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We introduce laser-assisted Time-Resolved SEM (TR-SEM), joining Scanning Electron Microscopy and laser light excitation, to probe the long-term temporal evolution of optically excited charge distributions at the surface of Metal Ammonium Lead Triiodide (MAPbI3) hybrid perovskite thin films. Laser-assisted TR-SEM relies on the optically induced local modification of Secondary Electron (SE) detection yield to provide mapping of photoexcited potentials and charge dynamics at surfaces, and qualifies as a complementary approach to near-field probe microscopies and nonlinear photoemission spectroscopies for photovoltage measurements. Real-time imaging of evolving field patterns are provided on timescales compatible with SEM scanning rates, so that temporal resolution in the millisecond range can be ultimately envisaged. MAPbI3 is an outstanding light-sensitive material candidate for applications in solar light harvesting and photovoltaics, also appealing as an active system for light generation. In this work, the real time temporal evolution of optically induced SE contrast patterns in MAPbI3 is experimentally recorded, both under illumination by a 405 nm blue laser and after light removal, showing the occurrence of modifications related to photoinduced positive charge fields at surface. The long term evolution of these surface fields are tentatively attributed to ion migration within the film, under the action of the illumination gradient and the hole collecting substrate. This optical excitation is fully reversible in MAPbI3 over timescales of hours and a complete recovery of the system occurs within days. Permanent irradiation damage of the material is avoided by operating the SEM at 5 keV of energy and 1–10 pA of primary current. Optical excitation is provided by intense above-bandgap illumination (up to 50 W/cm2). TR-SEM patterns show a strong dependence on the geometry of SE collection. Measurements are taken at different axial orientations of the sample with respect to the entrance of the in-column detection system of the SEM and compared with numerical modeling of the SE detection process. This enables to single out the information regarding the local potential distribution. Results are interpreted by combining data about the spectral distribution of emitted SEs with the configuration of the electric and magnetic fields in the specimen chamber. The present modeling sets a robust basis for the understanding of photoinduced SE electron contrast.

Key assets for transparent electric contacts in optoelectronic applications are high conductivity and large transparency over extended spectral range. Indium-Tin-Oxide and Aluminium-doped-Zinc-oxide are commercial examples, with their electrical conductivity resembling those of metals, despite, their transparency being limited up to 1.5µm. This work introduces smooth and compact amorphous thin films of n-type semiconducting WO3-x prepared by RF-sputtering followed by annealing in dry air, as optical layers of tailorable dielectric properties. We evaluate Figure of Merit, combining electrical conductivity and optical transparency, and rate the performances as a transparent conductive layer.

To leverage the outstanding photonic qualities of lead halide hybrid perovskites under high optical irradiance, their reliability and temporal stability must be assessed. Time-resolved scanning electron microscopy unveils that when illuminating the free surface of methylammonium lead iodide (MAPbI3) films at 500 Sun in vacuum, a giant photopotential locally develops in tens of seconds, differently evolving depending on charge-selective substrates. It is reversible on timescales of minutes in the case of hole-transporting poly(3,4-ethylenedi- oxythiophene) polystyrene sulfonate, while more persistent effects occurs, on the timescale of hours, in the case of electron-transporting TiO2. In addition, films grown on TiO2 show irreversible decay of photoluminescence measured in situ and photoinduced alteration of the work function at some grain bounda- ries. Different responses at high irradiance are ascribable to contact-dependent and light-induced spatial redistribution of charged defects, either ions or local- ized dipoles. It is also clear that photoexcited charges play different roles in the photochemistry of systems, depending on selective contacts and they are likely to mediate diverse photoassisted redox reaction paths. The TiO2 layer may act as a photocatalyst, leading to MAPbI3 degradation.

A seeded FEL driven by a linac based on super conducting cavities, generating 108–1010 coherent photons per shot at 2–5 keV with 0.2–1 MHz of repetition rate, can address the need of a source devoted to fine analysis of matter using the linear spectroscopy technique. The seeding scheme described hereafter is a multi-stage cascade upshifting the radiation frequency by a factor 20–40. The x-ray range can be achieved with a seed constituted by a coherent flash in the extreme ultraviolet range provided by an FEL oscillator operating at 12–14 nm. The whole chain of x-ray generation is described by means of start-to-end three-dimensional simulations.

