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We investigate the strain state of ensembles of thin and nearly coalescence-free self-assembled GaN nanowires prepared by plasma-assisted molecular beam epitaxy on Ti/Al2O3(0001) substrates. The shifts of Bragg peaks in high-resolution X-ray diffraction profiles reveal the presence of a homogeneous tensile strain in the out-of-plane direction. This strain is inversely proportional to the average nanowire radius and results from the surface stress acting on the nanowire sidewalls. The superposition of strain from nanowires with different radii in the same ensemble results in a broadening of the Bragg peaks that mimics an inhomogeneous strain on a macroscopic scale. The nanowire ensembles show a small blueshift of the bound-exciton transitions in photoluminescence spectra, reflecting the existence of a compensating in-plane compressive strain, as further supported by grazing incidence X-ray diffraction measurements carried out at a synchrotron. By combining X-ray diffraction and photoluminescence spectroscopy, the surface stress components fx and fz of the air-exposed GaN { 1 1 ¯ 00 } planes that constitute the nanowire sidewalls are determined experimentally to be 2.25 and − 0.7 N/m, respectively.

We study the cross-sectional shape of GaN nanowires (NWs) by transmission electron microscopy. The shape is examined at different heights of long NWs, as well as at the same height for NWs of different lengths. Two distinct trends in the evolution of the cross-sectional shape along the NW length are observed. At the top, merging NWs develop common {11̄00} side facets. At the bottom, the NWs acquire roundish shapes. This observation is explained by the entirely different NW environments at the top and the bottom of the NWs. At the top, NWs are exposed to the Ga and N atomic fluxes giving rise to axial growth, resulting in the equilibrium growth shape with zero growth rate at the {11̄00} facets. At the bottom, NWs are shadowed from the impinging fluxes and are only annealed, allowing them to eventually approach the equilibrium crystal shape. The study of identical samples by grazing incidence small-angle X-ray scattering independently confirms these trends in the shape evolution of the sidewall facets.

The intensity of small-angle X-ray scattering from GaN nanowires on Si(111) depends on the orientation of the side facets with respect to the incident beam. This reminiscence of truncation rod scattering gives rise to a deviation from Porod’s law. A roughness of just 3–4 atomic steps per micrometre-long side facet notably changes the intensity curves., Small-angle X-ray scattering from GaN nanowires grown on Si(111) is measured in the grazing-incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod’s law I(q) ∝ q −4. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavevectors q, the scattering intensity is reduced by surface roughness. A root-mean-square roughness of 0.9 nm, which is the height of just 3–4 atomic steps per micrometre-long facet, already gives rise to a strong intensity reduction.

We investigate the nucleation, growth, and coalescence of spontaneously formed GaN nanowires in molecular beam epitaxy combining the statistical analysis of scanning electron micrographs with Monte Carlo growth models. We find that (i) the nanowire density is limited by the shadowing of the substrate from the impinging fluxes by already existing nanowires, (ii) shortly after the nucleation stage, nanowire radial growth becomes negligible, and (iii) coalescence is caused by bundling of nanowires. The latter phenomenon is driven by the gain of surface energy at the expense of the elastic energy of bending and becomes energetically favorable once the nanowires exceed a certain critical length.

We investigate in detail the self-assembled nucleation and growth of GaN nanowires by molecular beam epitaxy on crystalline TiN films. We demonstrate that this type of substrate allows the growth of long and thin GaN nanowires that do not suffer from coalescence, which is in contrast to the growth on Si and other substrates. Only beyond a certain nanowire length that depends on the nanowire number density and exceeds here 1.5 {\mu}m, coalescence takes place by bundling, i.e. the same process as on Si. By analyzing the nearest neighbor distance distribution, we identify diffusion-induced repulsion of neighboring nanowires as the main mechanism limiting the nanowire number density during nucleation on TiN. Since on Si the final number density is determined by shadowing of the impinging molecular beams by existing nanowires, it is the difference in adatom surface diffusion that enables on TiN the formation of nanowire ensembles with reduced number density. These nanowire ensembles combine properties that make them a promising basis for the growth of core-shell heterostructures., Comment: The final publication is available at link.springer.com

To describe the annihilation of spatially separate electrons and holes in a disordered semiconductor, we suggest the use of a model based on the spatially inhomogeneous, nonlinear Smoluchowski equations with random initial distribution density. Furthermore, we present a Monte Carlo algorithm for solving this equation. Our approach provides a general method for the computation of the electron-hole kinetics in inhomogeneous media, taking into account both their radiative and nonradiative recombination by tunneling as well as their diffusion. To validate the simulation algorithm, we compare our model with a more general approach based on a statistical description and the Kirkwood closure of the Bogoliubov–Born–Green–Kirkwood–Yvon hierarchy equations. A comparison with recent experimental results is also discussed.

