X-ray microdiffraction analysis of semi-conductor nanostructures: beyond the ensemble average

by Cristian Mocuta

Friday 15 Feb 2008, 10:00 → 11:00 Europe/Berlin

Bldg. 25b/109

X-ray diffraction is a powerful non-destructive tool to determine the structural properties of nanostructures, in particular the size, spatial distribution, chemical composition and strain state; it can be applied to buried as well as uncapped structures. So far, in great part of x-ray studies, ensembles of nanostructures (e.g. quantum dots) have been investigated: using beam sizes of 0.1 to 1mm, typically 103-106 quantum dots are illuminated simultaneously. Consequently the obtained parameters are those of an average structure, and meaningful only if the ensemble is monodisperse (in size, composition, …). It is of high interest to measure (combining different methods) the structural, optical, electronic and/or mechanic properties of individual sub-micron sized objects, in order to understand the change in physical properties when the nanoscale is approached. Methods like photoluminescence, Raman, or local probe microscopes, can achieve high real-space resolutions and investigation can be performed on individual nanostructures (with eventual limitations like surface sensitivity, sample destruction, …). To perform a detailed characterization in inhomogeneous ensembles, it is necessary that an individual object can be analyzed, and that a specific object can be chosen for analysis, using various techniques. I will show here local probe x-ray diffraction experiments, using a setup developed at ID-01 beamline at ESRF : focused x-ray beams are used to localize and to characterize one by one particular sub-micron objects. Using the diffracted intensity to produce contrast , in a scanning mode, an image of the sample surface is recorded, which allows for the reproducible alignment of a specific object for analysis. I will first address microdiffraction results for single rolled-up semiconductor nanotubes (following the tube from the part attached to the substrate to its freestanding part). The lattice parameter distribution and the strain can be measured and then modeled inside the tube using elastic theory. In a second example I will show a similar approach for micron-sized SiGe pyramidal islands grown by Liquid Phase Epitaxy on Si(001). From experimental data on particular individual objects and using mathematical modeling (Finite Element Methods), the variation of structural parameters such as strain, composition and shape was measured from island to island, evidencing the presence of two kinds of objects. Complementary scanning electron microscopy (SEM) investigation was performed on the very same objects measured in diffraction. It is thus possible to combine different techniques (in this case SEM, AFM and diffraction) on exactly the same object; also a first test of using the AFM tip to interact with the islands will be shown. This approach opens up the possibility of combining x-ray microdiffraction technique with other micro-probe experiments on the same individual objects. 1 The probed volume has to be adapted to the typical feature size, in many cases in the micron range or below. 2 Any other x-ray induced signal (fluorescence, size broadening, coherent interference, …) could in principle be used for the microprobe imaging.