Orbital reconstruction in cases of trauma is usually performed using the unaffected side orbital volume as a reference, but this measurement does not fully consider the anatomical characteristics of orbital surfaces. We propose a novel procedure based on the registration of 3D orbital segmented surfaces. Reconstructed orbits from 20 patients and healthy orbits from 13 control subjects were segmented from the post-operative CT-scans. The 3D orbital model from the unaffected orbit was “mirrored” according to the sagittal plane and superimposed onto the reconstructed one, with calculation of volumes, asymmetry index and point-to-point RMS (root mean square) distances. Inter- and intra-observer errors were tested through Bland–Altman plot. Differences in volume, asymmetry index and RMS value between the control group and the treated patients were assessed through two-way ANOVA and Student's t-test (p < 0.05). According to Bland–Altman test, intra- and inter-operator repeatability was respectively 87% and 89%. No significant differences in volume or asymmetry index between the control group and the treated patients were observed (p > 0.05), but the RMS value was significantly larger in the latter ones (on average, 0.90 ± 0.26 mm vs. 0.67 ± 0.17 mm, p < 0.05). Results show that the reconstructed orbits present a morphologically different surface from the unaffected ones.

The design of lattice structures for additive manufacturing processes requires dedicated methods and tools able to reduce the computational heaviness of simulation and to predict accurately the global strength and strain energy dissipation and the stress distribution. Additionally, geometrical-related stress intensifications lead to local yielding regions and related plastic hinges which contribute to the deformation mechanisms. The use of cellular materials in heavy loaded structures has important role in lightered and hybrid components for aircrafts. In this paper, lattice structures are used as impact absorbers in aircrafts anti-icing systems to efficiently dissipate the kinetic energy. Some variants of lattice structures are analyzed by numerical simulations and experimental characterizations in the elastoplastic regime with the goal to correlate the equivalent material density, the structural strength, and the strain energy dissipation to cells shapes and dimensions.

The need of a fs-scale pulsed, high repetition rate, X-ray source for time-resolved fine analysis of matter (spectroscopy and photon scattering) in the linear response regime is addressed by the conceptual design of a facility called MariX (Multi-disciplinary Advanced Research Infrastructure for the generation and application of X-rays) outperforming current X-ray sources for the declared scope. MariX is based on the original design of a two-pass two-way superconducting linear electron accelerator , equipped with an arc compressor, to be operated in CW mode (1 MHz). MariX provides FEL emission in the range 0.2–8 keV with 1 0 8 photons per pulse ideally suited for photoelectric effect and inelastic X-ray scattering experiments. The accelerator complex includes an early stage that supports an advanced inverse Compton source of very high-flux hard X-rays of energies up to 180 keV that is well adapted for large area radiological imaging, realizing a broad science programme and serving a multidisciplinary user community, covering fundamental science of matter and application to life sciences, including health at preclinical and clinical level.

Ultrafast Scanning Electron Microscopy (USEM) merges photonics and electron microscopy, combining the temporal resolution of femtosecond laser spectroscopy with the nanoscale spatial resolution and surface selectivity typical of scanning electron microscopy, and provides imaging of dynamical photo induced processes at surfaces and in ultra-thin films. Our USEM apparatus is based on a UHV (10-9÷10-10 Torr) Field-Effect SEM, modified with optical windows at the tip of the electron gun and at the specimen chamber and optically coupled to an ultrafast pulsed fs laser source (300 fs,1030 nm) operated at 10 MHz repetition rate. The UHV controlled environment guarantees elevated sensitivity to the physical-chemistry of sample surfaces. Time-resolved operation is achieved by means of a pump-probe configuration, where the UV third harmonic (TH) beam of the laser source works as the optical pump, while the fourth harmonic (FH) beam promotes the emission of an ultrafast pulsed electron probe beam from the SEM tip. Pump and probe pulses are relatively delayed by an optical delay stage set on the optical path of the pump beam and the pump-probe relative delay is tuned over 4 ns with sub-ps resolution. SE detection is provided by an Everhart-Thorley detector, either directly operated in current mode for time resolved imaging or lock-in demodulated for time spectroscopy on selected areas. A signal rise time of about 10 ps has been demonstrated. We will discuss the state-of-the art of the technique, its key parameters and domains of application. We will present the temporal evolution of photoinduced SE contrast under strong UV optical pumping regime, providing information on optically excited dynamics of charge carriers and defects at surfaces of semiconductors and oxide overlayers.

Photo-assisted Ultrafast Scanning Electron Microscopy (USEM) maps the dynamics of surface photovoltages and local electric fields in semiconducting samples. Photovoltages and their gradients close to surface affect the emission yield and the detection efficiency of secondary electrons (SE), leading to photoexcited SE 2D patterns. In this work, we present a method to characterize the evolution of the patterns up to ultrafast regime. These results reveal the role of surface states in affecting the external field dynamics at picoseconds. Moreover, we show that tiny changes in surface preparation express deeply different photoexcited voltage signals. We investigate the relation between the surface chemistry of Si and photo-induced SE contrast.

