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This work presents first insights into the dynamics of free-surface release clouds from dynamically compressed polystyrene and pyrolytic graphite at pressures up to 200 GPa, where they transform into diamond or lonsdaleite, respectively. These ejecta clouds are released into either vacuum or various types of catcher systems, and are monitored with high-speed recordings (frame rates up to 10 MHz). Molecular dynamics simulations are used to give insights to the rate of diamond preservation throughout the free expansion and the catcher impact process, highlighting the challenges of diamond retrieval. Raman spectroscopy data show graphitic signatures on a catcher plate confirming that the shock-compressed PS is transformed. First electron microscopy analyses of solid catcher plates yield an outstanding number of different spherical-like objects in the size range between ten(s) up to hundreds of nanometres, which are one type of two potential diamond candidates identified. The origin of some objects can unambiguously be assigned, while the history of others remains speculative.

Extreme states of matter exist throughout the universe e.g. inside planetary cores, stars or astrophysical jets. Such conditions can be generated in the laboratory in the interaction of powerful lasers with solids. Yet, the measurement of the subsequent plasma dynamics with regard to density, temperature and ionization is a major experimental challenge. However, ultra-short X-ray pulses provided by X-ray free electron lasers (XFELs) allow for dedicated studies, which are highly relevant to study laboratory astrophysics, laser-fusion research or laser-plasma-based particle acceleration. Here, we report on experiments that employ a novel ultrafast method, which allows to simultaneously access temperature, ionization state and nanometer scale expansion dynamics in high-intensity laser-driven solid-density plasmas with a single X-ray detector. Using this method, we gain access to the expansion dynamics of a buried layer in compound samples, and we measure opacity changes arising from bound-bound resonance transitions in highly ionized copper. The presence of highly ionized copper leads to a temperature estimate of at least 2 million Kelvin already after the first 100 femtoseconds following the high-intensity laser irradiation. More specifically, we make use of asymmetries in small-angle X-ray scattering (SAXS) patterns, which arise from different spatial distributions of absorption and scattering cross sections in nanostructured grating samples when we tune an XFEL to atomic resonant energies of copper. Thereby, changes in asymmetry can be connected with the evolution of the plasma expansion and ionization dynamics. The potential of XFEL-based resonant SAXS to obtain three-dimensional ultrafast, nanoscopic information on density and opacity may offer a unique path for the characterization of dynamic processes in High Energy Density plasmas.

We report the temperature-dependent magnetic and structural properties of epitaxial Mn5Ge3 thin films grown on Ge substrates. Utilizing density-functional theory (DFT) calculations and various experimental methods, we reveal mechanisms governing the switching between collinear and noncollinear spin configuration in Mn5Ge3. The Mn atoms in Mn5Ge3 occupy two distinct Wyckoff positions with fourfold (Mn1) and sixfold (Mn2) multiplicity. The DFT calculations reveal that below a critical distance of approximately 3.002 Å the coupling between Mn2 atoms is antiferromagnetic (AFM) while ferromagnetic (FM) above that critical distance. The FM coupling between Mn1 atoms is weakly affected by the strain. The observed noncollinear spin configuration is due to the coexistence of AFM and FM coupling at low temperatures. The findings give insight in developing strain-controlled spintronic devices.

Resonant Small-angle x-ray scattering raw data obtained in measurements at MEC at LCLS and evalutation of the asymmetry in the scattering patterns. The data set is structured in case 1/Si-Cu-compound targets and case 2/Cu-only-targets as presented in the publication for on- and off-resonant XFEL probe energies.

Hafnium oxide was deposited from tetrakis(dimethylamino)hafnium (TDMAHf) and water by atomic layer deposition (ALD) on heated 400 Si wafers covered with native oxide in the temperature range from 100 C to 350 C. Optimized self-limiting ALD reaction and smallest hydrogen impurity level have been realized for a substrate temperature of 300 C. The stoichiometry of deposited films and hydrogen impurity level were measured by elastic recoil detection analysis. The hafnium to oxygen ratio showed the expected 1:2 value. Besides hydrogen, no other impurities could be detected. Furthermore, a strong correlation between the growth rate per cycle (GPC), film uniformity and level of hydrogen impurities was observed. Based on this result, the easily accessible GPC can be used as a first indication for the hydrogen impurity level in ALD grown thin films. Furthermore, the characterization of the crystal structure showed the appearance of some crystallites in an amorphous matrix already for a growth temperature of 250 C and a pure crystalline layer at a growth temperature of 350 C. The increased crystallinity with increasing growth temperature was attributed to a higher seed concentration and a constant crystal size.

