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The last missing piece of puzzle for the full functionalization of group IV optoelectronic devices is the direct band gap semiconductor made by CMOS compatible technology. Here we report on the fabrication of GeSn alloys with a Sn concentration of up to 6 % using ion implantation followed by ms-range explosive solid phase epitaxy. The n-type single crystalline GeSn alloys are made by co-doping of Sn and P into Ge. Both the activation of P and the formation of GeSn are performed during a single-step flash lamp annealing for 3 ms. The band gap engineering in ultra-doped n-type Ge and GeSn alloys is theoretically predicted by density functional theory and experimentally verified using ellipsometric spectroscopy. We demonstrate that both the diffusion and the segregation of Sn and P atoms in Ge are fully supressed by ms-range non-equilibrium thermal processing.

The catalytic graphitization with layer exchange (LE) of amorphous carbon in thin film stacks with Ni was investigated as a function of the initial stacking order. Bilayer and triple layer stacks were exposed to heating ramps up to 700 °C. Raman spectroscopy showed the formation of a layered graphitic structure after the annealing. During outwards LE, as C is transported towards the sample surface, a smooth layer with graphitic planes parallel to the interface with the substrate has formed through a 2D growth. A significant restructuring of the Ni layer appeared during inwards LE, as C is transported towards the substrate. Here, the fragmentation of the Ni layer, as well as the regions with turbulence-like and folding defects indicated a 3D growth. The degree of LE, quantified by ion beam analysis, is 95 % and 80 % for the outand inwards direction, respectively. Based on the calculation of surface and interface energies of the initial and final states, thermodynamic estimations pointed to the wetting of Ni grain boundaries by C atoms as the initial driving force for the LE and allowed a consistent understanding of the LE directionality and of the final thin film microstructure.

Free-standing nanowires are promising platforms for hosting three-dimensional heterostructures such as single quantum dots for quantum photonics, modulation doped heterostructures for gate-all-around high mobility transistors, etc. The peculiarity of heteroepitaxy in nanowires is the existence of multiple growth interfaces with different crystallographic orientations (usually one top- and six side-facets), where the growth can take place in different modes (e.g. vapor-liquid-solid on the top- and vapor-solid on the side-facets) and can be controlled independently. Furthermore, strained epilayers in nanowires can relax elastically both at the lateral free surface and by stretching the substrate, which in this case are the thin nanowires. All these features open up new possibilities for complex three-dimensional heterostructures with loose strain restrictions. Here, we have investigated the growth of radial and axial heterostructures, as well as combinations of the two, within III-As nanowires. The radial ones consist of thin GaAs nanowires overgrown all-around with an InxAl1-xAs layer in a core/shell fashion. In agreement with theory, the small volume of the core allows for strain engineering both in the core and the shell, and for realization of highly-mismatched/strained heterostructures (with misfits up to 4%) without dislocations. The manipulation of the growth kinetics is necessary in order to suppress strain-driven phenomena, such as the preferential shell-growth and In-incorporation on one side of the core. The axial heterostructures consist of single or multiple GaAs/AlxGa1-xAs quantum dots. The width and the thickness of these dots can be controlled independently. The AlxGa1-xAs segments were grown as digital alloys using pulsed epitaxy in order to achieve sharper interfaces and to avoid the formation of stacking faults. Finally, we realized complex heterostructures with single GaAs/AlxGa1-xAs quantum dots inside the core of core/shell nanowires, aiming at engineering the electronic properties of the dots depending on the composition and thickness of the shell. Besides MBE experiments, our investigations involved transmission electron microscopy (HR-TEM, STEM, EDX, STEM tomography), X-ray diffraction, Raman scattering spectroscopy, and photoluminescence spectroscopy.

High energy density physics 32, 63 - 69 (2019). doi:10.1016/j.hedp.2019.05.002, We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 10$^{19}$ W/cm$^2$, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index $(hk0)$, persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states., Published by Elsevier, Amsterdam [u.a.]

We study the photo-induced metallization process in niobium dioxide NbO2. This compound undergoes the thermal insulator-to-metal transition at the remarkably high temperature of 1080 K. Our optical pump ¬– terahertz probe measurements reveal the ultrafast switching of the film on a sub-picosecond timescale and the formation of a metastable metallic phase when the incident pump fluence exceeds the threshold of ~10 mJ/cm2. Remarkably, this threshold value corresponds to the deposited energy which is capable of heating NbO2 only up to 790 K, thus, evidencing the non-thermal character of the photo-induced insulator-to-metal transition. We also observe an enhanced formation of the metallic phase above the second threshold of ~17.5 mJ/cm2 which corresponds to the onset of the thermal switching. The transient optical conductivity in the metastable phase can be modeled using the Drude-Smith model confirming its metallic character. The present observation of non-thermal transition in NbO2 can serve as an important test bed for understanding photo-induced phenomena in strongly correlated oxides.

Niobium dioxide (NbO2) is an isovalent counterpart of VO2 with considerably higher transition temperature (Tc = 1080 K). We have performed time-resolved optical pump – THz probe measurements on NbO2 epitaxial thin film at room temperature. Notably, the pump energy required for the switching into a metastable metallic state is smaller than the energy necessary for heating NbO2 up to TC providing strong evidence for the non-thermal character of the photoinduced insulator-to-metal transition.

In Uranus and Neptune methane and other hydrocarbons are highly abundant. Their planetary interior conditions can be mimicked using high intensity lasers in the laboratory on a nanosecond timescale. Nanodiamond formation from shock-compressed polystyrene (~150GPa, ~5000K) was demonstrated via in situ X-ray diffraction with a XFEL. The lower size estimate is 4nm. 60% of the carbon atoms in the plastic are transferred to a diamond lattice. However, in total a maximum of ~16μg of nanodiamonds are expected from a 125nm CH foil and a 500μm focal spot. In order to understand the underlying hydrocarbon separation mechanism the physical recovery of nanodiamonds is pursued to learn from their shape, size, surface modifications and defects.