Nanoscale structures in silicon have been produced by means of a maskless plasma process that employs tetrafluoromethane and hydrogen. The influence of the radio-frequency power and process time on the surface texturing was studied. Desirable texturing effect has been achieved by applying an RF power in the range of 200-280 W and process time in the range of 20-30 min. The textured surface is characterized by nanopillars with lateral dimensions ranging from50 to 300 nm and with a depth in the 100-300 nm range. Depending on process parameters in the plasma etching recipe, the optical reflectance of the silicon surface is lowered and R < 5% is reached in the range going from the visible to the near-IR region.

Photon--assisted Ultrafast Scanning Electron Microscopy (USEM) is a novel stroboscopic pump-probe technique to probe charge carrier dynamics, featuring ps time resolution, nanoscale spatial resolution and surface sensitivity. It employs synchronized pulsed laser and electron beams, respectively to excite optical transitions and to probe their dynamical effects in terms of Secondary Electron (SE) contrast . The sample is excited by two ultrashort pulses, an optical one and an electronic one, and the effect on SE emission is measured as a function of the delay between the two pulses. We have developed a USEM setup for optimized surface sensitivity, by coupling a UV emitting laser with a customized SEM operating in Ultra-High Vacuum regime. The nanometer escape depth of the SE probe enables to address dynamics at surfaces and interfaces of today's nano-scale devices, where many applications rely on the interplay between semiconductors and insulators. In principle, SEM analysis is best performed onto conductive samples, since slow electrical charging effects are detrimental to reliable and reproducible results. In the case of dynamical USEM, it is also possible to probe charge dynamics at the surface of widegap semiconductors and oxides. . We have demonstrated the potentialities of USEM by recording the charge dynamics of Al2O3-on-Silicon thin films. Results can be explained in terms of optical and electronic excitation of color centers associated with oxygen vacancies . The laser-induced SE contrast shows a relevant CW contribution. Nevertheless, by a lock-in detection scheme, fast dynamical SE signal in the picosecond and nanosecond timescale can also be retrieved. The enhancement of SE emission at positive pump-probe delays is associated with optical excitation of F2+ center, with a t=12ns decay time; at negative delays the dynamics of the F+ centers, excited by the e-beam and decaying over TAU=7ns is recorded, by probing hot-electron photoemission. SE depletion at zero delay and with decay constant TAU=70ps was attributed to surface charging due to two-photon-photoemission. In this way USEM becomes a successful tool, complementary to time resolved PL and CL], to visualize the dynamics of optically active defects and charge traps, at the surface and interfaces of wide bandgap and insulating thin films.

The excitation dynamics of defects in insulators plays a central role in a variety of fields from Electronics and Photonics to Quantum computing. We report here a time-resolved measurement of electron dynamics in 100 nm film of aluminum oxide on silicon by Ultrafast Scanning Electron Microscopy (USEM). In our pump-probe setup, an UV femtosecond laser excitation pulse and a delayed picosecond electron probe pulse are spatially overlapped on the sample, triggering Secondary Electrons (SE) emission to the detector. The zero of the pump-probe delay and the time resolution were determined by measuring the dynamics of laser-induced SE contrast on silicon. We observed fast dynamics with components ranging from tens of picoseconds to few nanoseconds, that fits within the timescales typical of the UV color center evolution. The surface sensitivity of SE detection gives to the USEM the potential of applying pump-probe investigations to charge dynamics at surfaces and interfaces of current nano-devices. The present work demonstrates this approach on large gap insulator surfaces.

A bilayer WO3 photoelectrode was obtained by radio frequency (RF) plasma sputtering in a reactive 40% O-2/Ar atmosphere by depositing two successive WO3 coatings on a tungsten foil at two different total gas pressures (3 Pa and 1.7 Pa, respectively), followed by calcination at 600 degrees C. Two monolayer samples deposited at each of the two pressures and a bilayer sample deposited at inverted pressures were also prepared. Their photoelectrocatalytic (PEC) activity was evaluated by both Incident Photon-to-Current Efficiency (IPCE) measurements and separate evolution of H-2 and O-2 by water splitting in a two-compartment PEC cell. SEM analysis revealed that the photoanodes have a nanostructured porous double layer surmounting a columnar basement (Staffa-like morphology, after the name of the Scottish island). Mott-Schottky analysis showed that the single layer deposited at 3 Pa has a conduction flat band potential 0.1 V more positive than that deposited at 1.7 Pa. The equivalent n-n heterojunction at the interface of the double-layer creates a built-in electric field that facilitates the photopromoted electron transfer toward the lower lying conduction band material, while the columnar innermost layer introduces percolation paths for efficient electron transport toward the conductive tungsten foil. Both phenomena contribute to decrease the interfacial charge transfer resistance (R-ct) and lead up to a ca. 30% increase in the PEC performance compared to the monolayer and the inverted bilayer coatings and to a 93% faradaic efficiency, which is among the highest reported so far for WO3 photoanodes. Upon methanol addition an outstanding 4-fold photocurrent density increase up to 6.3 mA cm(-2) was attained over the bilayer WO3 photoanode, much larger than the usually observed current doubling effect.

