Characterization of nanowires by coherent x-ray diffractive imaging and ptychography

Imaging techniques are of paramount importance for our understanding of the universe. From galaxies and stars explored by huge telescopes down to micro and nanostructures studied by microscopes, imaging systems provide invaluable scientific information. When an object under investigation has a size of about 100 nanometers, x-rays become a perfect probe for non-destructive imaging. The manufacturing process of image forming lenses for x-rays becomes much more complicated comparing to optical ones. Therefore, ”lensless” techniques which rely on the coherent properties of radiation were developed. With third generation of synchrotron sources highly coherent and intense x-ray beams became widely accessible. They are used in new imaging methods such as coherent x-ray diffractive imaging (CXDI) and x-ray ptychography. Modern nanotechnology opens a wide spectrum of possible applications in different branches of physics, chemistry, biology and engineering. At the nanoscale, matter has different physical and chemical properties compared to the macroscale bulk material. Thecontinuing trend of miniaturization of functional components in semiconductor industry brings new challenges both in growth and characterization methods. This Thesis is focusedon application of coherent diffractive imaging methods to reveal the structure of single semiconductor nanowires (NWs). They have been attracting significant attention for a couple of decades due to their efficient strain relaxation properties. And since the strain plays a significant role in NW performance the projects carried out in this work are oriented on Bragg CXDI approaches. Three distinct projects were carried out during my research activity at DESY research center of the Helmholtz Association. Experimental work was performed at P06 and P10 beamlines at PETRA III synchrotron. The first part of this Thesis extends the application of the three-dimensional (3D) Bragg CXDI to strain field mapping in a single InP NW with a diameter of 100 nm. The measurement employed a nanofocused beam, which is characterized by transmission x-ray ptychography. It is shown that the separation of the object and probe functions is possible in the direct space after a 3D reconstruction. The influence of a catalyst particle at the tip of the NW on the strain distribution in InP part of the NW is revealed. The second part of the manuscript is dedicated to the development and application of two-dimensional x-ray Bragg ptychography (2D XBP) to studies of single NWs. This approach providesa larger field of view on the sample and the reliability of reconstruction results improves, due to the advantages of ptychography. The limitations of the technique are discussed bytheoretical analysis and finite element method modeling (FEM). Successful experimental implementation is demonstrated on a single InGaN/GaN core-shell NW. The third part isdevoted to 3D Bragg CXDI of strain evolution in a single GaNNWwith respect to applied voltage bias. A complicated tilting and defect formation process in the NW was revealedfrom the evolution of the Bragg peak. This study gives an insight into piezoelectrical properties of the sample, which dramatically influence electron-hole pair recombinationand may decrease the efficiency of optoelectronic devices based on GaN NWs