The ability to manipulate the metal-insulator transition (MIT) of metal oxides is of critical importance for fundamental investigations of electron correlations and practical implementations of power efficient tunable electrical and optical devices. Most of the existing techniques including chemical doping and epitaxial strain modification can only modify the global transition temperature, while the capability to locally manipulate MIT is still lacking for developing highly integrated functional devices. Here, lattice engineering induced by the energetic noble gas ion allowing a 3D local manipulation of the MIT in VO2 films is demonstrated and a spatial resolution laterally within the micrometer scale is reached. Ion-induced open volume defects efficiently modify the lattice constants of VO2 and consequently reduce the MIT temperature continuously from 341 to 275 K. According to a density functional theory calculation, the effect of lattice constant variation reduces the phase change energy barrier and therefore triggers the MIT at a much lower temperature. VO2 films with multiple transitions in both in-plane and out-of-plane dimensions can be achieved by implantation through a shadow mask or multienergy implantation. Based on this method, temperature-controlled VO2 metasurface structure is demonstrated by tuning only locally the MIT behavior on the VO2 surfaces.

The dendrite growth kinetics and morphology have been of great interest in the solidification science and casting industry. A detailed analysis of the particular solidification phenomena (coarsening, fragmentation etc.) requires X-ray techniques with a better spatial and temporal resolution. High resolution experimental data are also very important for the verification of the existing microstructural models. In this work, in-situ synchrotron X-ray radiography and diffraction methods have been combined to study the evolution of dendritic microstructures during the solidification of Ga - In alloys. The directional solidification experiments were performed at the ID19 and BM20 beamlines (ESRF, France) at a high spatial resolution of < 1 µm. The study is especially focused on the sidearm evolution, retraction and detachment, dendrite morphology and orientation. The sidearm evolution is quantified by an image analysis in a manner appropriated for comparison to simulations. Furthermore, we report on the reconstruction of crystallographic orientation of growing dendrites by using the CaRIne crystallography software. Our measurements show that the Indium dendrites grow along the orientation, typically observed in body-centered metals. Our future work will include the systematic study of the evolution of dendritic morphologies at finite cooling rates and under the impact of melt flow.

In the present work, we report on epitaxial growth of ferromagnetic Mn5Ge3 thin films on (001) Ge substrates induced by Mn in-diffusion during non-equilibrium flash lamp annealing for 20 ms. The ferromagnetic Mn5Ge3/Ge samples with very sharp interface between the Mn5Ge3 layer and the Ge substrate can be used to fabricate spintronic devices. Temperature-dependent magnetization reveals a Curie temperature of 282 K which can be tuned much above room temperature by strain engineering and/or co-doping with C. The microstructural properties of the fabricated films were investigated by X-ray diffraction, cross-sectional TEM and Rutherford backscattering spectrometry. Both used material and technology are highly compatible with complementary metal-oxide-semiconductor (CMOS) technology and can be used for spintronics.

We report a simple one‐pot method for the rapid preparation of sub‐10 nm pure hexagonal (v‐phase) NaYF4‐based upconverting nanoparticles (UCNPs). Using Therminol® 66 as a co‐solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd3+ and Yb3+ sensitizer ions and Er3+ activator ions). The size and phase‐purity of the as‐synthesized core and core-shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X‐ray diffraction, and small‐angle X‐ray scattering studies. In‐depth photophysical evaluation of the UCNPs was pursued by using steady‐state and time‐resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface‐related luminescence quenching effects.

In recent years, transition (refractory) metals such molybdenum (Mo) and zirconium (Zr) have been studied as infrared (IR) reflector in solar absorber applications. The sputter process parameters are very important for depositing a high quality thin film achieving the necessary low emittance. IR reflectance of the metal film is influenced by the film microstructure, presence of residual impurities and surface roughness. The main objective of the present study is to prepare Mo and Zr metallic thin films with improved optical properties by high power impulse magnetron sputtering at room temperature under high vacuum conditions. In comparison to the Mo and Zr thin films deposited by direct current magnetron at the same average power, thin films deposited by HiPIMS exhibits dense microstructure without voids, grown preferentially along c-axis, have smooth surface and are free of residual contaminants. Compared to the dcMS films we observed an element specific reduction of impurities measured by elastic recoil detection analysis (ERDA) by a factor 4/8 for N, 3/4 for H and 9/14 for O for Mo/Zr thin films respectively. The compositional effects are correlated with differences in the film morphology microstructure revealed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis.

A high-power, pulsed laser impacting the surface of a material can generate surface ablation, shock waves and crack propagation; while X-ray imaging can provide a time-resolved probe. Hard X-rays are perfectly suitable for visualizations of transient processes in optically opaque materials even for objects of several mm in size. The MHz pulsed time structure, tunable energy bandwidth, high brilliance, and the high degree of spatial coherence of hard X-rays (E > 30keV) from third generation synchrotron sources allowing transient processes to be tracked directly using ultra-high-speed image acquisition systems. We report on an in-situ real time investigation of ns single-pulsed laser-driven processes studied by combined diffraction-direct-space-imaging experiments exploiting the single bunch structure at the hard X-ray imaging beam line ID19 of the ESRF investigating the process of laser hole drilling into single crystalline silicium. Whereas macroscopic changes in bulk materials can be quite easily deduced from X-ray phase contrast imaging; information probing changes at the lattice level can be obtained using diffraction imaging. The whole process was followed for 120sec with a maximum frame rate of up to 100kHz. We have developed an experimental methods that allow to synchronize on the ns level the single shot laser operation with the high speed camera system matching the 4- and 16-bunch structure of the ESRF.

