SASE1 is the first beamline of the European XFEL that became operational in 2017. It consists of the SASE1 undulator system, the beam transport system, and the two scientific experiment stations: Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX), and Femtosecond X‐ray Experiments (FXE). The beam transport system comprises mirrors to offset and guide the beam to the instruments and a set of X‐ray optical components to align, manipulate and diagnose the beam. The SASE1 beam transport system is described here in its initial configuration, and results and experiences from the first year of user operation are reported.
Self-assembled GaAs quantum dots (QDs) have been grown on misoriented GaAs(111)B substrates using droplet epitaxy. Different droplet deposition temperatures, arsenization temperatures and times as well as postcrystallization annealing temperatures have been investigated, while the amount of deposited Ga was kept constant at two monolayers. Atomic force microscopy measurements reveal that unusually low Ga deposition and arsenization temperatures in comparison to the (100) and (111)A surfaces are necessary to obtain droplet/QD densities in the order of 108 cm−2. The formation of droplets and their crystallization result in QDs with approximately circular symmetry regardless of the anisotropy introduced by the miscut. Crystallized GaAs QDs show the thermal resistance comparable to those observed on (100) and (111)A. When the QDs are embedded in Al0.3Ga0.7 As, a spectrally broad luminescence around 790 nm can be observed.
Heteroepitaxy on nanopatterned substrates is a means of defect reduction at semiconductor heterointerfaces by exploiting substrate compliance and enhanced elastic lattice relaxation resulting from reduced dimensions. We explore this possibility in the InAs/GaAs(111)A system using a combination of nanosphere lithography and reactive ion etching of the GaAs(111)A substrate for nano-patterning of the substrate, yielding pillars with honeycomb and hexagonal arrangements and varied nearest neighbor distances. Substrate patterning is followed by MBE growth of InAs at temperatures of 150 -350 C and growth rates of 0.011 nm/s and 0.11 nm/s. InAs growth in the form of nano-islands on the pillar tops is achieved by lowering the adatom migration length by choosing a low growth temperature of 150 C at the growth rate 0.011 nm/s. The choice of a higher growth rate of 0.11 nm/s results in higher InAs island nucleation and the formation of hillocks concentrated at the pillar bases due to a further reduction of adatom migration length. A common feature of the growth morphology for all other explored conditions is the formation of merged hillocks or pyramids with well-defined facets due to the presence of a concave surface curvature at the pillar bases acting as adatom sinks.
We analyze the shape and position of heteroepitaxial InAs islands on the top face of cylindrical GaAs(111)A nanopillars experimentally and theoretically. Catalyst-free molecular beam epitaxial growth of InAs at low temperatures on GaAs nanopillars results in InAs islands with diameters < 30 nm exhibiting predominantly rounded triangular in-plane shapes. The islands show a tendency to grow at positions displaced from the center towards the pillar edge.Atomistic molecular statics simulations evidence that triangular-prismatic islands centered to the pillar axis with diameters smaller than that of the nanopillars are energetically preferred.Moreover, we reveal the existence of minimum-energy states for off-axis island positions, in 2 agreement with the experiment. These findings are interpreted by evaluating the spatial strain distributions and the number of broken bonds of surface atoms as a measure for the surface energy. The influence of surface steps on the energy of the system is addressed as well.
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