A full exploitation of the unique features of graphene and related 2D materials in micro and nano-scaled optoelectronic devices for high-data rate photonic systems calls for a deep understanding of their fundamental structural, electronic, photophysical properties and fast dynamics. We introduce some advanced spectroscopic techniques developed to this aim, while reviewing their specific potentialities and highlighting recent results. To provide absolute local nanoscale thickness metrology of as-grown 2D materials, Scanning Auger Electron Microspectroscopy has been calibrated. Results obtained for graphene and graphene oxide (GO) are reported, showing sub-monolayer resolution. Ultrafast optical transient absorption spectroscopy, with temporal resolution down to 10fs, unveils the relaxation dynamics of hot electrons in single layer graphene and the formation and radiative recombination via stimulated emission of biexcitons in ultra-narrow, structurally well-defined nanoribbons (GNRs). By recently developed ultrafast Time- and Angle-resolved Photo-Electron Spectroscopy (TR-ARPES) we investigate electronic states and spin dynamics close to the Dirac cone, in graphene and metal chalcogenide topological insulators (TI) under fs strong laser excitation, in view of spintronic applications.

Tungsten oxide (WO 3 ) thin films were deposited both on silicon, soda-lime glass and W foils by Ar/O 2 plasma diode sputtering process from a W metal target. The influence of O 2 content percentage on the structural and optical properties of the films, as well as the effects of post treatment annealing both in vacuum (400 °C) and in air (600 °C) have been investigated. X-ray diffraction studies revealed that the as-grown films are amorphous-like regardless of the oxygen percentage. The degree of crystallinity of films is increased by a post-growth thermal-annealing procedure. With respect to other plasma sputtering recipes, here a lower stress state is favoured by the slower deposition rate and the multi-step deposition. The optical band gap deduced from the absorbance spectra ranges from 3.1–3.3 eV for the amorphous samples and it decreases to 2.3–2.5 for the more crystalline films. The photoelectrochemical activity of WO 3 samples annealed at 600 °C in air have been investigated as a function of the O 2 content.

The ordered growth of self-assembled SiGe islands by surface thermal diffusion in ultra high vacuum from a lithographically etched Ge stripe on pit-patterned Si(100) surface has been experimentally investigated. The total surface coverage of Ge strongly depends on the distance from the source stripe, as quantitatively verified by Scanning Auger Microscopy. The size distribution of the islands as a function of the Ge coverage has been studied by coupling atomic force microscopy scans with Auger spectro-microscopy data. Our observations are consistent with a physical scenario where island positioning is essentially driven by energetic factors, which predominate with respect to the local kinetics of diffusion, and the growth evolution mainly depends on the local density of Ge atoms.

SiGe islands have been proposed for applications in the fields of microelectronics, optoelectronics and thermoelectrics. Although most of the works in literature are based on MBE, one of the possible advantages of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is a wider range of deposition rates, which in turn results in the possibility of growing islands with a high Ge concentration. We will show that LEPECVD can be effectively used for the controlled growth of ordered arrays of SiGe islands. In order to control the nucleation of the islands, patterned Si (001) substrates were obtained by e-beam lithography (EBL) and dry etching. We realized periodic circular pits with diameters ranging from 80 to 300 nm and depths from 65 to 75 nm. Subsequently, thin films (0.8–3.2 nm) of pure Ge were deposited by LEPECVD, resulting in regular and uniform arrays of Ge-rich islands. LEPECVD allowed the use of a wide range of growth rates (0.01–0.1 nm s−1) and substrates temperatures (600–750°C), so that the Ge content of the islands could be varied. Island morphology was characterized by AFM, while μ-Raman was used to analyze the Ge content inside the islands and the composition differences between islands on patterned and unpatterned areas of the substrate.

