Mario Keplinger

Unit Cell Structure of Crystal Polytypes in InAs and InSb Nanowires

The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.

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.

Coherence and wavefront characterization of Si-111 monochromators using double-grating interferometry

A study of the coherence and wavefront properties of a pseudo-channel-cut monochromator in comparison with a double-crystal monochromator is presented. Using a double-grating interferometer designed for the hard X-ray regime, the complex coherence factor was measured and the wavefront distortions at the sample position were analyzed. A transverse coherence length was found in the vertical direction that was a factor of two larger for the channel-cut monochromator owing to its higher mechanical stability. The wavefront distortions after different optical elements in the beam, such as monochromators and mirrors, were also quantified. This work is particularly relevant for coherent diffraction imaging experiments with synchrotron sources.

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.

Strain distribution in single, suspended germanium nanowires studied using nanofocused x-rays.

Within the quest for direct band-gap group IV materials, strain engineering in germanium is one promising route. We present a study of the strain distribution in single, suspended germanium nanowires using nanofocused synchrotron radiation. Evaluating the probed Bragg reflection for different illumination positions along the nanowire length results in corresponding strain components as well as the nanowires tilting and bending. By using these findings we determined the complete strain state with the help of finite element modelling. The resulting information provides us with the possibility of evaluating the validity of the strain investigations following from Raman scattering experiments which are based on the assumption of purely uniaxial strain.

InAs film grown on Si(111) by Metalorganic Vapor Phase Epitaxy

We report the successful growth of high quality InAs films directly on Si( 111) by Metal Organic Vapor Phase Epitaxy. A nearly mirror-like and uniform InAs film is obtained at 580 C for a thickness of 2 mu m. We measured a high value of the electron mobility of 5100 cm(2)/Vs at room temperature. The growth is performed using a standard two-step procedure. The influence of the nucleation layer, group V flow rate, and layer thickness on the electrical and morphological properties of the InAs film have been investigated. We present results of our studies by Atomic Force Microscopy, Scanning Electron Microscopy, electrical Hall/van der Pauw and structural X-Ray Diffraction characterization.

X-ray diffraction strain analysis of a single axial InAs1–xPx nanowire segment

Strain analysis of an axial InAs1–xPx hetero-segment in an InAs nanowire using nano-focused X-ray diffraction is presented.