Three-dimensional nanocomposite networks consisting of percolated Si nanowires in a SiOx matrix, Si:SiO2, were studied. The structures were obtained by reactive ion beam sputter deposition of SiOx (x~0.6) thin films at 450 °C and subsequent crystallization using conventional oven as well as millisecond line focus laser annealing. Rutherford backscattering spectrometry, Raman spectroscopy, X-ray diffraction, cross-sectional and energy-filtered transmission electron microscopy were applied for sample characterization. While oven annealing resulted in a mean Si wire diameter of 10 nm and a crystallinity of 72 % within the Si volume, almost single-domain Si structures with 30 nm in diameter and almost free of amorphous Si were obtained by millisecond laser application. The structural differences are attributed to the different crystallization processes: Conventional oven tempering proceeds via solid state, millisecond laser application via liquid phase crystallization of Si. The 5 orders of magnitude larger diffusion constant in the liquid phase is responsible for the three times larger Si nanostructure diameter. In conclusion, laser annealing offers not only significantly shorter process times but moreover a superior structural order of nano-Si compared to conventional heating.

Sputter deposited Mg2FexSi1-x films of 250 nm thickness have been investigated by means of Mössbauer spectroscopy. While non-hydrogenated films are characterized by a quadrupole split doublet, hydrogenated regions show a singlet with reduced isomer shift. The relative areas of the spectra reflect the relation between loaded and unloaded regions prepared at the same loading conditions.

Time-resolved in-situ or/and in-operando X-ray experiments open a very direct, natural way to study the formation and transformation of materials during relevant technological processes. High-brilliance, fs-pulsed X-rays generated by XFels demonstrate the highest temporal and spatial resolutions, but the maximum X-ray energy is currently limited to ~25 keV. Despite the possibility to illuminate macroscopic objects with large beams (~100mm2) synchrotron light sources produce X-rays pulses with much lower temporal resolution (~100ps), spatial coherence and brilliance but are able to reach X-ray energies higher than achieved at current FELs. MHz pulse repetition rates (ESRF:up to 5.6MHz in the 16 bunch mode) are characteristic to synchrotrons, allowing transient processes to be tracked using ultra-high-speed image acquisition systems with multiple frames, that are even able to visualize transient processes that are stochastic or a-periodic. Here, we report on an in-situ real time investigation into high-power (>1J), ns single-pulsed (Nd:YAG, = 532 nm; pulse length ~10 ns) laser-driven irradiation processes leading either to surface ablation, crack propagation or shock generation [1] studied by a combined diffraction-direct-space-imaging experiment exploiting the single bunch structure. Whereas macroscopic changes (i.e. density changes or cracks) in bulk materials can be quite easily deduced from X-ray phase contrast imaging, information probing changes at the lattice level can be obtained using diffraction imaging. As an example is in the figure shown such a combined experiment [2]: The first laser shot of an in-situ real-time laser hole-drilling experiment into a 0.50 mm thick Si (001) single crystalline wafer that was carried out for about 120sec. The sample was placed by about 45° with respect to the laser and the X-ray beam. Both beams intersect horizontally at the same height at an angle of 90° at the rotation center of the sample. The laser light was directed to the sample using a focusing lens. The synchronization of the cameras with laser and X-ray pulses are described in [1]. The X-ray beam fully illuminates the 10×10 mm2 wafer. The diffraction angle was tuned so that the Si (333) reflection in transmission geometry was recorded by the diffraction imaging detector.

Ultrafast insulator-to-metal transition in the correlated oxides such as vanadium dioxide (VO2) has been extensively explored for rich physics and potential applications. In this regard, its isovalent counterpart niobium dioxide (NbO2) with considerably higher transition temperature (Tc = 1080 K) can be envisaged as a potential candidate. We have performed time-resolved optical pump – terahertz (THz) probe measurements on NbO2 epitaxial thin at room temperature. The onset of the THz conductivity is followed by an exponential decay on a timescale of 400 fs. The photoinduced change in THz transmission at later delay times exhibits excitation threshold of 17.5 mJ/cm2. Notably, in contrast to VO2, the pump energy required for the switching into a metastable metallic state is smaller than the energy necessary for heating NbO2 up to Tc providing a strong evidence for the non-thermal character of the photoinduced insulator-to-metal transition in this system. The transient optical conductivity in the metastable state can be modelled using the Drude model confirming its metallic character.

We investigate the mechanical properties of freely suspended nanostrings fabricated from tensilestressed, crystalline In1-xGaxP. The intrinsic strain arises during epitaxial growth as a consequence of the lattice mismatch between the thin film and the substrate, and is confirmed by x-ray diffraction measurements. The flexural eigenfrequencies of the nanomechanical string resonators reveal an orientation dependent stress with a maximum value of 650 MPa. The angular dependence is explained by a combination of anisotropic Young's modulus and a change of elastic properties caused by defects. As a function of the crystal orientation, a stress variation of up to 50% is observed. This enables fine tuning of the tensile stress for any given Ga content x, which implies interesting prospects for the study of high Q nanomechanical systems.

III-V compound semiconductors have fueled many breakthroughs in physics and technology owing to their direct band gap and high electron mobility. It has also been very important that these fundamental properties can be tailored in ternary or quaternary alloys by selecting the chemical composition appropriately. Here we explore the great possibilities for strain engineering in core/shell nanowires as an alternative route to tailor the properties of III-V semiconductors without changing their chemical composition. In particular, we demonstrate that the GaAs core in GaAs/InₓGa₁₋ₓAs or GaAs/InₓAl₁₋ₓAs core/shell nanowires can sustain unusually large misfit strains that would have been impossible in equivalent thin-film heterostructures, and undergoes a significant modification of its electronic properties. Core/shell nanowires were grown in the self-catalyzed mode on SiOₓ/Si(111) substrates by molecular beam epitaxy. Strain analysis was performed using synchrotron X-ray diffraction and Raman scattering spectroscopy, and showed that for a thin enough core, the magnitude and the spatial distribution of the built-in misfit strain can be regulated via the composition and the thickness of the shell. Beyond a critical shell thickness, we obtain a heavily tensile-strained core and an almost strain-free shell. The tensile strain of the core exhibits a predominantly-hydrostatic character and causes the reduction of the GaAs band gap energy in accordance with our theoretical predictions using deformation-potential theory and first-principle calculations. For 7 % of strain (x = 0.54), the band gap energy was reduced to 0.87 eV at 300 K, i.e. a remarkable reduction of 40 %. Signatures of valence-band splitting were also identified in polarization-resolved photoluminescence measurements, as a result of the strain anisotropy in GaAs. Presuming a reduced effective mass of electrons in the tensile-strained core of GaAs/InₓAl₁₋ₓAs nanowires (core diameter = 22 nm, x = 0.39 - 0.49), the corresponding electron mobility was measured by time-domain terahertz spectroscopy to be in the range of 4000 cm²/V·s at 300 K. These values are the highest reported, even in comparison to GaAs/AlₓGa₁₋ₓAs nanowires with double the core thickness. Our results demonstrate that strained GaAs in core/shell nanowires can resemble the electronic properties of InₓGa₁₋ₓAs, surmounting issues with phase separation, surface segregation or alloy disorder that typically exist in ternary alloys and limit the device performance.