The local dynamics of dendritic sidearms during the growth stage are studied by in-situ radiography observations at high spatial resolution of < 1 µm. A flat sample of a Ga-In alloy is solidified top-down applying a vertical temperature gradient. The evolving dendritic microstructure is visualized using synchrotron X-ray imaging at the beamline ID19 (ESRF, France). The experimental investigations on the dendrite evolution revealed a transition from a four-fold symmetry to a hyperbranched dendritic morphology. Both, the sidearm-splitting phenomena – responsible for this morphological transition – as well as the arm growth dynamics are characterized by image processing.

Ferromagnetism in certain B2 ordered alloys such as Fe60Al40 can be switched on, and tuned, via antisite disordering of the atomic arrangement. The disordering is accompanied by a 1% increase in the lattice parameter. Here we performed a systematic disordering of B2 Fe60Al40 thin films, and obtained correlations between the order parameter (S), lattice parameter (a0), and the induced saturation magnetization (Ms). As the lattice is gradually disordered, a critical point occurs at 1-S=0.6 and a0=2.91 A, where a sharp increase of the Ms is observed. DFT calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, whereas above, it is largely the increase of a0 in the disordered state that determines the Ms. The insights obtained here can be useful for achieving tailored magnetic properties in alloys through disordering.

In thin amorphous carbon (a-C) films being in contact with a thin nickel layer, metal-induced crystallization and layer exchange (LE) occur at temperatures lower than 700 °C. Analysis of thin film stacks with different architectures (a-C/Ni, Ni/a-C and Ni/a-C/Ni) by means of ion beam analysis, Raman spectroscopy, X-ray diffraction and transmission electron microscopy revealed that the degree of LE and the structural quality of the crystallized carbon layers depend on the initial layer sequence. A LE degree of approx. 93 % was found for Ni/a-C bilayers, where graphenic layers formed on the Ni surface, whereas in a-C/Ni bilayers only 83 % of carbon was transferred from the surface towards the fused silica substrate. The diffusion of carbon in the outward direction produces turbostratic carbon with basal planes oriented parallel to the Ni surface, while for the inward direction planar and curved turbostratic structures coexist. The crystallization and the LE are driven by the crystallization energy of a-C. The LE is mediated by the wetting of the Ni grain boundaries by carbon. The directionality of the LE was explained primarily by the difference in the surface and interface energies in the a-C/Ni and Ni/a-C stacks that were obtained from thermodynamic considerations.

The local dynamics of dendritic tips and side arms during the growth and coarsening stages are studied by X-ray synchrotron radiography and by numerical simulation. The direct investigation of dendritic side arm evolution 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 were performed at the ID19 beamline (ESRF, France) at a spatial resolution of < 1 µm. A flat sample of a Ga-In alloy is solidified from top to down applying a vertical temperature gradient. The present measurements provide real-time in-situ data on three phenomena that are of major importance in dendrite morphology evolution: side arm coarsening, side arm-splitting and dendrite tip evolution. The combination of numerical modeling and synchrotron experiments has allowed to improve the understanding of coarsening of dendritic side arms and to obtain material information that is relevant for quantitative modeling. Another interesting effect can be observed during in situ solidification experiments: a transition from a four-fold symmetry to a hyperbranched dendritic morphology. This morphological transition originating from the splitting of dendrite side arms has been detected.

Ge with a quasi-direct band gap can be realized by strain engineering, alloying with Sn, or ultrahigh n-type doping. In this work, we use all three approaches together to fabricate direct-band-gap Ge-Sn alloys. The heavily doped n-type Ge-Sn is realized with CMOS-compatible nonequilibrium material processing. P is used to form highly doped n-type Ge-Sn layers and to modify the lattice parameter of P-doped Ge-Sn alloys. The strain engineering in heavily-P-doped Ge-Sn films is confirmed by x-ray diffraction and micro Raman spectroscopy. The change of the band gap in P-doped Ge-Sn alloy as a function of P concentration is theoretically predicted by density functional theory and experimentally verified by near-infrared spectroscopic ellipsometry. According to the shift of the absorption edge, it is shown that for an electron concentration greater than 1x10^20 cm-3 the band-gap renormalization is partially compensated by the Burstein-Moss effect. These results indicate that Ge-based materials have high potential for use in near-infrared optoelectronic devices, fully compatible with CMOS technology., 20 pages, 6 figures

Single quantum dots in the core of freestanding semiconductor nanowires is a promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. Here, we demonstrate that complex quantum-dots can be grown in self-catalyzed III-As nanowires and their emission can be tuned in a wide range of wavelengths. The quantum dots are formed inside self-catalyzed GaAs nanowires (grown on Si substrates by molecular beam epitaxy) by first growing an axial AlxGa1-xAs/GaAs/AlxGa1-xAs heterostructure in pulsed mode . The AlxGa1-xAs segments are grown as digital alloys with a precise control of the composition, the thickness, and the crystal structure (absence of stacking faults). Then, the nanowires are overgrown all-around with an InxAl1-xAs layer in a core/shell fashion. Owing to the large lattice-mismatch with the shell, the thin core develops tensile hydrostatic strain and the emission from the dot is strongly red-shifted. Furthermore, distinct exciton-biexciton features are identified in photoluminescence measurements.