International audience; Spectroscopy is the characterization of matter through themeasure-ment of its interaction with light. The discovery by Isaac Newton ofthe sprawl of colors (dispersion) of sunlight by a prism has beenthe foundation of further developments of spectroscopy. Joseph vonFraunhofer used a diffraction grating for the same goal and observeddark bands in the spectrum of the sunlight. Walter N. Hartleycontributed to make the link between the position of atoms in theperiodic table and the observed bands in spectra. The invention ofthe spectroscopy has induced a fantastic advanced in knowledgeof universe due to its ability to determine the composition of thesun and stars. It has been then possible to analyze not only remotematerials, but also the light sources, gas, and liquids in laboratoriessince the end of the nineteenth century. The twentieth centurysaw the development of new spectroscopic techniques and theimprovement of the sensitivity of the devices in order to detectsmaller and smaller concentrations of chemical components. Thebasis of spectroscopy is given considering the distinction betweenelastic and inelastic scattering, before investigating the way toimprove the sensitivity of devices by field enhancement.

Tungsten oxide nanowires have been synthesized by vacuum annealing in the range 500-710 °C from amorphous-like tungsten films, deposited on a Si(100) substrate by pulsed laser deposition (PLD) in the presence of a He background pressure. The oxygen required for the nanowires formation is already adsorbed in the W matrix before annealing, its amount depending on deposition parameters. Nanowire crystalline phase and stoichiometry depend on annealing temperature, ranging from W18O49-Magneli phase to monoclinic WO3. Sufficiently long annealing induces the formation of micrometer-long nanowires, up to 3.6 ?m with an aspect ratio up to 90. Oxide nanowire growth appears to be triggered by the crystallization of the underlying amorphous W film, promoting their synthesis at low temperatures.

In this chapter, we describe the determination of the stress profile in 150-nm-wide SiGe nano-stripes embedded into a Si matrix by using oblique incidence tip-enhanced Raman spectroscopy (TERS) with a spatial resolution of ~20 nm. The TERS spectra of the stripes exhibit a number of locally enhanced phonon modes that are absent when the tip is positioned out of the stripes. The hydrostatic stress component across the nano-stripe width is evaluated from the strain-induced frequency shift of the Si-Ge mode at ~380 cm -1. The stress magnitude is found to be largest in the nano-stripe center and decreases monotonously on each side down to zero at the boundaries. This behavior is quantitatively described by a classic continuous medium model. These results demonstrate the applicability of the TERS technique to stress determination in novel semiconductor structures at the nanometer scale.

The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by the surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices.

Resonant inelastic X-ray scattering (RIXS) spectra for Ni 2 p to 3 d excitation and 3 d to 2 p de-excitation of NiO are studied both theoretically and experimentally. Theoretical calculations with a single impurity Anderson model (SIAM) describe the charge transfer (CT) and d – d excitations in RIXS, and detailed study is made for the CT energy. High resolution RIXS measurements reveal the precise d – d excitation structure and its polarization dependence, and they are well reproduced by the SIAM calculation.

Resonant inelastic x-ray scattering (RIXS) at the L2,3 edges of 3d transition metal compounds has recently become a high resolution spectroscopic technique thanks to improvements in the instrumentation. We have chosen the prototypical case of NiO to explore the various levels of interpretation applicable to L3 RIXS spectra of strongly correlated electron systems. Starting from a set of experimental data measured across the Ni L3 absorption edge with 550 meV combined energy resolution, we analyse the rich spectral structure within an atomic framework. The spectra can be separated into dd and charge transfer excitation regions. The dd excitations can be interpreted and well reproduced within a crystal field model. The charge transfer excitations are analysed through the comparison with calculations made in the Anderson impurity model. A series of parameters belonging to the proposed models (crystal field strength, charge transfer energy, hybridization integrals) can thus be extracted in a very direct and unambiguous way.

We present a numerical study of the applicability of the sum rule analysis used, in X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS), to evaluate the atomic spin moment of 3d transition metals. Magnetic circular dichroism (MCD) is present both in XAS and RIXS at the L2,3 absorption edges, although the two sum rules presented here assume different experimental setups: in XAS-MCD the magnetic field is parallel to the circularly polarised incident photons, in RIXS-MCD the two are orthogonal (perpendicular geometry). For Mn2+ we explore XAS- and RIXS-MCD by performing atomic model calculation in the single ion limit with inclusion of the appropriate crystal and magnetic fields. The comparison of the XAS-MCD spectra with those measured from a MnFe2O4 sample is used to fix the calculation parameters. The applicability of the XAS- and RIXS-MCD sum rules to Mn2+ is numerically checked within the chosen calculation model.

