Andrei Y. Nikulin

Experimental characterization of the coherence properties of hard x-ray sources.

The experimental characterization of the coherence properties of hard X-ray sources is reported and discussed. The source is described by its Mutual Optical Intensity (MOI). The coherent-mode decomposition is applied to the MOI described by a Gaussian-Schell model. The method allows for a direct, quantitative characterization of the degree of coherence of both synchrotron and laboratory sources. The latter represents the first example of characterizing a low coherence hard x-ray source.

Atomistic structure of SiO2∕Si∕SiO2 quantum wells with an apparently crystalline silicon oxide

Thermal oxidation of a silicon-on-insulator substrate produces evidence that an ordered SiO2 structure can exist on thermally oxidized SiO2–Si interfaces. An apparently ordered SiO2 layer was observed by a high-resolution transmission electron microscope (HRTEM) when a thin silicon layer enclosed by SiO2 was less than 3.0nm thick. X-ray diffraction of the ultrathin Si (<3nm) samples showed diffractions from an ordered SiO2 phase, first-order Bragg reflection peaks with a lattice spacing of 4.1±0.15A, and second-order Bragg reflection peaks with 2.03±0.15A, in addition to the peaks from the Si substrate and the thin Si layer. Even in samples with thick Si layers enclosed by SiO2, which did not show the apparently ordered silicon oxide layer by the HRTEM, x-ray results showed a weak diffraction as if from a crystalline silicon oxide. The disappearance of the second-order Bragg reflection at higher energies indicates that the lattice structure of any crystalline SiO2 phase is far from perfect.

X-ray diffraction imaging of Al2O3 nanoparticles embedded in an amorphous matrix

An experimental “momentum transfer” x-ray diffraction imaging technique is suggested and tested for nondestructive determination of the shape and size of Al2O3 nanoscale particles embedded in an amorphous polymer matrix. An advantage of the proposed technique is that it does not require coherent x rays or a beamstop to block the direct beam. A two-dimensional image of an average Al2O3 nanoparticle has been reconstructed using the Gerchberg–Saxton method, with a spatial resolution of 2.5 nm. The shape of the resulting image yielded the average size of the nanoparticle to be approximately 50 nm, which is in excellent agreement with the expected value.

X-ray evaluation of microroughness of mechanochemically polished silicon surfaces

Microroughness of mechanochemically polished (100) silicon surfaces has been determined from X-ray reflectivity data. An optical interferometer showed equally smooth surfaces (3~4 A rms) for all samples examined. X-ray Fresnel reflectivity data revealed distinct decay features of the scattering profiles from samples polished using different relative strengths of mechanical and chemical factors. Profile fits were used to evaluate the Gaussian rms roughness at 11~12 A under oxide layers of a density close to 2.0 g/cm3. A stoichiometry not far from that of SiO2 was found for the overlayer oxide.

Radiation-induced melting in coherent X-ray diffractive imaging at the nanoscale

Coherent X-ray diffraction techniques play an increasingly significant role in imaging nanoscale structures which range from metallic and semiconductor samples to biological objects. The conventional knowledge about radiation damage effects caused by ever higher brilliance X-ray sources has to be critically revised while studying nanostructured materials.

Application of the Phase‐Retrieval X‐Ray Diffractometry to an Ultra‐High Spatial Resolution Mapping of SiGe Films near the Absorption Edge of Ge

A recently developed new experimental analytical X-ray diffraction method for the direct non-destructive characterization of single-crystal alloys is applied to map the complex structure-factor of SiGe layers with an ultra-high spatial resolution of 5.8 A. The technique is based on analytical measurements of X-ray phase and amplitude changes in a narrow polychromatic region of synchrotron radiation near the absorption edge of the alloy impurity. These atomic spatial resolution studies have allowed observation and preliminary analysis of surface and interface nanoscale sublayers, where the crystal structure-factor may noticeably differ from the bulk material.

Early detection of nanoparticle growth from x-ray reciprocal space mapping

The potential for nondestructive in situ detection of the formation of weakly diffracting nanoparticles has been confirmed by a combination of experiment and simulation. A triple axis diffractometer was used to collect two-dimensional reciprocal space maps of diffracted synchrotron x-rays from nanoscale Al2Cu precipitates embedded in a bulk metallic matrix. The appearance and asymmetric profile of the monochromator pseudostreaks are demonstrated to be indicative of the sensitivity of the technique to both the presence and orientation of the nanoparticles. This is a fundamental step toward in situ detection of sparsely dispersed, embedded nanoparticles and to quantitative temporal studies of particle number, scale, and dispersion.

Columnar structure in porous silicon: influence of etching time on pore dynamics and ordering.

Applications of porous silicon are ranging from drug delivery vehicles to micro fuel cells. The size of the pores and their distribution plays critical role in the final properties of the devices manufactured on their base. We performed nondestructive quantitative experimental studies of selected porous silicon samples with gradient porosity. We were able to determine the average size of the pores and its dynamics as a function of the etching time. We also were able to determine the statistical parameters of the pore formation.

Real-time in situ nanoclustering during initial stages of artificial aging of Al–Cu alloys

We report an experimental demonstration of real-time in situ x-ray diffraction investigations of clustering and dynamic strain in early stages of nanoparticle growth in Al–Cu alloys. Simulations involving a simplified model of local strain are well correlated with the x-ray diffraction data, suggesting a redistribution of point defects and the formation of nanoscale clusters in the bulk material. A modal, representative nanoparticle size is determined subsequent to the final stage of artificial aging. Such investigations are imperative for the understanding, and ultimately the control, of nanoparticle nucleation and growth in this technologically important alloy.

X-ray diffraction profiling of metal-metal interfaces at the nanoscale

Several samples containing interfaces between dissimilar metals were examined using diffraction of synchrotron radiation. The complex refractive index profile in the vicinity of the interface for each sample was reconstructed with spatial resolution of about 40 nm by the phase retrieval x-ray diffractometry technique. A series of computer simulations related to the analysis of various configurations of interfaces between dissimilar materials were performed. A practical algorithm of experimental data collection for a detailed examination of internal interfaces was suggested. An estimation of the minimal size of the interface structure modulations, which can be analyzed by the phase retrieval x-ray diffractometry technique, was suggested from the results of computer simulations. It was shown that interface modulations of about 50-100 nm in bimetals can be readily reconstructed by the technique.