E. Wintersberger

Phonon strain shift coefficients in Si1−xGex alloys

A comprehensive study of the biaxial strain-induced shift of the Si1−xGex Raman active phonon modes is presented. High-resolution Raman measurements of Si1−xGex/Si heterostructures have been compared to x-ray diffraction data. Our approach, unlike previous works, is effective to decouple and quantify separately the effect of strain and composition on the phonon frequencies, yielding an accurate determination of the phonon strain shift coefficients in the entire composition range. Our results show that the strain shift coefficients are independent of the composition, a result which is in good agreement with theoretical calculations, performed within the framework of valence force-field theory.

X-ray Nanodiffraction on a Single SiGe Quantum Dot inside a Functioning Field-Effect Transistor

For advanced electronic, optoelectronic, or mechanical nanoscale devices a detailed understanding of their structural properties and in particular the strain state within their active region is of utmost importance. We demonstrate that X-ray nanodiffraction represents an excellent tool to investigate the internal structure of such devices in a nondestructive way by using a focused synchotron X-ray beam with a diameter of 400 nm. We show results on the strain fields in and around a single SiGe island, which serves as stressor for the Si-channel in a fully functioning Si–metal–oxide semiconductor field-effect transistor.

xrayutilities: a versatile tool for reciprocal space conversion of scattering data recorded with linear and area detectors

Algorithms for the reciprocal space conversion of linear and area detectors are implemented in an open-source Python package.

Structural Investigations of Core-shell Nanowires Using Grazing Incidence X-ray Diffraction.

The fabrication of core-shell structures is crucial for many nanowire device concepts. For the proper tailoring of their electronic properties, control of structural parameters such as shape, size, diameter of core and shell, their chemical composition, and information on their strain fields is mandatory. Using synchrotron X-ray diffraction studies and finite element simulations, we determined the chemical composition, dimensions, and strain distribution for series of InAs/InAsP core-shell wires grown on Si(111) with systematically varied growth parameters. In particular we detect initiation of plastic relaxation of these structures with increasing shell thickness and/or increasing phosphorus content. We establish a phase diagram, defining the region of parameters leading to pseudomorphic nanowire growth. This is important to avoid extended defects which are detrimental for their electronic properties.

The NeXus data format

A description is presented of the NeXus data format for X-ray and neutron scattering and muon spectroscopy.

Unit cell parameters of wurtzite InP nanowires determined by x-ray diffraction

High resolution x-ray diffraction is used to study the structural properties of the wurtzite polytype of InP nanowires. Wurtzite InP nanowires are grown by metal-organic vapor phase epitaxy using S-doping. From the evaluation of the Bragg peak position we determine the lattice parameters of the wurtzite InP nanowires. The unit cell dimensions are found to differ from the ones expected from geometric conversion of the cubic bulk InP lattice constant. The atomic distances along the c direction are increased whereas the atomic spacing in the a direction is reduced in comparison to the corresponding distances in the zinc-blende phase. Using core/shell nanowires with a thin core and thick nominally intrinsic shells we are able to determine the lattice parameters of wurtzite InP with a negligible influence of the S-doping due to the much larger volume in the shell. The determined material properties will enable the ab initio calculation of electronic and optical properties of wurtzite InP nanowires.

In situ investigation of the island nucleation of Ge on Si(001) using x-ray scattering methods

The growth of Ge on Si(001) is investigated in situ at 500 and 600°C, combining grazing incidence diffraction, multiple wavelength anomalous diffraction, and small angle scattering. This allows probing simultaneously the island shape, strain state, composition, and the transition from wetting layer to island growth. At 500°C no intermixing occurs. The wetting layer is found to decrease by one atomic layer at the onset of island nucleation. At 600°C interdiffusion plays an important role in strain relaxation leading to a more stable wetting layer. Small angle scattering yields the island morphology and shows the transition from pyramids to multifacetted domes.

Analysis of periodic dislocation networks using x-ray diffraction and extended finite element modeling

We combine the extended finite element method with simulations of diffracted x-ray intensities to investigate the diffusely scattered intensity due to dislocations. As a model system a thin PbSe epitaxial layer grown on top of a PbTe buffer on a CdTe substrate was chosen. The PbSe film shows a periodic dislocation network where the dislocations run along the orthogonal ⟨110⟩ directions. The array of dislocations within this layer can be described by a short range order model with a narrow distribution.

Self-assembled Si0.80Ge0.20 nanoripples on Si(1 1 10) substrates

Si0.8Ge0.2 heteroepitaxy on vicinal Si(1 1 10) substrates leads to the formation of a nanoscale ripple morphology. Atomic force microscopy, and grazing incidence small angle x-ray scattering reveal that these SiGe structures are essentially prisms of triangular cross section bounded by two adjacent {105} facets. Transmission electron microscopy shows the existence of a wetting layer. X-ray diffraction in combination with finite element simulations was performed to extract strain distribution maps. The stabilization of the prism structure is attributed to the strain-dependence of the {105} surface energy.

3D SiGe QUANTUM DOT CRYSTALS: STRUCTURAL CHARACTERIZATION AND ELECTRONIC COUPLING

We report on the growth of SiGe quantum dot crystals which are realized by depositing Ge on a two-dimensionally pit-patterned Si substrate and subsequent growth of Si spacer and Ge island layers. Lateral periods of 100 nm are obtained by employing deep UV lithography using synchrotron radiation. The vertical period of the typically 10 period dot superlattices was of the order of 10 nm. Ordering of the islands was investigated by atomic force microscopy as well as by high resolution x-ray diffraction studies. From the quantitative evaluation of the x-ray diffraction data a mean Ge content of about 60% in the quantum dots was obtained and an rms. deviation from ideal lattice sites of about 3 nm was found. A simulation of the eigenenergies based on the nextnano3 simulation package was used to interpret the measured photoluminescence data.