The dendrite coarsening kinetics and dendrite morphology have been of great interest in the solidification science and casting industry. A detailed analysis of particular solidification phenomena (coalescence, fragmentation etc.) requires X-ray techniques with a high spatial and temporal resolution. High resolution experimental data are also very important for the verification of the existing microstructural models. The synchrotron radiography experiments with solidifying Ga - In alloys were performed at BM20 and ID19 (ESRF, Grenoble) at a spatial resolution of < 1 µm. The temporal dynamics of morphological transitions such as retraction, coalescence and pinch-off of the sidearms were studied in-situ. Recently, we showed that the combination of numerical modelling [1] and experiments [2,3] performed at the ESFR synchrotron X-ray source in Grenoble has allowed to improve the understanding of the pinch-off process of dendritic sidearms and to obtain material information that is relevant for quantitative modelling. In this work, a Ga–In alloy was solidified in vertical direction starting from the top of the solidification cell at a controlled cooling rate of 0.002 K/s and at a vertical temperature gradient of ~2 K/mm. In general, all fragmentation events are located in the deceleration zone that is formed during the initial phase of solidification. Behind an advancing growth front, under slow growth conditions that are almost close to steady state conditions, the coarsening in the mushy zone does not involve a significant detachment of sidearms. A detailed and advanced image analysis in combination with the high temporal and spatial resolution data of the experiment, allowed us to identify an additional migration process that is influenced by the existing temperature gradient. This Temperature Gradient Zone Melting (TGZM) process is characterized by a sidearm migration rate of 0.01 µm/s. Interestingly, the results of our analysis suggest that this process does not play a significant role for the sidearm detachment process itself. References 1. H. Neumann-Heyme, et al. PHYS. REV. E, 92 (2015) 060401 2. Shevchenko et al., IOP Conf. Series: Mat. Sci. and Eng. 228 (2017), 012005 3. Neumann-Heyme, Shevchenko et al., Acta Mater. 146 (2018) 176

A high-power, nanosecond-pulsed laser impacting the surface of a material can generate an ablation plasma that drives a shock wave into it; while in situ X-ray imaging can provide a time-resolved probe of the shock-induced material behaviour on macroscopic lengths scales. Here, we report on an investigation into laser-driven shock compression of a polyurethane foam and a graphite rod by means of single-pulse synchrotron X-ray phase-contrast imaging with a MHz frame rate. A 6-J, 10-ns-pulsed laser was used to generate shock compression. Physical processes governing the laser-induced dynamic response such as elastic compression, compaction, pore collapse, fracture, and fragmentation have been imaged; and the advantage of exploiting the partial spatial coherence of a synchrotron source for studying low-density, carbon-based materials is emphasized. The successful combination of a high-energy laser and ultra-high-speed X-ray imaging using synchrotron light demonstrates the potentiality of accessing complementary information from scientific studies of laser-driven shock compression.

A nanofabrication method for the production of ultra-dense planar metallic nanowire arrays scalable to wafer-size is presented. The method is based on an efficient template deposition process to grow diverse metallic nanowire arrays with extreme regularity in only two steps. First, III-V semiconductor substrates are irradiated by a low-energy ion beam at an elevated temperature, forming a highly ordered nanogroove pattern by a “reverse epitaxy” process due to self-assembly of surface vacancies. Second, diverse metallic nanowire arrays (Au, Fe, Ni, Co, FeAl alloy) are fabricated on these III-V templates by deposition at a glancing incidence angle. This method allows for the fabrication of metallic nanowire arrays with periodicities down to 45 nm scaled up to wafer-size fabrication. As typical noble and magnetic metals, the Au and Fe nanowire arrays produced here exhibited large anisotropic optical and magnetic properties, respectively. The excitation of localized surface plasmon resonances (LSPRs) of the Au nanowire arrays resulted in a high electric field enhancement, which was used to detect phthalocyanine (CoPc) in surface-enhanced Raman scattering (SERS). Furthermore, the Fe nanowire arrays showed a very high in-plane magnetic anisotropy of approximately 412 mT, which may be the largest in-plane magnetic anisotropy field yet reported that is solely induced via shape anisotropy within the plane of a thin film.

Nanostructured materials will be key components in future technological solutions of our society’s present challenges: They can enhance the efficiency of energy harvesting from renewable sources, increase the sensitivity of diagnostical tools in medicine, or enable novel information technology. For making substantial contributions to these fields by applying nanostructured materials, we must be able to fabricate them easily and reproducibly on industrially relevant scales. This can be achieved by the bottom-up approach of templated growth on substrates nanopatterned by ion irradiation: The required technologies of low-energy ion irradiation, polymer chemistry, and physical vapor deposition are well-established. In this contribution, we outline the mechanism of self-assembly of vacancies and adatoms on crystalline semiconductor surfaces induced by low-energy ion irradiation [1,2]: At temperatures above the material’s recrystallization temperature, the substrate crystallinity is retained. Thus, diffusion of vacancies and adatoms on the surface is highly anisotropic, which leads to the formation of surface nanopatterns reflecting the crystal symmetry of the substrate material. The various resulting pattern morphologies and the influence of external process parameters will be presented. We hope to initiate discussion and collaboration by highlighting potential applications based on these ion-induced nanopatterns, such as growing epitaxial nanowire arrays by shadowing effects at oblique incidence deposition, or inducing long-range order in copolymer thin films to fabricate chemically nanopatterned templates for nanostructure growth [3]. Acknowledgement We thank K. Potzger and A. Henschke (HZDR) for their support in MBE for templated nanowire growth. References [1] X. Ou et al., Nanoscale 7, 18928 (2015) [2] Q. Jia et al., Nano Research 15, 1 (2017) [3] D. Erb et al., Science Advances 1, e1500751 (2015)

