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lll - V Nanostructures Project


The research of our group focuses on the growth and characterization of semiconductor nanostructures in the III/V material system. Employing X-ray scattering techniques, we investigate their crystalline properties (e.g. composition, crystallographic phases, lattice strain,and defects) based on the intensity distribution in the vicinity of a set of reciprocal lattice points G.

 

nanostructures

 

Together with the Theory and Simulation group of the IPS and in collaboration with the Solid State Physics Group of the University of Siegen,  and the Epitaxy Group of the Paul-Drude Institut für Festkörperelektronik in Berlin we currently aim at the growth of self-catalyzed III/V nanowires and their characterization using X-ray scattering methods, especially in-situ.

Our main research is focused on GaAs nanowires. GaAs nanowires are small free standing crystalline “matchsticks” usually up to several µm in length with a diameter ranging from tens of nm to hundreds of nm. Promising applications of these nanowires could increase the efficiency of e.g. LEDs, solar cells, sensors and transistors, but nanowires are also interesting for fundamental research, e.g. on qu-bits or Majorana fermions.

 

 

The evolution of the crystalline structure of these nanowires during the growth or growth-related processes such as annealing, is highly interesting since insight into the growth mechanism might offer ways for controlling the final physical properties of the nanowires. More specifically, we aim at investigating the so-called Wurtzite – Zinc-Blende polytypism in GaAs nanowires. The Wurtzite – Zinc-Blende polytypism refers to the occurrence of two crystalline phases of the same material (in our case GaAs can occur in the hexagonal Wurtzite and the cubic Zinc-Blende structure) in the same nanowire. Polytypism is naturally observed in GaAs nanowires (and III/V nanowires in general) as a rather random stacking sequence of Wurtzite and Zinc-Blende segments of various lengths along the nanowire growth axis. Since the two phases differ in their structural and electronic properties, polytypism strongly affects the electronic efficiency of possible nanowire-based devices. Thus considerable effort is made in order to understand and finally control polytypism in nanowires.

 

One of the key techniques in our research on polytypism, we employ X-ray radiation provided by synchrotron facilities such as the Angrstomquelle Karlsruhe (ANKA) at the Karlsruhe Institute of Technology, the European Synchrotron Radiation Facility (ESRF) in Grenoble or PETRA III in Hamburg. By taking advantage of versatile measurement geometries and the high photon flux we can investigate e.g. structure-sensitive Bragg – reflections which provide unique insight in the evolution of polytypism when applied in-situ.

 

in-situ research

 

 

The in-situ investigation of growth processes with X-ray radiation provided by a synchrotron source is a challenging experimental task that requires both dedicated equipment and growth compatible non-destructive characterization methods. In our research we combine growth methods for achieving epitaxial nanostructures and non-destructive, time-resolved  X-ray scattering methods.

For the fabrication of the GaAs nanowires, we employ a portable molecular beam epitaxy (PMBE) system based at ANKA (KIT), which is dedicated for X-ray investigations at a synchrotron beamline during the growth process.

 

 

Previous topics

  • Investigations of strain and composition of InGaAs/GaAs during high temperature annealing by time-resolved grazing incidence X-ray diffraction
  • Determination of composition and strain of InAlGaAs/GaAs quantum dots by evaluation of coherent X-ray scattering features and comparison with numerical calculations