We have monitored the evolution of the structure of multilayered-granular films deposited by alternately growing thin cobalt layers and thicker copper layers by RF-sputtering. In particular, we have observed how the reduction of the Co layer thickness affects the overall structure of the samples and the morphology of the magnetic particles. Synchrotron radiation has been used to do diffraction, reflectivity and small angle scattering measurements, performed in anomalous configuration, in order to increase the low Co–Cu electron density contrast. We find that the samples are made of large columnar grains whose size depends on Co nominal thickness. A good agreement between the results of the X-ray analysis and the magnetic properties of the samples has been found.

We have investigated the magnetic structure of a Sr2FeMoO6 single crystal by X-ray magnetic circular dichroism at the L2,3 edges of Fe and Mo sites. The spin magnetic moments we find on Fe (3.05μB) and Mo ( − 0.32μB) give, for the first time, a direct confirmation of their ferrimagnetic ordering. The presence of a finite spin moment on Mo together with only very small orbital moments on both Fe and Mo confirms that the predicted half-metallicity of the Sr2FeMoO6 compound is due to a configuration with five localized d electrons forming a high-spin moment on Fe and one s antiparallel delocalized electron shared between the Mo and the other sites.

We have measured polarization effects by 4f resonant inelastic X-ray scattering in the M 5 region of intermetallic compounds of cerium (CeRh 2 , CePd 3 , Ce 7 Rh 3 , and CeAl 2 ). The experiment was performed using linearly polarized radiation with direction either perpendicular or parallel to the scattering plane. The dichroic effect is strong and depends on the degree of hybridization of the compound. We show that there is a monotonous relationship between the measured dichroism integrated along the emitted energies, and the radius of the cerium ion in different compounds. Since the atomic size of cerium is directly related to its valence, we conclude that the linear dichroism in inelastic X-ray scattering is a bulk-sensitive indicator of the relative weight of the 4f 0 and 4f 1 ground state configurations of cerium compounds.

In this paper we experimentally study the growth of self-assembled SiGe islands formed on Si(001) by exploiting the thermally activated surface diffusion of Ge atoms from a local Ge source stripe in the temperature range 600-700 °C. This new growth strategy allows us to vary continuously the Ge coverage from 8 to 0 monolayers as the distance from the source increases, and thus enables the investigation of the island growth over a wide range of dynamical regimes at the same time, providing a unique birds eye view of the factors governing the growth process and the dominant mechanism for the mass collection by a critical nucleus. Our results give experimental evidence that the nucleation process evolves within a diffusion limited regime. At a given annealing temperature, we find that the nucleation density depends only on the kinetics of the Ge surface diffusion resulting in a universal scaling distribution depending only on the Ge coverage. An analytical model is able to reproduce quantitatively the trend of the island density. Following the nucleation, the growth process appears to be driven mainly by short-range interactions between an island and the atoms diffusing within its vicinities. The islands volume distribution is, in fact, well described in the whole range of parameters by the Mulheran's capture zone model. The complex growth mechanism leads to a strong intermixing of Si and Ge within the island volume. Our growth strategy allows us to directly investigate the correlation between the Si incorporation and the Ge coverage in the same experimental conditions: higher intermixing is found for lower Ge coverage. This confirms that, besides the Ge gathering from the surface, also the Si incorporation from the substrate is driven by the diffusion kinetics, thus imposing a strict constraint on the initial Ge coverage, its diffusion properties and the final island volume © 2014 IOP Publishing Ltd.

Polycrystalline InP was grown on Si(001) and Si(111) substrates by using indium (In) metal as a starting material in hydride vapor phase epitaxy (HVPE) reactor. In metal was deposited on silicon substrates by thermal evaporation technique. The deposited In resulted in islands of different size and was found to be polycrystalline in nature. Different growth experiments of growing InP were performed, and the growth mechanism was investigated. Atomic force microscopy and scanning electron microscopy for morphological investigation, Scanning Auger microscopy for surface and compositional analyses, powder X-ray diffraction for crystallinity, and micro photoluminescence for optical quality assessment were conducted. It is shown that the growth starts first by phosphidisation of the In islands to InP followed by subsequent selective deposition of InP in HVPE regardless of the Si substrate orientation. Polycrystalline InP of large grain size is achieved and the growth rate as high as 21 ?m/h is obtained on both substrates. Sulfur doping of the polycrystalline InP was investigated by growing alternating layers of sulfur doped and unintentionally doped InP for equal interval of time. These layers could be delineated by stain etching showing that enough amount of sulfur can be incorporated. Grains of large lateral dimension up to 3 ?m polycrystalline InP on Si with good morphological and optical quality is obtained. The process is generic and it can also be applied for the growth of other polycrystalline III-V semiconductor layers on low cost and flexible substrates for solar cell applications. © 2014 AIP Publishing LLC.