We built the honeycomb networks of misfit dislocations by combining angular dislocations. Two different models of the disorder in the dislocation network are explored, the random shifts of nodes and random sources of distortion. The displacement fields of disturbed dislocation networks are used to simulate the X-ray diffraction patterns by Monte Carlo method. We find that substantial disorder is needed for random shifts to be experimentally observable as diffuse scattering. The model of random distortion sources produces comparable diffuse scattering with the dislocation network looking closer to the initial perfect structure. The diffraction patterns due to distorted honeycomb dislocation networks are compared with the ones due to triangular networks of straight dislocations. The disorder parameters of the latter networks can be chosen to produce diffraction patterns similar to the former.

Diffraction profiles for different models of dislocation arrangements are calculated directly by the Monte Carlo method and compared with the strain distributions for the same arrangements, which corresponds to the Stokes–Wilson approximation. It is shown that the strain distributions and the diffraction profiles are in close agreement as long as long-range order is absent. Analytical calculation of the strain distribution for uncorrelated defects is presented. For straight dislocations, the Stokes–Wilson and the Krivoglaz–Wilkens approximations give the same diffraction profiles, with the Gaussian central part and ∝q−3power law at the tails.

Single GaN nanowires formed spontaneously on a given substrate represent nanoscopic single crystals free of any extended defects. However, due to the high area density of thus formed GaN nanowire ensembles, individual nanowires coalesce with others in their immediate vicinity. This coalescence process may introduce strain and structural defects, foiling the idea of defect-free material due to the nanowire geometry. To investigate the consequences of this process, a quantitative measure of the coalescence of nanowire ensembles is required. We derive objective criteria to determine the coalescence degree of GaN nanowire ensembles. These criteria are based on the area-perimeter relationship of the cross-sectional shapes observed, and in particular on their circularity. Employing these criteria, we distinguish single nanowires from coalesced aggregates in an ensemble, determine the diameter distribution of both, and finally analyze the coalescence degree of nanowire ensembles with increasing fill factor.

The reciprocal space map of a heteroepitaxial single-crystal GaN(0001) film on 6H-SiC(0001) is analyzed. The film contains a high density of threading dislocations which intersect the film parallel to the surface normal. The strain field associated with these dislocations is expected to broaden all reciprocal lattice points to discs perpendicular to the dislocation lines,i.e.parallel to the surface. Experimentally, however, the reflection is observed to be broadened also perpendicular to the surface and is rotated towards the surface normal. Using Monte Carlo simulations, it is shown that both of these features are a natural consequence of the presence of misfit dislocations at the film/substrate interface.

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial InxGa1-xN quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the InxGa1-xN/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures.

The coalescence in dense arrays of spontaneously formed GaN nanowires proceeds by bundling: adjacent nanowires bend and merge at their top, thus reducing their surface energy at the expense of the elastic energy of bending. We give a theoretical description of the energetics of this bundling process. The bending energy is shown to be substantially reduced by the creation of dislocations at the coalescence joints. A comparison of experimental and calculated x-ray diffraction profiles from ensembles of bundled nanowires demonstrates that a large part of the bending energy is indeed relaxed by plastic deformation. The residual bending manifests itself by extended tails of the diffraction profiles.

The X-ray diffraction peak profiles due to misfit dislocations are studied for the dislocations correlated on distances much smaller than the film thickness. Simple analytical expressions for the correlation parameter are obtained when the dislocations are generated as a Markov chain. Monte Carlo calculations show the peak profile evolution as order in the dislocation positions increases.