X-ray radiography is an effective tool for investigating flow phenomena and solidification processes in opaque metallic alloys. This work is devoted to complex interaction between dendritic growth and melt flow during solidification of Ga-In alloys under natural and forced convection. Natural convection is caused by density variations within the solidifying alloys. Forced convection was produced by electromagnetic stirring. The conventional X-ray radioscopic experiments with sufficient spatial resolution (5-10 µm) deliver simultaneous information of both the dendrite structure and the flow patterns ahead of the solidification front and especially near the mushy zone. Melt convection alters the solutal field near the solidification front leading to different microstructures or even to the formation of freckle defects. The coarsening stage of dendritic structure is characterized by transformation of the sidearm morphology present after growth. The direct investigation of dendritic sidearm evolution during coarsening appears to be rather complex and impose high requirements with respect to the spatial and temporal resolution and sensitivity of the detector. The synchrotron imaging experiments with solidifying Ga-In alloys were performed at the BM20 and ID19 beamlines (ESRF, France) at a spatial resolution of < 1 µm. The present measurements provide real-time in-situ data on three phenomena that are of major importance in coarsening of dendrites: sidearm retraction, pinch-off and coalescence of neighboring sidearms. Using an advanced image analysis of high temporal and spatial resolution experimental data allows us to verify existing microstructural models.

Metal-induced crystallization with layer exchange (MIC w LE) can reduce the crystallization temperature of group 14 elements by several hundred degrees. This is especially interesting for device fabrication on substrates with limited thermal stability. Ideally, the process allows the transfer of defined amounts of an initially amorphous material onto a randomly selected substrate. In this contribution, MIC w LE is studied for Ni/ a-C thin films with different stacking sequences in order to quantify the influence of the stacking sequence on the layer exchange degree αLE and on the degree of graphitic ordering. The process was monitored in situ by temperature-dependent Rutherford backscattering spectrometry (RBS) and Raman spectroscopy up to 700 °C. RBS, Raman, elastic recoil detection, X-ray diffraction and transmission electron microscopy were applied for ex situ characterization. The highest αLE of 96% was found for the initial Ni/a-C stacking sequence. In contrast, the inverse sequence resulted in an incomplete LE. The formation of 2D-layered carbon structures occurred independently of the initial stacking sequence.1 Beyond the threshold of 580 °C, increasing the temperature to up to 700 °C had a negligible impact on the degree of 2D-ordering. Since LE and graphitization occur simultaneously at high temperatures, MIC w LE rather than dissolution/ precipitation is proposed as responsible mechanism for carbon crystallization.

Magnetic materials become more significant for future data storage devices and spintronic applications. In certain alloy thin films like Fe60Al40, nano-sized ferromagnetic structures can be created by means of focused ion irradiation.[1,2,3] Fe60Al40 shows a disorder induced phase transition from the thermodynamically stable, chemically ordered B2 to the metastable A2 phase going along with an evolution of ferromagnetism and an increase of the lattice parameter (Figure 1). This can be explained with the higher local number of Fe-Fe nearest neighbors in the disordered state. The correlation between structural and magnetic properties in the phase transition regime, consisting of A2 and B2 phases, is uncertain as well as the influence of the ion type or the temperature treatment. The effects of ion implantation on the structural and magnetic properties of 250 nm thick Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction (XRD) and Vibrating sample magnetometry. From XRD measurements, the order parameter S and the peak shift due to the change of the lattice parameter have been derived and correlated with the magnetization. The irradiation of paramagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of the saturation magnetization (MS) which was expected to be directly related to the number of displacements per atom (dpa) by using the simulation program TRIM [4], independent on the ion species. However, unlike than expected, the induced magnetization differed but correlated directly with the measured lattice parameter. A significant change of lattice parameter and MS did not appear for proton irradiation at elevated temperatures (250 °C) where the ordered B2 phase was retained. Upon low temperature (LN2) hydrogen implantation of disordered A2 Fe60Al40 films, on the other hand, unlike for helium or neon irradiation, the lattice parameter and the saturation magnetization decreased indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of chemically disordered alloy thin films well below the ordering temperature. Furthermore, the studies show that the structural and magnetic properties of 250 nm thick Fe60Al40 films are directly linked with each other (Figure 2) and do not depend on the type of treatment. The chemical disorder induced evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak represented by the order parameter dropping to 0. Nevertheless, the role of defects remains uncertain since ion irradiation leads besides the structural disordering also to an increase of the defect concentration and a temperature treatment to structural ordering and an annealing of defects. However, as described beforehand, H+ implantation causes little ordering but also an increase of the open volume defect concentration, which was characterized by means of Positron annihilation spectroscopy. This offers the opportunity to differentiate between structural disorder and defect concentration. Given the fact that the proton implanted films follow the shown general behavior a dependence on the structural order only can be assumed.

