This project is a special research programme with members from JKU Linz, TU Vienna, Uni Vienna, Uni Jena, and TU Munich, with the goal to develop solid state radiation sources and detectors in the mid-IR range, based on semiconductor nanostructures. More information on the SFB in general can be found at the IR-On website.
The project part P7 of the SFB, "Next generation x-ray techniques", deals with the structural analysis of semiconductor nanostructures using x-ray scattering, which is a prerequisite to optimize the sample growth, but also provides valuable input data for theoretical modeling of electronic, optic, and transport properties of devices produced within the SFB.
Development of a focusing setup
The main aim of the project part is the development of analysis schemes using a focused x-ray beam. For this purpose a collaboration with the European Synchrotron Radiation facility (ESRF) in Grenoble, France, has been established. Together with the team of beamline ID01, a new dedicated instrument has been developed, with the ability to focus x-rays in the 10 keV energy range down to about 100 nm, and hence illuminate single nanoobjects. The diffractometer has been installed in the winter shutdown 2010/2011, and is currently in the commissioning phase. For focusing, both compound refractive lenses as well as Fresnel zone plates can be used. The sample stage consists of a high-precision hexapod and a piezo translation stage, allowing sample positioning in the nm range.
Investigation of nanostructures
Using the setup developed within the project, several nanostructures have been investigated. Among them are a single SiGe quantum dot embedded into a fully functional MOS transistor structure, single nanowires, and even single quantum dots embedded into a nanowire (this is the image on top of the page).
Beside conventional analysis using simulations of the scattered intensity distribution in reciprocal space, the coherence of the focused beam can be exploited to achieve a direct inversion of the measured intensity to obtain the real space sample structure. The scattered wave field is in kinematical approximation given by the Fourier transform of the electron density distribution in the sample. Since only the scattered intensity averaged over the detector response time can be measured, the phase inforamtion of this Fourier transform is lost. Using iterative algorithms, this phase information can be reconstructed, given some basic information like the overall size of an object are known, and the intensity pattern can be samples finer than the reciprocal space frequency given by that size. While this "phase retrieval" algorithms work well for unstrained objects, they do so far not converge reliably for strained objects. The development of the respective algorithms is a second major goal of the x-ray project within the SFB.