X-ray diffraction is a powerful non-destructive tool that can be used to analyse strain fields and composition profiles in condensed matter. With the development of small structures for micro- and nanoelectronics, local probes are gaining popularity for the understanding of novel properties related to the small size. We present here a scanning X-ray diffraction microscope (SXDM) for local probe X-ray diffraction experiments. The approach combines X-ray diffraction with strong focused X-ray beams to localise the micro/nanostructure(s) and analyse their strain and composition. The volume probed has to be adapted to the typical feature size, in many cases in the range of micrometres or smaller. Using epitaxial semiconductor islands grown on Si(001) [1] as a model system, we were able to identify and probe individual objects one by one. The results below were obtained for SiGe islands of 2-3 µm lateral size. The resolution is determined by the focus size. The ongoing refurbishment of the ID01 beamline has permitted us to reach a submicrometre focus size thus making the technique applicable to individual nanostructures.

The principle of SXDM is shown in Figure 95: the X-ray beam is focused by X-ray lenses [2] onto a small spot (3 x 5 µm2) on the sample, from where it is elastically scattered into the detector (a). The intensity distribution at different positions in reciprocal space is probed (b, c). If the focused beam is scattered at the bare sample surface (b), the intensity distribution shows a sharp Si substrate peak and a streak perpendicular to the sample surface, the crystal truncation rod (CTR). If the X-ray beam hits an island, additional scattering occurs (c). While measuring at this island-sensitive position in reciprocal space, the sample’s lateral position is scanned, and the intensity is recorded as a function of the real space position in the fashion of a scanning probe microscope (d). By scanning the sample in the two lateral directions, an image of the island distribution on the sample is recorded (e), with the local strain being responsible for the observed contrast in the image. Any signal characteristic of the object (fluorescence, different lattice parameter, size broadening or coherent interference, etc.) could be used to produce this contrast: the setup works like a scanning probe microscope with tuneable probe signal.

Fig. 95: Scheme of the scanning X-ray diffraction microscope (details in text).

 

Overlaid to the SXDM color map (panel e) are the island positions (squares) extracted from an optical microscopy image, showing a perfect agreement and proving the capabilities to distinguish and identify single micrometre-sized objects. Once the individual islands are identified, entire reciprocal space maps (RSM, see Figure 96) were measured for the particular islands marked by arrows. The red arrows mark islands identified as having a different structure than the majority (~97-98%) of the islands. Comparing the recorded signals, major differences concerning the distribution of the scattered intensity is found. On the (004) RSM (a,c), the position of the maximum intensity changes, pointing to a different relaxation (average lattice parameter) in the direction perpendicular to the surface. Even more information can be accessed from the non-symmetric (115) reflections (b,d): the scattered intensity close to the relaxation line is a signature of relaxation in the epitaxial islands (IL1), while intensity along the CTR represents pseudomorphically strained material (IL2). The Ge concentration distribution is then obtained from fitting the experimental data and displayed in panels (e,f). The detailed analysis (finite element method (FEM) simulation, not shown here) also gives access to the strain field in the islands. The flat islands are found to represent the bottom part of the truncated pyramids; this morphology is confirmed by Scanning Electron Microscopy (SEM), imaging of the very same objects.

Fig. 96: RSM of SiGe islands around (004) and (115) reciprocal lattice points (top and bottom rows respectively) on particular single islands: (a,b) truncated pyramid-shaped islands and (c,d) flat islands respectively. The position of the substrate peak (cross) and the maximum scattered signal (IL1, IL2) are marked. The inset shows a SXDM map with the particular measured single islands pointed at by arrows. On panels (b,d), the vertical dotted line is the CTR, the continuous lines represent the relaxation lines. The insets show SEM images of the very same islands. (e,f) Ge concentration distribution in the islands (from FEM simulations).

 

The approach presented here combines local resolution with strain and composition sensitivity. The method is non-destructive, depth-sensitive (works both for free surfaces and buried structures) and requires no particular sample preparation. It also opens the possibility of combining X-ray diffraction and complementary microprobe techniques (atomic-force microscopy, SEM, micro-photo-luminescence, near-field microscopies) on exactly the same micro- or nanostructure. In this example, differences between the morphology, strain and chemical composition of individual objects (truncated and flat pyramids) are clearly correlated and resolved, reaching sensitivities hardly achievable by other techniques, and particularly not by ensemble averaging measurements.

 

References

[1] M. Schmidbauer, X-ray Diffuse Scattering from Self-organized Mesoscopic Semiconductor Structures, Springer Tracts in Modern Physics 199, Springer Berlin Heidelberg (2004).
[2] C.G. Schroer et al., Phys. Rev. Lett. 94, 054802 (2005); Appl. Phys. Lett. 87, 124103 (2005).

Authors

C. Mocuta (a), J. Stangl (b), K. Mundboth (a,b), T.H. Metzger (a), G. Bauer (b), I. Vartanyants (c), M. Schmidbauer (d), T. Boeck (d).
(a) ESRF
(b) Institut fuer Halbleiter- und Festkoerperphysik, Johannes Kepler Universitaet Linz (Austria)
(c) Hasylab at DESY, Hamburg (Germany)
(d) Institute for Crystal Growth, Berlin (Germany)