres are conducted. Additionally, X-ray Diffraction (XRD), UV-VIS spectrometry, and Hall Effect measurements are performed. It is shown that structural parameters like e.g. grain sizes and dopant concentration result in different electrical properties and as a result determine the optical behavior like spectral selectivity of the TCO. Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

The detailed knowledge of composition and microstructure is essential for the understanding of processes and properties of new materials for applications at high temperatures. To ensure materials functionality under in operando conditions, new concepts for analysis and process monitoring are necessary. In this contribution, selected PVD deposited thin film material systems were studied in situ at temperatures up to 830°C by Rutherford backscattering spectrometry (RBS), Raman spectroscopy, and spectroscopic ellipsometry (SE) within a cluster tool. Metal-induced crystallization with and without layer exchange (MIC w/o LE) is an emerging technique for processing of amorphous group IV elements below their isothermal crystallization temperatures. In this study, a bilayer system of 60 nm amorphous Si covered by 30 nm Ag (a-Si/Ag) was annealed at temperatures of 380 to 700°C by the combination of the above mentioned in situ techniques. The process comprised a relatively long-term incubation period followed by a fast MIC w/o LE step. More than 90% of the initial a-Si could be crystallized on top of the Ag-layer for optimized process conditions with temperatures of about 550°C. The as-formed Si consisted of up to 95% crystalline Si. As an example for high-temperature solar selective coatings for thermo-solar applications, AlTiN and AlTiN1-xOx (x = 0 - 0.2) thin films were investigated in order to understand the influence of the oxygen/nitrogen ratio on the optical properties and failure mechanisms at high temperatures. The elemental depth profiles and the phase structure of both coatings do not change during annealing in high vacuum at temperatures up to of 750°C, as revealed by unchanged RBS and Raman spectra, respectively. SE and RBS results showed the influence of the initial oxygen content on high temperature stability of AlTiN and AlTiN1-xOx thin films. The low emittance of AlTiN1-xOx, allowed performing in situ RBS analysis at temperatures up to 830°C for the first time.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature solar-selective coatings due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. The composition, morphology, phase structure and microstructure of the films were characterized by elastic recoil detection (ERD), scanning and transmission electron microscopy and X-ray diffraction.

Metal-induced crystallization with and without layer exchange (MIC w/o LE) is a method to decrease the crystallization temperature of amorphous group 14 elements (G14E) by up to several hundred degrees. In situ experiments are expected to provide new insights into thin film evolution and elementary process steps of MIC w/o LE and to improve existing models of this type of phase transformation. While MIC w/o LE has been widely studied for Si and Ge in contact with catalytic metals, there exist only a few studies for the crystallization of amorphous carbon. Therefore, in this contribution in situ Rutherford backscattering spectrometry (RBS), Raman spectroscopy and spectroscopic ellipsometry studies were performed during annealing of amorphous carbon/nickel (a-C/Ni) layer stacks at temperatures up to 750°C. Due to its small lattice mismatch with the basal plane of graphite and high diffusivity of C atoms, Ni is a suitable catalyst for the growth of graphene and crystalline graphitic nanostructures. During the annealing of an a-C/Ni layer stack covalent bonds between the carbon atoms at the catalyst interface are weakened. Liberated carbon atoms can move along the interface and diffuse along the grain boundaries into the Ni layer towards the catalyst surface, where nucleation and grain growth of graphitic crystallites occur. Our in situ studies showed a change in the stacking sequence between C and Ni layers under defined experimental conditions. According to in situ Raman measurements, this mechanism occurs independent of the stacking sequence, while the velocity of the LE differs significantly. As observed in time and temperature resolved Raman spectra, the position of the G peak and the I(D)/I(G) ratio changed according to the Three-Stage-Model by Ferrari and Robertson, confirming the transformation of amorphous carbon to nc-graphite. With the in situ RBS measurements more insight into LE was given. Here peak positions of C and Ni were shifted, indicating a change of the energy of the scattered ions for both layers respectively and proving the combination of the observed graphitization process with LE during annealing. The thickness of the synthesized crystalline graphitic layer is controlled by the finite carbon source – the deposited a-C film, which is a decisive advantage of this process compared to CVD. It is demonstrated that the structure and the crystallite size of the metallic catalyst layer has a strong influence on the crystallite size and the quality of the graphitic film. LE is potentially interesting for industrial applications, as it allows the formation of polycrystalline thin films of G14E at much lower temperatures - than during thermal annealing without the metallic catalyst. Depending on the initial stacking sequence, the crystalline graphitic film can be deposited on a suitable device-ready substrate or transferred to another substrate after the dissolution of the transition metal catalyst.

