Mojmír Meduňa

Comparative study of multilayers used in monochromators for synchrotron-based coherent hard X-ray imaging

A systematic study is presented in which multilayers of different composition (W/Si, Mo/Si, Pd/B(4)C), periodicity (from 2.5 to 5.5 nm) and number of layers have been characterized. In particular, the intrinsic quality (roughness and reflectivity) as well as the performance (homogeneity and coherence of the outgoing beam) as a monochromator for synchrotron radiation hard X-ray micro-imaging are investigated. The results indicate that the material composition is the dominating factor for the performance. By helping scientists and engineers specify the design parameters of multilayer monochromators, these results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal-based devices; i.e. larger spectral bandwidth and high photon flux density, which are particularly useful for synchrotron-based micro-radiography and -tomography.

Perfect crystals grown from imperfect interfaces

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.

Highly Mismatched, Dislocation-Free SiGe/Si Heterostructures

Defect-free mismatched heterostructures on Si substrates are produced by an innovative strategy. The strain relaxation is engineered to occur elastically rather than plastically by combining suitable substrate patterning and vertical crystal growth with compositional grading. Its validity is proven both experimentally and theoretically for the pivotal case of SiGe/Si(001).

GaAs/Ge crystals grown on Si substrates patterned down to the micron scale

Monolithic integration of III-V compounds into high density Si integrated circuits is a key technological challenge for the next generation of optoelectronic devices. In this work, we report on the metal organic vapor phase epitaxy growth of strain-free GaAs crystals on Si substrates patterned down to the micron scale. The differences in thermal expansion coefficient and lattice parameter are adapted by a 2-μm-thick intermediate Ge layer grown by low-energy plasma enhanced chemical vapor deposition. The GaAs crystals evolve during growth towards a pyramidal shape, with lateral facets composed of {111} planes and an apex formed by {137} and (001) surfaces. The influence of the anisotropic GaAs growth kinetics on the final morphology is highlighted by means of scanning and transmission electron microscopy measurements. The effect of the Si pattern geometry, substrate orientation, and crystal aspect ratio on the GaAs structural properties was investigated by means of high resolution X-ray diffraction. The th...

Intersubband absorption of strain-compensated Si1−xGex valence-band quantum wells with 0.7⩽x⩽0.85

Strain-compensated, p-type SiGe quantum wells with a high Ge concentration of up to 85% have been grown on commercially available Si0.5Ge0.5 pseudosubstrates by molecular-beam epitaxy. Structural investigations by transmission electron microscopy and high-resolution x-ray reflection and diffraction showed that at a growth temperature around T=300°C, samples in excellent compliance with the design parameters, comparatively sharp interfaces, and negligible increase of growth-induced surface roughness can be grown. Comparison of polarization-dependent intersubband absorption measurements with simulated intersubband absorption spectra shows that for the quantum wells investigated in this work, the hole eigenstates, their in-plane dispersion, and the polarization-dependent intersubband transition matrix elements are accurately described by a strain-dependent, six-band k∙p Luttinger-Kohn Hamiltonian in which only one fitting parameter—the intersubband transition linewidth—is used.

X-ray reflectivity of self-assembled structures in SiGe multilayers and comparison with atomic force microscopy

We have studied the interface morphology of SiGe/Si multilayers by means of specular and nonspecular x-ray reflectivity under grazing incidence. The samples were grown by molecular beam epitaxy on silicon substrates with (001) surface orientation and with different directions of the surface misorientation. X-ray reflectivity measurements in different azimuths are compared to data from atomic force microscopy, which are used to simulate the x-ray experiments. With this combination of experimental techniques we have determined the structural properties, in particular the ordering of different features present at the sample surface and inside the multilayer at the SiGe/Si layer interfaces.

Three-dimensional Ge/SiGe multiple quantum wells deposited on Si(001) and Si(111) patterned substrates

In this work we address three-dimensional heterojunctions, demonstrating that photoluminescence from defect-free, Ge/SiGe multiple quantum well (MQW) micro-crystals grown on deeply patterned Si(001) and Si(111) substrates exhibit similar radiative intensity and analogous spectral shape.

Reconstruction of crystal shapes by X‐ray nanodiffraction from three‐dimensional superlattices

Quantitative nondestructive imaging of structural properties of semiconductor layer stacks at the nanoscale is essential for tailoring the device characteristics of many low-dimensional quantum structures, such as ultrafast transistors, solid state lasers and detectors. Here it is shown that scanning nanodiffraction of synchrotron X-ray radiation can unravel the three-dimensional structure of epitaxial crystals containing a periodic superlattice underneath their faceted surface. By mapping reciprocal space in all three dimensions, the superlattice period is determined across the various crystal facets and the very high crystalline quality of the structures is demonstrated. It is shown that the presence of the superlattice allows the reconstruction of the crystal shape without the need of any structural model.

Stacking Fault Analysis of Epitaxial 3C-SiC on Si(001) Ridges

The stacking faults (SFs) in 3C-SiC epitaxially grown on ridges deeply etched into Si (001) substrates offcut towards [110] were quantitatively analyzed by electron microscopy and X-ray diffraction. A significant reduction of SF density with respect to planar material was observed for the {111} planes parallel to the ridges. The highest SF density was found in the (-1-11) plane. A previously observed defect was identified as twins by electron backscatter diffraction.

Lattice bending in three-dimensional Ge microcrystals studied by X-ray nanodiffraction and modelling

Extending the functionality of ubiquitous Si-based microelectronic devices often requires combining materials with different lattice parameters and thermal expansion coefficients. In this paper, scanning X-ray nanodiffraction is used to map the lattice bending produced by thermal strain relaxation in heteroepitaxial Ge microcrystals of various heights grown on high aspect ratio Si pillars. The local crystal lattice tilt and curvature are obtained from experimental three-dimensional reciprocal space maps and compared with diffraction patterns simulated by means of the finite element method. The simulations are in good agreement with the experimental data for various positions of the focused X-ray beam inside a Ge microcrystal. Both experiment and simulations reveal that the crystal lattice bending induced by thermal strain relaxation vanishes with increasing Ge crystal height.