The geometry and high surface-to-volume ratio of nanowires offer unique possibilities for strain engineering in epitaxial semiconductor heterostructures with large lattice mismatch. In addition, the possibility to grow nanowires of high crystal quality epitaxially on Si substrates adds to their technological significance. In this work, we have investigated the growth of free-standing GaAs/InxGa1-xAs and GaAs/InxAl1-xAs core/shell nanowires on Si(111) substrates by molecular beam epitaxy, the accommodation of the lattice mismatch therein, and its effect on the nanowire properties. Very thin GaAs core nanowires (20-25 nm in diameter) were grown in the self-catalyzed mode with a sufficiently low number density (to avoid beam shadowing effects) on SiOx/Si(111) substrates, after an in situ treatment of the latter with Ga droplets. This resulted in zinc blende nanowires with their axis along the [111] crystallographic direction and six {1-10} sidewalls. Subsequently, conformal overgrowth of the InxGa1-xAs or InxAl1-xAs shell was obtained only under kinetically limited growth conditions that suppressed mismatch-induced bending phenomena. The strain in the core and the shell was studied systematically as a function of the shell composition and thickness. To that end, we used Raman scattering spectroscopy, transmission electron microscopy and synchrotron X-ray diffraction, and compared the results with theoretical predictions based on continuum elasticity and density functional theories. Our results demonstrate that highly mismatched core/shell nanowires with defect-free interface can be obtained beyond what is possible in thin film heterostructures. More interestingly, nanowires with strain-free shell and fully strained core can be grown under certain conditions. The large strain in the GaAs core is expected to have a strong effect on its fundamental properties. Here, we demonstrate a large shrinkage of the band gap, which can be as high as 35 % depending on the composition of the shell.

Compound semiconductors are versatile materials due to the possibility to tailor their (opto)electronic properties by selecting their composition appropriately. When grown heteroepitaxially, though, this possibility is constrained by the lattice mismatch with the substrate. InxGa1-xAs is a good example because it can have, depending on x, a suitable direct optical band gap for optoelectronic applications in the infrared (e.g. telecommunication wavelengths) or high electron mobility for high-speed transistors. However, the practical choices of x are limited by the available substrates, typically GaAs for low x or InP for x≈0.53. Nanowires are a promising alternative for the realization of epitaxial heterostructures with high lattice mismatch due to their unique geometry and high surface-to-volume ratio. In addition, the possibility of monolithic integration in Si-CMOS platforms adds to their technological significance. In this work, we have investigated the growth of free-standing GaAs/InxGa1-xAs core/shell nanowires on Si(111) substrates by molecular beam epitaxy and the accommodation of lattice mismatch therein. Specifically, we have concentrated on highly lattice mismatched heterostructures (x=0.20-0.80) and very thin cores (diameter < 25 nm). Self-catalyzed growth of very thin GaAs core nanowires with a sufficiently low number density (to avoid beam shadowing during the shell growth) was possible on native-oxide/Si(111) substrates, after an in situ treatment of the latter with Ga droplets. This resulted in zinc blende nanowires with their axis along the [111] crystallographic direction and six {1-10} sidewalls. Subsequently, conformal overgrowth of the InxGa1-xAs shell was obtained only under kinetically limited growth conditions that suppressed mismatch-induced bending phenomena. The strain in the core and the shell was studied systematically as a function of the shell composition and thickness. To that end, we used Raman scattering spectroscopy, transmission electron microscopy and (synchrotron/lab source) X-ray diffraction, and compared the results with theoretical predictions based on continuum elasticity and density functional theories. All findings point to the existence of anisotropic tensile strain in the core that increases (as quantified by Raman measurements) with increasing the shell thickness, whereas the corresponding compressive strain in the shell decreases to zero. Our work demonstrates the opportunity to grow not only relaxed InxGa1-xAs shells with high structural quality (as adopted from the GaAs core) in a wide, if not the whole, compositional range, but also highly strained (tensile) GaAs cores with (opto)electronic properties that remain to be explored.

Thin films of nano-structured crystalline silicon (nc-Si) are potential absorber and supporting layers for next-generation Si solar cells. As one candidate, Si-SiO2 nanocomposites with percolated nc-Si have been fabricated by rapid thermal annealing (RTA) of sputter-deposited SiOx films (x≈1). A percolated silicon network has been formed by solid state phase separation into nc-Si and SiO2 [1, 2]. In the present study, SiO0.6 layers of ~500nm thickness are grown on quartz by ion beam sputter (IBS) as well as by reactive magnetron sputter (RMS) deposition. Formation of percolated Si-SiO2 nanocomposites is achieved by two different modes of thermal treatment: (i) Furnace annealing at 950°C and (ii) scanning laser processing. In case (ii), a diode laser with dwell times in the ms range, power densities of ~30 kW/cm², a wavelength of λ= 808nm and a line focus of 100µm x 11mm is applied. This process is ~106 times faster than isothermal treatment and ~103 times faster than RTA. Another advantage of this method is the usability of temperature sensitive substrates and maintaining homogeneous processing. Rutherford backscattering spectra of as-deposited and processed SiO0.6 reveals a compositional change in thin surface and interface layers, but no significant change in the bulk composition. Raman spectroscopy and X-ray diffraction show that the crystallinity of the nc-Si is higher for the laser-treated sample. High resolution- and energy-filtered transmission electron microscopy (HTEM, EFTEM) show additionally, that in both cases the as-deposited SiO0.6 is transformed into a percolated nanocomposite consisting of amorphous SiO2 and nc-Si. In more detail, laser processing of IBS-deposited layers leads to isotropic morphologies self-similar to furnace-annealed samples, but scaled up by a factor of ~5. This is explained by a phase separation in the liquid state and the solid state, respectively, which cause diffusion coefficients differing by several orders of magnitude. During the deposition by RMS, phase separated filament-like morphologies form. Here, furnace annealing leads to enhanced phase separation accompanied by crystallization. In contrast laser processing erases the as-deposited filaments and produces isotropic morphologies similar to IBS-deposited and laser-processed samples [1] Friedrich, D. et al. Sponge-like Si-SiO2 nanocomposite - Morphology studies of spinodally decomposed silicon-rich oxide. Appl. Phys. Lett. 103, 131911 (2013). [2] Ilday, S. et al. Multiscale Self-Assembly of Silicon Quantum Dots into an Anisotropic Three-Dimensional Random Network. Nano Lett. 16, 1942–1948 (2016).