Titanium dioxide (TiO2) is used in many applications as a photocatalyst. However, TiO2 activity is mostly limited to the UV spectral region due to its wide band-gap (~3eV). For this reason, many efforts1 have been focused on band-gap narrowing to achieve visible-light (VISL) response in TiO2, mostly by doping. Metal (cation) doping increases VISL absorption significantly but, unfortunately, it introduces structural distortions in the host matrix that result in a large number of defects acting as carrier recombination centers.1 Post-processing thermal treatments are normally employed here to improve the structural order.2 In this work, we study the impact of rapid non-contact thermal processes as flash-lamp annealing (FLA) on the electronic structure of Cr-doped TiO2. For this purpose, (amorphous) thin films with different Cr contents were produced at room temperature by magnetron co-sputtering. The dopant concentration was quantified by Rutherford backscattering spectrometry (RBS) whereas the resulting structural phases after FLA were assessed by Raman and X-ray diffraction (XRD). Due to the disordered nature of the samples, the structural characterization has been complemented with local-order information around host and dopant sites from the X-ray near-edge structure (XANES). Finally, the optical properties have been studied by spectroscopic ellipsometry (SE). It is found that FLA can selectively tune the anatase/rutile phase formation in pure TiO2. In addition, films with low doping (Cr < 6 at.%) display a rutile structure. For higher doping level, the formation of high-valence Cr sites is observed, which seems to be detrimental for the structural promotion. Nonetheless, these sites are thermally unstable and annihilated upon FLA. REFs: 1Asahi et al. SCI 293, 269 (2001); 2W. Zhu et al. PRL 103, 226401 (2009).

The detailed knowledge of composition and structure is essential for the understanding of processes and properties of functional materials at elevated temperatures. To ensure materials functionality under in operando conditions, new concepts for analysis and process monitoring are necessary. In this contribution, selected layered material systems were studied in situ at temperatures up to 830°C by Rutherford backscattering (RBS), Raman spectroscopy, and ellipsometry within a cluster tool. Metal induced crystallization (MIC) is a promising technique for hydrogen-free synthesis of two-dimensional materials. Here, Si/Ag bilayers are studied as model system. The Si/Ag layer stacks are annealed at temperatures of 380 to 700°C. simultaneously, depth profiles of the elements are investigated by RBS revealing the diffusion kinetics. The changes in the phase structure and the degree of crystallinity are analyzed by Raman spectroscopy. Both the quick initial nucleation and ensuing growth processes are investigated. MIC is observed for all temperatures under study, while layer exchange occurs only for optimized process conditions. As an example for high-temperature functional coatings, AlTiOxN1-x thin films were investigated in order to understand the influence of the oxygen to nitrogen ratio on the optical properties and their failure mechanisms at high temperatures. Ellipsometry and RBS results showed the influence of the initial oxygen content in the sample, inward diffusion of oxygen into the coating, and the high temperature stability of AlTiOxN1-x thin films. The low emittance of AlTiOxN1-x, allowed performing in situ RBS analysis at temperature up to 830°C for the first time. Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

