Tomas Stankevic – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Research > PhD theses 2016 > 2016 > Tomas Stankevic

 

Tomas StankevicTomas Stankevic

A thesis submitted October 2015 for the degree of Doctor of Philosophy and defended February 25, 2016.

The PhD School of Science
Faculty of Science, Niels Bohr Institute, X-ray and Neutron Science, University of Copenhagen, Denmark

Academic supervisor:
Robert Feidenhans'l

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Structural investigations of nanowires using X-ray diffraction

Advancements in growth of the nanowire-based devices opened another dimension of possible structures and material combinations, which find their applications in a widevariety of fields, including everyday life. Characterization of such devices brings its own challenges and here we show that X-rays offer large possibilities to analyze the structural properties.

In the present work we used three different techniques to characterize a large spectrum of different nanowire heterostructures from the structural point of view:

(i) First, we measured high resolution three dimensional reciprocal space maps averaged over large number of nanowires. Knowing the precise positions of multiple Bragg peaks in reciprocal space we could calculate the average strain and composition.

(ii) In the second technique we used a nanofocused X-ray beam of 100 nm in diameter to measure the local variation of strain and tilt on a scale of a few nanowires with high spatial resolution. Next, we combined it with three dimensional reciprocal space mapping and while scanning across a single nanowire with the nanofocused beam, we measured three dimensional intensity distributions around the Bragg peaks at every step. This allowed a very accurate measurement of strain at every point of the single nanowire. We showed that in critical heterostructures the strain distribution can be very inhomogeneous.

(iii) Lastly, we have studied in-situ the nanowire growth by molecular beam epitaxy at the synchrotron beamline. With this, we could grow the nanowires and measure X-ray diffraction in real time. We studied the initial stage of pure WZ InAs nanowire growth. By measuring the interference fringes in the scattering signal, raising from the finite length of the NWs it was possible to precisely determine the nanowire length evolution at each time step. Next, we formed a hybrid axial and radial heterostructure with InAsSb and observed how bending of the nanowires takes place in real time.

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