Dendrites are common microstructures that are formed during industrial casting or welding. The processes involved in the formation of different dendrite morphologies and the orientation selection during dendritic growth are rather complex and still far from being fully understood [1-3]. In this work, in situ synchrotron X-ray radiography and diffraction methods have been combined to study the evolution of dendritic microstructures during the solidification of Ga - In alloys. The in situ directional solidification experiments were performed at the ID19 and BM20 beamlines (ESRF, France) at a high spatial resolution of < 1 µm. Solidification processes are affected by natural convection as soon as instable density stratification arises in the melt. Melt flow induces various effects on the dendrite and grain morphology primarily caused by the convective transport of the solute. Usually, the morphologies of these dendrites differ from those developing under purely diffusive condition. Our observations show a facilitation of the growth of primary trunks or lateral branches, a suppression of side branching, dendrite remelting and fragmentation [4]. The final microstructure reveals dendrites with random and complicated morphologies. The flow-induced variations of the local solute concentration may result in the changes of dendrite crystal orientations. According to theoretical predictions, the dendrite growth directions should not follow necessarily low indexed crystallographic directions [1]. Therefore coupling of in situ X-ray imaging with X-ray diffraction provides additionally information of the crystallographic orientation of the growing dendrites. Our measurements show that majority of the Indium dendrites grow along the orientation, typically observed in body-centered metals. The analysis of the diffraction patterns obtained from the complex dendritic structures shows that a further improvement towards a 3D imaging experiment is needed. These first results demonstrate that the combination of these X-ray techniques can provide new data about the solidification processes and help to validate microstructural solidification models. This work is financially supported by the Helmholtz Association “LIMTECH”.

We study the local dynamics of dendritic side arms during coarsening by combining in-situ radiography observations with numerical and analytical models. A flat sample of a Ga-In alloy is partially solidified and then held isothermally in a vertical temperature gradient. The evolving dendritic microstructure is visualized by synchrotron X-ray imaging at the BM20 beamline (ESRF, France). The resulting 2D images provide a high resolution in space and time at low noise levels, enabling accurate dynamical measurements. Throughout the initial growth stage there is evidence of solutal natural convection, which however vanishes towards the subsequent coarsening processes. During the coarsening stage, the time evolution of essential geometrical features of side branches was captured by automated image processing. This data is used to quantify the dynamics of three basic evolution scenarios for side branches: retraction, pinch-off and coalescence. We exploit the universal dynamics of sidearm necks during pinch-off to determine the product of liquid diffusivity and capillarity length, 𝐷𝑑0, as a parameter that is crucial in the calibration of quantitative reference models. By employing an idealized phase-field model for the evolution of a single side branch, we are able to predict the behaviour of selected side branches from the experiment in a consistent way.

Ge/Si core/shell quantum dots (QDs) recently received extensive attention due to their specific properties induced by the confinement effects of the core and shell structure. They have a type II confinement resulting in spatially separated charge carriers, the electronic structure strongly dependent on the core and shell size. Herein, the experimental realization of Ge/Si core/shell QDs with strongly tunable optical properties is demonstrated. QDs embedded in an amorphous alumina glass matrix are produced by simple magnetron sputtering deposition. In addition, they are regularly arranged within the matrix due to their self-assembled growth regime. QDs with different Ge core and Si shell sizes are made. These core/shell structures have a significantly stronger absorption compared to pure Ge QDs and a highly tunable absorption peak dependent on the size of the core and shell. The optical properties are in agreement with recent theoretical predictions showing the dramatic influence of the shell size on optical gap, resulting in 0.7 eV blue shift for only 0.4 nm decrease at the shell thickness. Therefore, these materials are very promising for light-harvesting applications.

The integration of high-mobility III-V compound semiconductors emerges as a promising route for Si device technologies to overcome the limits of further down-scaling. In this paper, a non-conventional approach of the combination of ion beam implantation with short-time flash lamp annealing is employed to fabricate InxGa1-xAs nanocrystals and to study their crystallization process in thin Si layers. The implantation fluence ratio of Ga and In ions has been varied to tailor the final nanocrystal composition. Raman spectroscopy and X-ray diffraction analyses verify the formation of ternary III-V nanocrystals within the Si layer. Transmission electron microscopy reveals single-crystalline precipitates with a low number of defects. A liquid epitaxy mechanism is used to describe the formation process of III-V nanocrystals after melting of the implanted thin Si layer by flash lamp annealing. The fabricated InxGa1-xAs nanocrystals are mainly Ga-rich with respect to the implanted Ga/In ratio.

Strain distribution in GaAs/InxGa1-xAs core/shell nanowires grown by molecular beam epitaxy on Si(111) substrates

Magnetic materials become more significant for future data storage devices and spintronic applications. In certain alloy thin films like Fe60Al40, nano-sized ferromagnetic structures can be created by means of focused ion irradiation.[1,2,3] Fe60Al40 shows a disorder induced phase transition from the thermodynamically stable, chemically ordered B2 to the metastable A2 phase going along with an evolution of ferromagnetism and an increase of the lattice parameter (Figure 1). This can be explained with the higher local number of Fe-Fe nearest neighbors in the disordered state. The correlation between structural and magnetic properties in the phase transition regime, consisting of A2 and B2 phases, is uncertain as well as the influence of the ion type or the temperature treatment. The effects of ion implantation on the structural and magnetic properties of 250 nm thick Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction (XRD) and Vibrating sample magnetometry. From XRD measurements, the order parameter S and the peak shift due to the change of the lattice parameter have been derived and correlated with the magnetization. The irradiation of paramagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of the saturation magnetization (MS) which was expected to be directly related to the number of displacements per atom (dpa) by using the simulation program TRIM [4], independent on the ion species. However, unlike than expected, the induced magnetization differed but correlated directly with the measured lattice parameter. A significant change of lattice parameter and MS did not appear for proton irradiation at elevated temperatures (250 °C) where the ordered B2 phase was retained. Upon low temperature (LN2) hydrogen implantation of disordered A2 Fe60Al40 films, on the other hand, unlike for helium or neon irradiation, the lattice parameter and the saturation magnetization decreased indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of chemically disordered alloy thin films well below the ordering temperature. Furthermore, the studies show that the structural and magnetic properties of 250 nm thick Fe60Al40 films are directly linked with each other (Figure 2) and do not depend on the type of treatment. The chemical disorder induced evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak represented by the order parameter dropping to 0. Nevertheless, the role of defects remains uncertain since ion irradiation leads besides the structural disordering also to an increase of the defect concentration and a temperature treatment to structural ordering and an annealing of defects. However, as described beforehand, H+ implantation causes little ordering but also an increase of the open volume defect concentration, which was characterized by means of Positron annihilation spectroscopy. This offers the opportunity to differentiate between structural disorder and defect concentration. Given the fact that the proton implanted films follow the shown general behavior a dependence on the structural order only can be assumed.

