Growth, structure and morphology of epitaxial Fe(0 0 1) films on GaAs(0 0 1)c(4 × 4).

We studied epitaxy, growth, structure and morphology of thin Fe(0 0 1) films on the As-rich GaAs(0 0 1)-c(4 × 4) surface, deposited by molecular beam epitaxy at growth temperatures between room temperature and 250° C. Electron and x-ray diffraction (XRD) techniques evidence epitaxial growth with Fe(0 0 1)[1 0 0] ∥ GaAs(0 0 1)[1 0 0]. The residual strain derived from the XRD results is consistent with recent stress measurements. Cross-sectional transmission electron microscopy reveals an abrupt interface for room-temperature films and the formation of a ∼10 nm thick crystalline Fe-Ga-As intermediate layer at 250° C. The dependence of the surface morphology on growth temperature and annealing evidences a kinetic roughening of the Fe surface at growth temperatures of 100-200° C due to the presence of step-edge barriers.

Structural investigation of GaInP nanowires using X-ray diffraction.

In this work the structure of ternary Ga x In1 - x P nanowires is investigated with respect to the chemical composition and homogeneity. The nanowires were grown by metal-organic vapor-phase epitaxy. For the investigation of ensemble fluctuations on several lateral length scales, X-ray diffraction reciprocal space maps have been analyzed. The data reveal a complicated varying materials composition across the sample and in the nanowires on the order of 20%. The use of modern synchrotron sources, where beam-sizes in the order of several 10 µm are available, enables us to investigate compositional gradients along the sample by recording diffraction patterns at different positions. In addition, compositional variations were found also within single nanowires in X-ray energy dispersive spectroscopy measurements.

Strain relief and shape oscillations in site-controlled coherent SiGe islands.

Strain engineering and the crystalline quality of semiconductor nanostructures are important issues for electronic and optoelectronic devices. We report on defect-free SiGe island arrays resulting from Ge coverages of up to 38 monolayers grown on prepatterned Si(001) substrates. This represents a significant expansion of the parameter space known for the growth of perfect island arrays. A cyclic development of the Ge content and island shape was observed while increasing the Ge coverage. Synchrotron-based x-ray diffraction experiments and finite element method calculations allow us to study the strain behavior of such islands in great detail. In contrast to the oscillatory changes of island shape and average Ge content, the overall strain behavior of these islands exhibits a clear monotonic trend of progressive strain relaxation with increasing Ge coverage.

Strain distribution in Si capping layers on SiGe islands: influence of cap thickness and footprint in reciprocal space.

We present investigations on the strain properties of silicon capping layers on top of regular SiGe island arrays, in dependence on the Si-layer thickness. Such island arrays are used as stressors for the active channel in field-effect transistors where the desired tensile strain in the Si channel is a crucial parameter for the performance of the device. The thickness of the Si cap was varied from 0 to 30 nm. The results of high resolution x-ray diffraction experiments served as input to perform detailed strain calculations via finite element method models. Thus, detailed information on the Ge distribution within the buried islands and the strain interaction between the SiGe island and Si cap was obtained. It was found that the tensile strain within the Si capping layer strongly depends on its thickness, even if the Ge concentration of the buried dot remains unchanged, with tensile strains degrading if thicker Si layers are used.

Particle-assisted Ga(x)In(1-x)P nanowire growth for designed bandgap structures.

Non-tapered vertically straight Ga(x)In(1-x)P nanowires were grown in a compositional range from Ga(0.2)In(0.8)P to pure GaP in particle-assisted mode by controlling the trimethylindium, trimethylgallium and hydrogen chloride flows in metal-organic vapor phase epitaxy. X-ray energy dispersive spectroscopy in transmission electron microscopy revealed homogeneous radial material composition in single nanowires, whereas variations in the material composition were found along the nanowires. High-resolution x-ray diffraction indicates a variation of the material composition on the order of about 19% measuring an entire sample area, i.e., including edge effects during growth. The non-capped nanowires emit room temperature photoluminescence strongly in the energy range of 1.43-2.16 eV, correlated with the bandgap expected from the material composition.