Materials used in the receiver tubes of a solar thermal power plant must exhibit several properties, e.g. high temperature stability, high absorption in the solar region and low thermal emittance. Nowadays, temperatures of up to 450°C and up to 550°C are reached using parabolic trough arrays and solar tower absorbers, respectively, whereas temperatures up to 800°C or higher could be reached if the receiver materials were stable enough. Previous R&D approaches for high temperature solar receiver materials include multilayer coatings deposited by PVD or sol-gel techniques. Here, a new concept for solar-selective coating is presented. A transparent conductive oxide (TCO) is deposited as a solar selective transmitter on a black body absorber to implement both, high absorption (from the black body) in the ultraviolet, visible and near infrared spectral range (300 nm – 2500 nm) as well as high reflectivity (from the TCO) in the infrared (> 2500 nm) in a relative simple material design. Therefore SnO2:Ta and TiO2:Ta thin films are reactively magnetron co-sputtered from tantalum doped and undoped metal targets at high temperatures (400°C - 700°C). By changing dopant concentration, oxygen flux, process pressure and deposition temperature the optical properties of these films can be tailored to meet the requirements of a solar selective transmitter coating. It is also shown that the electrical properties of the TCO, namely charge carrier concentration and mobility, determine the optical behavior. The correlation between structural, optical, and electrical properties is analyzed by Raman Spectroscopy and Spectroscopic Ellipsometry (SE) both at room- and especially (in situ) at hightemperatures simulating the conditions where the functional coating is supposed to operate. Additionally, Rutherford Backscattering Spectroscopy (RBS), X-ray Diffraction (XRD), UV-VIS spectrometry, and Hall Effect measurements are performed. Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

Estructura selectiva solar con autolimpieza resistente a altas temperaturas. La presente invención se dirige a una estructura formada por una sección superior que comprende una capa superior que comprende TiO2 dopado que presenta una alta transmitancia en el espectro visible y una alta reflectancia en la región IR y propiedades de autolimpieza, una sección intermedia absorbente y un sustrato. Debido a las propiedades mencionadas y a la resistencia a altas temperaturas, la estructura es útil como estructura selectiva solar para receptores de torre en sistemas de energía solar por concentración (CSP).

AlTiN, AlTiO, and AlTiO(x)N(y) thin films were investigated in order to understand the influence of the oxygen to nitrogen ratio on the failure mechanisms at high temperatures. The thin films were deposited by cathodic vacuum arc and characterized in-situ following the methodology proposed for comprehensive environmental testing of optical properties in thin films using the HZDR cluster tool [1]. This multi-chamber material processing and analysis system enables the detailed analysis of the temperature dependence of composition, chemical bonding, and optical properties of thin films. The methodology combines the sequential study of the optical constants by spectroscopic ellipsometry, compositional analysis using ion beam analysis techniques and structure analysis by Raman spectroscopy. All characterizations of AlTiO(x)N(y) thin films were carried out in situ without sample exposure to undefined atmospheres. The samples were heated in vacuum from room temperature to 800°C inside the different chambers and in parallel, to elucidate the influence of the ambience on the degradation process. Moreover, ex-situ annealing in air was performed. Ellipsometry, Raman and ERDA results show the influence of the initial oxygen content in the sample with the inward diffusion of oxygen into the coating and the oxidation resistance at high temperatures. [1] I. Heras, E. Guillén, R. Wenisch, M. Krause, R. Escobar Galindo, J.L. Endrino - Comprehensive environmental testing of optical properties in thin films. Procedia CIRP. 22 (2014) 271–276

A systematic study of TiO2 films deposited by dc filtered cathodic vacuum arc (FCVA) was carried out by varying the deposition parameters in a reactive oxygen atmosphere. The influence of the oxygen partial pressure on film properties is analyzed. Composition was obtained by Rutherford backscattering spectroscopy (RBS) measurements, which also allow us to obtain the density of the films. Morphology of the samples was studied by scanning electron microscopy (SEM) and their optical properties by ellipsometry. Transparent, very dense and stoichiometric TiO2 films were obtained by FCVA at room temperature. Unión Europea H2020 FRIENDS2

Metal induced crystallization (MIC) is a promising technique for thin film transistor fabrication and graphene synthesis. In MIC, a transition metal catalyzes the crystallization of the amorphous phase of a group IV element by bond screening near the interface and facilitation of nucleation. So far, in situ studies have been performed using X-ray diffraction which is sensitive to the degree of crystallinity. However this technique lacks depth resolution and is therefore unable to track diffusion and layer exchange. Here, the Si/Ag and C/Ni bilayer systems are studied. The samples are annealed at temperatures of up to 750 °C. Simultaneously, depth profiles of the elements are investigated by in situ Rutherford backscattering spectroscopy revealing the diffusion kinetics. The changes in the phase structure are explored by in situ Raman spectroscopy. Both the quick initial nucleation and ensuing growth processes are investigated. Scanning electron microscopy provides insight to the surface morphology.