Silicides have been widely used for CMOS devices in order to provide a stable Ohmic contact with a low contact resistivity. With the integration of Ge on Si the focus also shifted to germanides as a low resistivity contact material. In addition, ferromagnetic germanides may serve as spin injector materials for Ge-based spintronic devices. Usually, germanides have been fabricated by furnace or rapid thermal annealing in literature. In this contribution we investigate the formation process of Ni germanides using a combination of magnetron sputtering and flash lamp annealing (FLA). Three different types of Ge served as a substrate for the deposition of the transition metal: amorphous Ge made by magnetron-sputtering on a SiO2-Si substrate, polycrystalline Ge made by magnetron-sputtering followed by FLA, and monocrystalline Ge in the form of a (100) Ge wafer. After metal deposition samples are in-situ annealed by FLA without breaking the vacuum, which triggers the formation of germanides and prevents a possible, but unwanted oxidation. In order to investigate the crystallization behavior, the structures have been characterized by Raman spectroscopy, X-ray diffraction, ellipsometry, current-voltage and Hall effect measurements.

There is a broad palette of applications for thin Si and Ge films ranging from photovoltaics over various microelectronic devices to sensor applications. Both amorphous and polycrystalline thin films are of interest for thin film photovoltaics, and thin film poly-Si transistors are the heart piece for driving LCDs and OLEDs [1]. In addition, the ability to deposit SiO2 and Si layers in an alternating order and to process them allows to extend the device density without further downscaling [2]. Amorphous thin film deposition methods are the most cost-effective ones, the subsequent crystallization is the most critical process step with regard to microstructure, defect density, and electrical properties. Potentially, flash lamp annealing (FLA) is a very suitable method due to the short process time, the qualification for temperature-sensible substrates and the possibility to take advantage of non-equilibrium crystallization modes [3]. In this work thin amorphous Si and Ge films have been deposited on SiO2 by DC-magnetron sputtering and crystallized by in-situ FLA in a new FLA sputter tool recently installed by the Rovak GmbH at HZDR (Fig. 1). The in-situ-processing suppresses the influence of surface oxidation effects after deposition prior to FLA. In order to investigate the crystallization behaviour, the thin films have been characterized by Raman spectroscopy, X-ray diffraction, ellipsometry, current-voltage and Hall effect measurements. Based on these results and in combination with temperature simulations, a model for the crystallization of thin amorphous Si and Ge films is derived.

III-As semiconductors in the form of free-standing nanowires have exhibited new potentials for a wide variety of future applications in nanotechnology, ranging from energy-efficient electronic switches to entangled-photon-pair sources for quantum information technology, including the possibility for monolithic integration in the mainstream Si technology. Using molecular beam epitaxy, we developed an in situ procedure (substrate annealing – Ga deposition – substrate annealing) in order to modify the surface of Si substrates and, thus, to achieve highly synchronized nucleation of self-catalyzed GaAs nanowire ensembles with well controlled dimensions and number density. Specifically, the radius can be as low as 10 nm, the distribution of lengths can be sub-Poissonian (due to the so-called nucleation anti-bunching), and the number density can be varied from 10 ⁶ to 10 ⁹ cm-2. Furthermore, the GaAs nanowires can be hydrostatically strained when they are overgrown all-around with lattice-mismatched shells. The high surface-to-volume ratios allow for growing highly mismatched combinations without dislocations, beyond to what is possible in thin-film heterostructures. Here, we show that the mismatch strain inside the GaAs core can be engineered via the composition and the thickness of an (In, Ga)As or (In, Al)As shell. As a result, the electronic properties of GaAs can be widely tuned; the band gap and the electron effective mass can be reduced down to 60% of the strain-free values, rendering GaAs nanowires suitable for photonic devices across the near-infrared (800 – 1400 nm) range or for high-speed transistors.

Der Vortrag stellt das Helmholtz Innovation Lab für Ultrakurzzeitausheilung vor. Im zweiten Teil werden experimentelle Ergebnisse bei der Kristallisation von dünnen amorphen Halbleiterschichten (Si, Ge, NiGe) mittels magnetron sputtering und Blitzlampenausheilung diskutiert.