Structural analysis of infrared active nanostructures by next generation x-ray techniques      


J. Stangl

G. Bauer

 

 

The aim of P07 is the development of new X-ray diffraction techniques to perform a full structural characterization of specific, single nanostructures. For this purpose a dedicated diffractometer has been developed in the first two IR-ON periods, which became operational by the end of 2010. First studies of single nanowires and SiGe islands have been very succesful, reaching real-space resolutions in the 10 nm range. In the third period, we will still improve and tune the setup, to reach sub-100 nm X-ray focus sizes and extend the lower limit of nanostructure size we can investigate. Depending on the incidence aperture to the focusing element, the X-ray beam can be optimized for full coherence or maximum flux. Hence, the setup can be used both for phase retrieval experiments requiring a high dynamic range and work well for sufficiently strong scatterers, as well as for studies of very small and weakly scattering nanostructures using simulations of the scattered intensity, which requires less dynamic range.

The structural results will be combined to those obtained by optical and electrical methods, as well as to ab initio calculations to achieve a full understanding of the structure-function relationship. In the last period we will further develop phase retrieval algorithms in diffraction geometry, to overcome some of the difficulties and limits detected in the second period. To study nanowires, which are longer than the X-ray focus diameter, and strained SiGe islands on an inhomogeneously strained substrate, “conventional” oversampling methods cannot be used. We will explore the possibilities of ptychography, where data oversampling in reciprocals space is replaced by sampling several overlapping regions in real space, so that large objects can be investigated without loss of resolution. This approach has been succesfully applied to imaging application in small angle scattering, but only few studies using it in diffraction geometry exist. We will apply the method to study single Si and Ge nanowires embedded into straining devices (P08), SiGe islands inside detector structures (P02), as well as regular nanocrystal arrangements in single and multiple layers for IR detector applications (P05, partly in combination with P04).

Combining the strain results from nanowires and SiGe islands to ab-initio calculations of band structure and band alignment, and optical measurements on the very same nanoobject investigated by X-ray scattering, we will be able to understand the microscopic origin of certain properties, enabling otimization of growth and fabrication techniques. To push structural characterization to the limits, we will combine the direct analysis schemes (phase retrieval and ptychography) with modelling approaches based on finite element simulations, depending on the obtainable data quality regarding dynamic range and signal to noise ratio.

 

Report 2005-2009 (.pdf)

 

Report 2009-2012 (.pdf)