Edwin Fohtung

Strain field in (Ga,Mn)As/GaAs periodic wires revealed by coherent X-ray diffraction

An experimental and simulation study of the full strain tensor and of strain-induced magnetocrystalline anisotropies in arrays of lithographically patterned (Ga,Mn)As on GaAs(001) is performed using a coherent diffraction lensless microscopy technique. We demonstrate the ability of our technique to get an insight into the strain field propagating in the crystal part belonging to the substrate. The experimentally reconstructed strain fields are in good agreement with those obtained from simulations based on elasticity theory.

Lateral ordering, strain, and morphology evolution of InGaAs/GaAs(001) quantum dots due to high temperature postgrowth annealing

The effect of postgrowth annealing on shape and ordering of a single layer of InGaAs/GaAs(001) quantum dots is investigated by three dimensional grazing incidence small angle x-ray scattering. A transition from disordered dots to two-dimensional lateral ordering is found. This transition is accompanying a quantum dot shape transformation. Grazing incidence diffraction measurements relate the observed ordering type to strain driven self organization. The role of different growth conditions leading to lateral correlation is discussed by comparing the results to recent experimental achievements in the field.

A Shack-Hartmann Sensor for Single-Shot Multi-Contrast Imaging with Hard X-rays

An array of compound refractive X-ray lenses (CRL) with 20x20 lenslets, a focal distance of 20 cm and a visibility of 0.93 is presented. It can be used as a Shack-Hartmann sensor for hard X-rays (SHARX) for wavefront sensing and permits for true single-shot multi-contrast imaging the dynamics of materials with a spatial resolution in the micrometer range, sensitivity on nanosized structures and temporal resolution on the microsecond scale. The objects absorption and its induced wavefront shift can be assessed simultaneously together with information from diffraction channels. This enables the imaging of hierarchical materials. In contrast to the established Hartmann sensors the SHARX has an increased flux efficiency through focusing of the beam rather than blocking parts of it. We investigated the spatiotemporal behavior of a cavitation bubble induced by laser pulses. Furthermore, we validated the SHARX by measuring refraction angles of a single diamond CRL, where we obtained an angular resolution better than 4 microrad.

Three-dimensional reciprocal space profile of an individual nanocrystallite inside a thin-film solar cell absorber layer

The strain profile of an individual Cu(In,Ga)Se2 nanocrystallite in a solar cell absorber layer is accessed using synchrotron radiation. We find that the investigated crystallite is inhomogeneously strained. The strain is most likely produced by a combination of intergranular strain and composition variations in nanocrystals inside the polycrystalline semiconductor film and carries information about the intercrystalline interaction. The measurements are made nondestructively and without additional sample preparation or x-ray beam nanofocusing. This is the first step towards measurements of strain profiles of individual crystallites inside a working solar cell.

Bragg Coherent Diffraction Imaging Techniques at 3rd and 4th Generation Light Sources

Although X-ray crystallography is established as a state of the art imaging technique that has been revolutionary across materials sciences, physics, chemistry, biology and medicine, the imaging of non-crystalline objects is inaccessible by this method. A promising approach that can overcome this challenge is coherent diffractive imaging (CDI). CDI is a lensless microscopy technique that can provide nanoscale images of both non-crystalline and crystalline objects. The morphology, structure and evolution of an object of interest is probed using a coherent source of photons (often X-rays, visible light) or electrons. Coherency is needed for the interference to produce a usable diffraction pattern. While the diffraction pattern contains the magnitude information of the object in reciprocal space, the phase information can be recovered using iterative feedback algorithms, allowing the reconstruction of the image of an object. As no lenses are used, the image is free of aberrations and hence the resolution is limited only by the wavelength of the probe, exposure, and the robustness of the reconstruction algorithm. This technique has proven crucial for imaging of variety of samples, from nanostructures to bio-tissues and individual cells. The aim of this chapter is to provide a clear picture of recent state-of-the-art developments in CDI techniques, and particularly in Bragg Coherent Diffraction Imaging, applied to oxide nanostructures.