Three-dimensional high-resolution quantitative microscopy of extended crystals.

Hard X-ray lens-less microscopy raises hopes for a non-invasive quantitative imaging, capable of achieving the extreme resolving power demands of nanoscience. However, a limit imposed by the partial coherence of third generation synchrotron sources restricts the sample size to the micrometer range. Recently, X-ray ptychography has been demonstrated as a solution for arbitrarily extending the field of view without degrading the resolution. Here we show that ptychography, applied in the Bragg geometry, opens new perspectives for crystalline imaging. The spatial dependence of the three-dimensional Bragg peak intensity is mapped and the entire data subsequently inverted with a Bragg-adapted phase retrieval ptychographical algorithm. We report on the image obtained from an extended crystalline sample, nanostructured from a silicon-on-insulator substrate. The possibility to retrieve, without transverse size restriction, the highly resolved three-dimensional density and displacement field will allow for the unprecedented investigation of a wide variety of crystalline materials, ranging from life science to microelectronics.

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.

Stress and interdiffusion during molecular beam epitaxy of Fe on As-rich GaAs(001).

Interdiffusion during growth of Fe on As-rich GaAs(001) substrates has been investigated by real time stress measurements. Compared to Ga-rich GaAs(001), interdiffusion processes are decisively reduced. The optimum growth temperature (characterized by abrupt interfaces, pseudomorphic growth and negligible intermixing) is found to lie below 50 °C. At higher growth temperatures interdiffusion effects increase and eventually lead to the formation of a compact crystalline alloy layer of presumably Fe(2 + x)Ga(1 - x), as evidenced by transmission electron microscopy and x-ray diffraction.

Three-dimensional x-ray Fourier transform holography: the Bragg case.

A novel approach to determine the structure of nanoscale crystals in three dimensions is proposed by the use of coherent x-ray Fourier transform holography in Bragg geometry. The full internal description is directly obtained by a single Fourier transform of the 3D intensity hologram. Together with the morphology, Bragg geometry gives access to the 3D displacement field within the crystal. This result opens great possibilities for the investigation of strain fields inside nanocrystals in a simple way.

Collective shape oscillations of SiGe islands on pit-patterned Si(001) substrates: a coherent-growth strategy enabled by self-regulated intermixing.

The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays very uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower "dome" and steeper "barn" morphologies. Correspondingly, the average Ge content in the alloyed islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the islands to keep growing in size while avoiding plastic relaxation.

X-ray diffraction as a local probe tool.

For the structural characterization of nanoscale objects, X-ray diffraction is widely used as a technique complementing local probe analysis methods such as scanning electron microscopy and transmission electron microscopy. Details on strain distributions, chemical composition, or size and shape of nanostructures are addressed. X-ray diffraction traditionally obtains very good statistically averaged properties over large ensembles-provided this averaging is meaningful for ensembles with sufficiently small dispersion of properties. In many cases, however, it is desirable to combine different analysis techniques on exactly the same nano-object, for example, to gain a more detailed insight into the interdependence of properties. X-ray beams focused to diameters in the sub-micron range, which are available at third-generation synchrotron sources, allow for such X-ray diffraction studies of individual nano-objects.

Strain induced changes in the magnetic phase diagram of metamagnetic heteroepitaxial EuSe/PbSe(1-x)Tex multilayers.

Multilayers of strained metamagnetic EuSe intercalated with nonmagnetic PbSe1-xTex were grown by molecular beam epitaxy under conditions optimized by electron diffraction. From detailed structural and magnetic characterization using anomalous synchrotron x-ray diffraction and magnetization measurements, the phase transition temperatures and the magnetic phase diagrams of strained EuSe as a function of the in-plane lattice constant are determined. In this way, it is demonstrated that the magnetic properties of the samples can be significantly changed by applying biaxial strain on EuSe in superlattice structures.