We discuss the application of resonant inelastic X-ray scattering to the study of magnetic systems in the soft X-ray range. To this end we distinguish two broad areas. In the first the layout of the experiment is such that the absorption magnetic circular dichroism (MCD) is not zero. In the second the magnetisation is perpendicular to the incident helical beam so that the absorption MCD is zero. In the first area we summarise published results on Fe-Co alloys and we present new data on Mn impurities in Ni together with calculations. In the second area we summarise published results on Ni in Ni-ferrite with final 3s shell excitation and we present new results on Co-metal and Co in Co-ferrite measured with a new approach. This is based on the incident energy dependence of the integral of the Raman spectrum in inner shell excitation (integrated resonant Raman scattering). The potentialities and the limitations of the above methods are critically presented.

The intensity of the O 1s core photoemission peak has been measured for CuO and NiO as the photon energy is scanned across the L-3 absorption edge of the metal ion. In CuO, the O 1s peak shows a typical antiresonant behavior, with a sizable decrease of its cross section at photon energies below the Cu L-3 threshold. No detectable effect is observed for NiO. The CuO data are well reproduced by a multiatom resonant photoemission model based on resonant light scattering.

We present experimental results on resonant Raman scattering in Te, La, and Gd with ${M}_{5}$ and ${M}_{4}$ excitation. The spectra show clearly the importance of the final-state interaction between the configuration having one $4p$ hole and that having two $4d$ holes. The weight of the spectral region at higher transferred energies is very much dependent on Z and it is higher at low Z. Moreover, the sensitivity to the change of the excitation energy is quite different in the three cases, while the energy extension of the spectra is not very different in the three cases (typically 50\char21{}55 eV with ${M}_{5}$ excitation). We show how these findings can be rationalized in a simple and expressive way by considering the interplay between all energy scales characteristic of the process. These are the multiplet splitting, the spin orbit, the strength of the configuration interaction, and the energy separation between the two configurations.

We present results on inelastic photon scattering in NiO and CoO in the L 3 threshold region ending with a final 3s hole i.e. 2p 6 3d n →2p 5 3d n +1 →2p 6 3d n +1 3s 1 . The spectra show the presence of Raman features dispersing with the incident photon energy and of a strong feature at constant outgoing photon energy. In CoO the results show that this constant energy feature is present only when the core excitation is 2–3 eV above the core absorption thresholds. This supports a simple model based on the energy loss due to the creation of electron hole pairs across the gap. This mechanism can convert the intermediate excited state into a state of lower energy decaying with the emission at constant photon energy. The implications of these results are briefly discussed.

We measured circular dichroism in resonant x-ray scattering 3d{sup n}{r_arrow}2p{sup 5}3d{sup n+1}{r_arrow}3s{sup 1}3d{sup n+1} with incidence perpendicular to the magnetization where the absorption dichroism vanishes. The advantages of photon scattering over other techniques make it possible to study a wide range of materials. The Ni L{sub 3} dichroism in NiFe{sub 2}O {sub 4} is (28{plus_minus}5){percent} in agreement with a localized model. In the metal Co the dichroism is reduced to (10.4{plus_minus}1){percent} (L{sub 3}) and (6.8{plus_minus}1.5){percent} (7.5 eV above L{sub 3} ), indicating a large sensitivity to the nature of the valence states despite the fact that this spectroscopy is based on inner shell transitions. {copyright} {ital 1999} {ital The American Physical Society }

We report on two independent measurements of the spin polarization at the Fe 3 O 4 / γ - Al 2 O 3 interface. Fe 3 O 4 / γ - Al 2 O 3 (1 1 1) bilayers have been epitaxially grown by oxygen-assisted molecular beam epitaxy onto α - Al 2 O 3 (0 0 0 1) substrates and some were covered ex situ by a cobalt electrode. Spin resolved photoemission yield to a negative spin polarization ( ≃ - 40 %) while transport measurements on Fe 3 O 4 / γ - Al 2 O 3 / Co magnetic tunnel junctions show a positive TMR, which suggest that the tunnel spin polarization at the Fe 3 O 4 / γ - Al 2 O 3 interface is positive.

We present results on resonant inner-shell scattering in Gd across the M5 threshold; the scattering channel with formally a 4p hole in the final state is studied. Two scattering channels are in competition: one at constant transferred energy and another at constant outgoing energy. The branching ratio of the process at constant transferred energy is about 5%. We isolate the many-body satellite structure of the formally 4p3/2 final hole state and we discuss the importance of the multiplet splitting and of the super Coster-Kronig conversion of this state into another final state with two 4d holes. The results with resonant M5 excitation are also compared with those of non-resonant excitation well above the M4 threshold. Guidelines for future research are briefly presented.