Metal induced crystallization (MIC) is a promising technique for low temperature thin film transistor fabrication and graphene synthesis. In MIC, a transition metal acts as seed for the crystallization of an amorphous group IV element. Bond screening near the interface and facilitation of nucleation are recently discussed as mechanisms for MIC. So far, in situ studies have been performed using X-ray diffraction, which is sensitive to the degree of crystallinity but lacks depth resolution. A better insight into the MIC mechanisms requires depth resolved in situ studies in order to determine the concentration profiles of the diffusing components. Here, the Si/Ag and C/Ni bilayer systems are studied. They are annealed at temperatures of up to 750 °C. Simultaneously, the layer composition and the compositional profiles are investigated with in situ Rutherford backscattering spectroscopy revealing the diffusion kinetics of the components. Both, the quick initial nucleation and the ensuing growth processes are investigated. Further characterization is performed employing in vacuo Raman spectroscopy revealing the phase structure of the resulting films and scanning electron microscopy to investigate the surface structure.

Transparent conductive oxides (TCOs) are already widely used in the optoelectronic industry e.g. as electrodes for liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), or thin film solar cells. Less attention has been devoted to their optical properties and thermal stability until now. In this work, Tantalum doped TiO2 and SnO2 TCO films are investigated with respect to their structural, optical, and electrical properties at temperatures from RT to 700°C. The films are prepared at room temperature by direct current reactive magnetron sputtering from metallic as well as ceramic targets and subsequently isothermally annealed at temperatures of 425°C. For compositional and structural analysis x-ray diffraction (XRD), Raman spectroscopy, and Rutherford backscattering spectroscopy (RBS) are used. The optical properties are determined by spectroscopic ellipsometry, spectral photometry, and subsequent modelling. Hall effect measurements are used to determine the electrical properties of the TCO films. The as-deposited layers are amorphous and isolating. By thermal annealing they are activated and become conductive.

In solar thermal energy conversion systems, receivers containing the heat transfer fluid are coated by a solar selective coating which must exhibit high absorption in the solar region and low thermal emittance. Additionally, the coating materials have to be structurally, optically, and mechanically stable at high temperatures. Nowadays, temperatures of up to 450 °C and up to 550°C are reached using parabolic trough arrays and solar tower absorbers, respectively, whereas temperatures up to 800 °C could be reached if the receiver materials were stable enough. Solar selective coatings can be formed by a transparent conductive oxide (TCOs) film deposited on a black body absorber to have both, high absorption in the ultraviolet, visible and near infrared spectral range (300 nm – 2500 nm) as well as high reflectivity in the infrared (> 2500 nm). The former is to absorb as much sunlight as possible, the latter for preventing thermal radiation losses from the system to the environment. In this work Ta:TiO2 and Ta:SnO2 TCOs thin films are reactively magnetron sputtered from tantalum doped metallic targets. The oxygen flow during deposition is precisely controlled by a plasma emission unit which is crucial to obtain optimal electrical and therefore also optical properties by maintaining high sputtering rates. While the as-deposited films are amorphous and non-conductive, they are crystallized and therefore electrically activated upon a subsequent thermal treatment at 425 °C for 1 hour. The correlation between structural, optical, and electrical properties is shown by Rutherford Backscattering Spectroscopy (RBS), X-ray Diffraction (XRD), Raman Spectroscopy, Spectroscopic Ellipsometry (SE) (both at room- and high- temperatures), UV-VIS spectrometry, and Hall Effect measurements. Preliminary tests show that optical constants of Ta:TiO2 films are maintained after annealing at 700ºC.