[The frequency-doubled double-pulse Neodym:YAG laser lithotripter (FREDDY) in lithotripsy of urinary stones. First clinical experience]. / Der frequenzverdoppelte Doppelpuls-Neodym:YAG-Laser (FREDDY) bei Urolithiasis. Erste klinische Erfahrungen.

Laser lithotripsy does not play an important role in urinary stone treatment, mostly due to ineffective fragmentation efficiency, and high purchase and maintenance costs. The aim of the following retrospective study was to show the clinical significance and efficiency of an innovative laser lithotripsy system for urinary stone treatment. Between November 1998 and October 1999, 48 patients were treated with the innovative frequency- doubled double-pulse Neodym: YAG laser lithotripter FREDDY. A total of 50 renal units were treated, 43 ureteroscopically, four ureterorenoscopically, three percutaneous-nephroscopically, and one bladder stone cystoscopically. With a median laser operation time of 5 min (range: 1-30 min) and a total procedure duration of 60 min (range: 15-180 min), a stone-free rate of upper ureteral stones of 62%, middle ureteral stones of 91% and distal ureteral stones of 100% were documented on the first day after treatment. In an observation period of 6 months, no complications were seen. In our experience Laser lithotripsy with FREDDY is an effective, simple and reliable method for the treatment of ureteral stones, with low purchase and maintenance costs. The extremely thin and highly flexible quartz fibre may extend the endoscopic spectrum to otherwise poorly accessible upper ureteral stones, the renal pelvis and renal calix stones. Therefore, a prospective validation study for comparison with ballistic lithotriptors is of great interest.

Direct determination of strain and composition profiles in SiGe islands by anomalous x-Ray diffraction at high momentum transfer.

Anomalous x-ray scattering is employed for quantitative measurements of the Ge composition profile in islands on Si(001). The anomalous effect in SiGe is enhanced exploiting the dependence of the complex atomic form factors on the momentum transfer. Comparing the intensity ratios for x-ray energies below and close to the K edge of Ge at various Bragg reflections in the grazing incidence diffraction setup, the sensitivity for the Ge profile is considerably enhanced. The method is demonstrated for SiGe dome-shaped islands grown on Si(001). It is found that the composition inside the island changes rather abruptly, whereas the lattice parameter relaxes continuously.

Nanometer-scale resolution of strain and interdiffusion in self-assembled InAs/GaAs quantum dots

Tomographic nanometer-scale images of self-assembled InAs/GaAs quantum dots have been obtained from surface-sensitive x-ray diffraction. Based on the three-dimensional intensity mapping of selected regions in reciprocal space, the method yields the shape of the dots along with the lattice parameter distribution and the vertical interdiffusion profile on a subnanometer scale. The material composition is found to vary continuously from GaAs at the base of the dot to InAs at the top.

Metastability of Si1-yCy epilayers under 2 MeV alpha-particle irradiation

In this work we present some recent results concerning the alpha-particles irradiation of Si1-yCy alloy epitaxially grown on silicon. The study of the damage process is interesting because of the extensive use of backscattering technique as a tool of characterisation of this kind of materials and because of the possibility of adding information about the transformations that this metastable material undergoes. We point out that the irradiation damage process causes a change in the material structure different from that due to the thermal treatments. The irradiation damage occurs at a rate much higher than in Si, however it involves only a silicon atom fraction that appears to be proportional to the substitutional carbon content.

Environmental influences on self-stimulatory behavior.

Effects of passive environmental conditions (quiet, radio, and television) and manipulable objects (no toys, toys, toys plus staff interaction) on stereotypic behavior were evaluated. Four institutionalized, mentally retarded residents with high rates of stereotypic behavior were observed in a multipurpose room of their cottage. Results showed statistically significant lower levels of stereotypic behavior under the quiet and music conditions as compared to the television condition. There were no significant differences between the three manipulable-object conditions.