PoLIFEMo (Polarized Light Improved Flux Efficiency Monochromator) is the name of a spherical grating monochromator designed with the purpose of maximizing the photon intensity on a sample. The device is mainly conceived to improve performances of an x-ray emission spectrometer, whose limiting factor is often the counting rate. By keeping the exit slit fixed, it is possible to get a small spot size (∼35 μm vertically) on a sample placed just near the exit slit, without the need of postgrating refocusing optics. No entrance slit is used because of the low beam divergence, allowing the use of a virtual source in a converging beam geometry. The tuning of the output energy is done by translating and pitching the spherical grating. The resolving power is higher than 1000 over the whole energy range (500–1500 eV). A detailed analysis of the optical performances is given, mostly in comparison with the Dragon-type monochromator. The actual realization of the device is described, together with the specific motivati...

We report on the use of recently developed spin-polarized electron sources with very high polarization for electron spectroscopy. In particular we present data of spin-resolved inverse photoemission from Fe(100) films excited by polarized electrons produced by a strained GaAsP negative electron affinity photocathode. This highly polarized source (beam polarization P=65%) allows a direct and almost complete decoupling of the majority and minority empty states in Fe(100), much better than the standard GaAs sources (P=20%–30%). The preparation and characterization of the strained photocathode is also discussed.

The continued downscaling in SiGe heterostructures is approaching the point at which lateral confinement leads to a uniaxial strain state, giving high enhancements of the charge carrier mobility. Investigation of the strain relaxation as induced by the patterning of a continuous SiGe layer is thus of scientific and technological importance. In the present work, the strain in single lithographically defined low-dimensional SiGe structures has been directly mapped via nanobeam x-ray diffraction. We found that the nanopatterning is able to induce an anisotropic strain relaxation, leading to a conversion of the strain state from biaxial to uniaxial. Its origin is fully compatible with a pure elastic deformation of the crystal lattice without involving plastic relaxation by injection of misfit dislocations.

The extension of SiGe technology towards new electronic and optoelectronic applications on the Si platform requires that Ge-rich nanostructures be obtained in a well-controlled manner. Ge deposition on Si substrates usually creates SiGe nanostructures with relatively low and inhomogeneous Ge content. We have realized SiGe nanostructures with a very high (up to 90%) Ge content. Using substrate patterning, a regular array of nanostructures is obtained. We report that electron microscopy reveals an abrupt change in Ge content of about 20% between the filled pit and the island, which has not been observed in other Ge island systems. Dislocations are mainly found within the filled pit and only rarely in the island. Selective chemical etching and electron energy-loss spectroscopy reveal that the island itself is homogeneous. These Ge-rich islands are possible candidates for electronic applications requiring locally induced stress, and optoelectronic applications which exploit the Ge-like band structure of Ge-rich SiGe.

We investigated the surface diffusion and island nucleation of Ge on Si(100) in presence of a submonolayer coverage of carbon as surfactant by using scanning Auger microscopy and atomic force microscopy. Ge stripes have been deposited and lithographically etched on a Si substrate and used as sources for the surface diffusion of Ge promoted by annealing at 600, 650, and $700\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The diffusion coefficient has been determined by fitting the postannealing coverage profiles measured by Auger microscopy with a one-dimensional continuous model. The carbon coverage has been spatially modulated on a single sample, allowing the measurement of the diffusion coefficient as a function of the C thickness at $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. We show that the reduction in the diffusion coefficient while increasing the surfactant coverage is described by a linear dependence of the diffusion activation energy on the C coverage. This dependence is discussed in terms of the chemical interactions among Si, C, and Ge, of the surface roughness and the local strain field induced by the C surfactant. Spontaneous nucleation of SiGe islands coexists with the continuous surface diffusion of Ge. The transition of the island nucleation as a function of the carbon coverage is observed to be continuous from the Stranski-Krastanov mode to the Volmer-Weber regime. We propose a consistent scenario correlating diffusion and nucleation parameters within a diffusion limited growth regime and show the existence of a threshold for C coverage below which no effect is observed.

W–(1%)La 2 O 3 has been investigated after thermal exposure in the Quasi-Stationary Plasma Accelerator facility in order to obtain information regarding its surface damage and morphological modification. The profilometry measurements and the Scanning Electron Microscopy analysis showed that surface erosion and corrugation become more pronounced with increasing the thermal load. The La 2 O 3 particle density inside the sample has been measured by Scanning Auger Microscopy. It decreases with increasing the thermal load and presents a negative gradient from the bulk to the surface.