In this paper, we analyze the correlation of the magnetism and the carrier concentration with the shift of the spectroscopic critical points for low compensated GaMnAs samples with a high Curie temperature of around 150 K. The GaMnAs layers were grown by low-temperature molecular beam epitaxy. The low-temperature annealing leads to a reduction of Mn interstitials from 0.8 to 0.4% and an enhancement in the hole concentration. The saturation magnetization is 51 emu/cm and the Curie temperature is 150 K after post-growth annealing, while those of as-grown layers are 37 emu/cm and 80 K. The resistivity dropped significantly after the post-growth annealing, due to the fact that the number of Mn, which act as double donors and compensate holes, was significantly reduced by the low-temperature and long-time annealing. The electronic band structure is investigated by spectroscopic ellipsometry. The transition energies of critical points show redshift after post-growth annealing due to the annealing-induced enhancement of the hole concentration. Our results support the valence band picture (the Zener model) in ferromagnetic GaMnAs.

The processes involved in the formation of different dendrite morphologies are rather complex and still far from being fully understood. To verify existing micro-structural models in situ, in high resolution, in time and space, X-ray techniques are needed. In this work, in situ synchrotron X-ray radiography and diffraction methods have been combined to study the evolution of dendritic microstructures during the solidification of Ga - In alloys. The in situ directional solidification experiments were performed at the ID19 beamline (ESRF, France) at a high spatial resolution of < 1 µm. The dendritic growth and essential dynamics of the sidearm development were quantified. Melt flow induces various effects on the dendrite and grain morphology primarily caused by the convective transport of the solute. Usually, the morphologies of these dendrites differ from those developing under purely diffusive condition. Our observations show a facilitation of the growth of primary trunks or lateral branches, a suppression of side branching, dendrite remelting and fragmentation. The dendrite morphologies with their random character are difficult to analyse by means of the 2D radiographic projections. The flow-induced variations of the local solute concentration may result in the changes of dendrite crystal orientations. The coupling of in situ X-ray imaging with X-ray diffraction provides additionally information of the crystallographic orientation of the growing dendrites. These measurements show that the Indium dendrites grow along the orientation, typically observed in body-centered metals. The analysis of the diffraction patterns with its complex morphology show that further improvements towards a 3D imaging experiment are needed. These first results demonstrate that the combination of different X-ray techniques can provide new data about the solidification process and help to validate different solidification models.

Transition metal oxides represent an exotic class of correlated systems in which a complex interplay between the spin, charge, orbital and lattice degrees of freedom may result in colossal magnetoresistance, superconductivity; charge ordered (CO) phases, etc. The half-doped Pr0.5Sr0.5MnO3 manganite represents a unique stripe type CO-orbital order that induces transport and magnetic anisotropy whereas the CO in Nd0.5Sr0.5MnO3 is charge-exchange (CE)-type which is isotropic in nature. We have systematically explored ~ 200nm epitaxial manganite thin films grown on (100), (110), and (111) oriented (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates by pulsed laser deposition technique. Our Terahertz (THz) time-domain spectroscopic data reveal charge density wave (CDW) resonance centered around 5-6 meV for (110) oriented films and Drude-like conductivity for (100) and (111) oriented films. The CDW resonance in the optical conductivity spectra can be tuned from 4 meV to 6 meV for (110) oriented films and depends on the amount of ferromagnetic phase fraction in the CO matrix and corroborates well with the magnetization measurements. The nonlinear conductivity related to the sliding of the pinned CDW character makes the studied systems promising candidates for ultrafast coherent control of charge transport by resonant THz pumping.

The core/shell nanowire (NW) geometry is suitable for the pseudomorphic growth of highly mismatched semiconductor heterostructures, where the shell thickness can exceed significantly the critical thickness in equivalent planar heterostructures. We have investigated the accommodation of misfit strain in self-catalyzed GaAs/(In,Ga)As core/shell NWs grown on Si (111) substrates by molecular beam epitaxy. The NWs have their axis along the [111] crystallographic direction, six {11 ̅0} sidewalls, and their crystal structure is predominantly zinc blende. For strain analysis, we used Raman scattering spectroscopy, transmission electron microscopy, X-ray diffraction and photoluminescence spectroscopy. Within a certain range of core/shell dimensions and shell composition, our findings reveal that the elastic energy in NWs without misfit dislocations can be confined exclusively inside the core, allowing for the shell to be strain-free. The experimental results are also compared with theoretical simulations of the strain (continuum elasticity theory) and phonon energy (density functional theory).

The effects of ion implantation on the structural and magnetic properties of Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction and Vibrating sample magnetometry. The studies show that the magnetic properties of the 250 nm thick Fe60Al40 films, showing a phase transition from para- to ferromagnetic state, depend on the structural order only. The evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak. The irradiation of superparamagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of saturation magnetization (MS) going along with a lattice expansion induced by structural disorder. This effect didn’t appear for proton irradiation at elevated temperatures (250 ∘C) where the ordered B2 was retained. The low temperature hydrogen implantation of disordered A2 Fe60Al40 films on the other hand decreased, unlike helium or neon irradiation, the lattice parameter and MS indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of A2 Fe60Al40 thin films well below the ordering temperature.