The work is focused on understanding the physical mechanism leading to limited free electron density and mobility by variation of the oxygen content in polycrystalline tantalum doped TiO2 films. The films were prepared on glass substrates using a two-step approach involving direct-current magnetron sputtering of non-conducting amorphous films followed by annealing in vacuum. It is demonstrated that that fine tuning of the oxygen content during growth is crucial to ensure the formation of anatase films with low resistivity in the range of 10-3 Ωcm and high optical transmittance after the annealing process. An increase of the oxygen content in the anatase film leads to a decrease of the free electron density and the mobility. This dependence of the film electrical properties is discussed in terms of the effective electrical activation of the Ta dopant taking into account the formation of negatively charged acceptor like defects. Doppler broadening positron annihilation spectroscopy was used to determine the density of negatively charged open-volume defects as a function of oxygen deficiency of the Ta-doped anatase TiO2 films. It is observed that the density of these negatively charged defects increases with increasing oxygen content in the films, which is attributed to the formation of Ti-related vacancies. These acceptor like defects are considered to counteract n-type doping by Ta resulting in a decreasing electron density with increasing oxygen content. Furthermore, due to their maximum charge state of q = -4, Ti vacancies are effective scattering centers for free electrons [1]. Thus, their presence is believed to contribute to the observed decrease of the free electron mobility with increasing oxygen content in the films. These experimental results are consistent with previously reported first-principles calculations [1] of the point defect formation enthalpies for Ti substitution by Ta and for intrinsic Ti-vacancies in anatase TiO2 in dependence of the oxygen chemical potential. 1. J. Osorio-Guillen, S. Lany, and A. Zunger, Phys. Rev. Lett. 100, 036601 (2008).

Moderne Werkstoffe sind bei vielen Anwendungen extremenUmgebungsbedingungen ausgesetzt. Dazu zählen hohe und tiefe Temperaturen bzw. große Temperaturschwankungen, die teilweise in Verbindung mit korrosiven oder reaktiven Atmosphären oder hohen mechanischen Belastungen auftreten. Sie stellen hohe Anforderungen an die Beständigkeit und Stabilität der verwendeten Werkstoffe. Als Beispiele seien solarselektive Absorber, Komponenten des Antriebsstranges von Verbrennungsmotoren und Rotoren von Turboladern und Turbinen genannt. Die Gewährleistung der Werkstofffunktionalität über die gesamte Lebensdauer erfordert neue Konzepte der Analyse und Prüfung. Dazu wurde am 6MV Ionenbeschleuniger des HZDR mit Fördermitteln des Impuls- und Vernetzungsfonds des Präsidenten der HGF sowie des KFSI ein Cluster-Tool zur in situ Modifizierung und Analyse von Werkstoffen bei Temperaturen von bis zu 1000°C, unter korrosiven Atmosphären und mechanischem Verschleiß aufgebaut. Kernbestandteil des Cluster-Tools ist eine zentrale Hochvakuum (HV)-Probenaufnahme- und Transferkammer. Diese ist mit weiteren HV-Kammern verbunden, in denen die sequentielle Probensynthese und -modifizierung, die Element- und Strukturanalytik und die optische Charakterisierung erfolgt, ohne das Vakuum zu brechen und die Werkstoffe undefinierten Umgebungsbedingungen auszusetzen. In allen Kammern besteht die Möglichkeit, in situ Heizexperimente bei Temperaturen von bis zu 1000°C durchzuführen. Die mit dem 6MV Tandem-Ionenbeschleuniger verbundene Kammer zur Ionenstrahlanalytik ermöglicht die Untersuchung der Elementzusammensetzung durch Rutherford Rückstreuung und Nuklearer Reaktionsanalyse. Die Schichtzusammensetzung kann bis in eine Tiefe von ca. 1μm auf bis zu 10 nm genau bestimmt werden. Bei Hochtemperaturuntersuchungen reduziert sich die Tiefenauflösung auf etwa 25 nm. Zur strukturellen Charakterisierung der Proben dient ein fasergekoppeltes Ramanspektrometer an der Analysenkammer des Cluster-Tools, dessen Probenkopf über eine Kamera auch die visuelle Beurteilung der Proben ermöglicht. Die Untersuchung der optischen Eigenschaften und deren Abhängigkeit von Temperatur und Atmosphäre erfolgt durch spektroskopische Ellipsometrie in einer Umweltkammer, in der korrosive Umgebungen simuliert werden können. Für 2015 ist die Installation eines in situ Tribometers in Vorbereitung, mit dem das Reib- und Verschleißverhalten von Werkstoffen unter definierten Atmosphären untersucht werden kann. Zusammengefasst entsteht mit dem Cluster-Tool am 6 MV Ionenbeschleuniger des HZDR ein Messplatz zur umfassenden in situ Modifizierung und Analyse von Werkstoffen, mit Hilfe dessen komplexe Probenbehandlungsprotokolle unter extremen Umgebungsbedingungen bearbeitet werden können.