Crystal truncation rods (CTRs) from thin Fe3Si films grown on GaAs(001) by molecular beam epitaxy (MBE) are measured at different stages of deposition. The films do not develop their own surface roughness but are conformal to the substrate, so that the substrate roughness governs the whole system. A factor that describes the roughness of a zinc blende structure in the β model of terrace height probabilities is derived and applied to describe the experimental curves. We show that the β model adequately describes the CTRs while the model of continuous Gaussian fluctuations of the surface height notably underestimates the root-mean-squared (rms) roughness.

The thin films of lattice matched materials Fe 3 Si and Ge are grown on GaAs(001) by molecular beam epitaxy and studied by grazing incidence X-ray diffraction. In the initial phase of Fe 3 Si growth, the surface is rough due to a formation of small islands. Two-dimensional layer-by-layer growth arises later on. Germanium grows on GaAs in a layer-by-layer growth mode right from the beginning. A substantial improvement of the surface smoothness of the Ge film during the growth is found.

We study the nucleation and growth kinetics on the Ge(0 0 1) surface at elevated temperatures using in situ surface X-ray diffraction. The time evolution of characteristic length scales on the surface is analyzed through the widths of the different components of the integer-order (morphology sensitive) and fractional-order (reconstruction sensitive) diffraction peaks. We find an activation energy of 0.58 eV for Ge island nucleation during homoepitaxy, which implies a diffusion activation energy higher than that obtained for both adatom and dimer diffusion on Ge(0 0 1) in previous studies. Sub-monolayer homoepitaxial Ge islands coarsen according to a power law, with a relatively low time exponent of n =0.2. The coarsening of small 2×1 reconstruction domains on a flat surface prepared by deposition of an integer number of layers shows a strong temperature dependence, whereby the coarsening exponent decreases from 0.41 to 0.2 as the temperature is increased.

We derive analytic expressions for the built-in electrostatic potential arising from piezo- and pyroelectricity in a cylindrical axial In(x)Ga(1-x)N/GaN nanowire (NW) heterostructure. Our simulations show that, for sufficiently thin NWs, a significant reduction of the built-in potential is reached in comparison to the planar heterostructure of the same In content, thickness, and orientation. This specific feature of axial NW heterostructures makes the aspect ratio of the embedded In(x)Ga(1-x)N/GaN disks an important additional degree of freedom to control the recombination energies. We furthermore show that the magnitude of the polarization potential decreases again above a certain value of the aspect ratio and that the extrema of the potential move from the central axis of the NW towards the side facets when the thickness of the disk is increased.

Molecular beam epitaxy of Fe3Si on GaAs(001) is studied in situ by grazing incidence x-ray diffraction. Layer-by-layer growth of Fe3Si films is observed at a low growth rate and substrate temperatures near 200 degrees Celsius. A damping of x-ray intensity oscillations due to a gradual surface roughening during growth is found. The corresponding sequence of coverages of the different terrace levels is obtained. The after-deposition surface recovery is very slow. Annealing at 310 degrees Celsius combined with the deposition of one monolayer of Fe3Si restores the surface to high perfection and minimal roughness. Our stoichiometric films possess long-range order and a high quality heteroepitaxial interface., 8 pages, 3 figures

The first order phase transition between ferromagnetic α-MnAs and paramagnetic β-MnAs in nanoscale disks etched out of an epitaxial MnAs film on GaAs(001) is studied by means of X-ray diffraction and magnetic force microscopy. The temperature hysteresis loop is observed to be expanded with decreasing disk size. The hysteresis is absent in the original continuous epitaxial film from which the disks are fabricated. The hysteresis is explained as a competition between the energy gain due to the phase transition and the energy loss due to the elastic strain.

We propose a common description of the full widths at half maximum of X-ray diffraction peaks obtained in different scans of triple-crystal diffractometry and as well as for glancing incidence and glancing exit double-crystal measurements. Calculations are compared with measurements of GaAs/Si(001) heteroepitaxial films. We show that the diffraction peak broadening is entirely due to random 60° misfit dislocations that provide only a minor part of the misfit relaxation. The remaining relaxation is due to periodic edge misfit dislocations that do not contribute to the peak broadening.

Molecular beam epitaxy of Fe 3 Si films on GaAs (001) is studied in situ by grazing incidence X-ray diffraction. Fe 3 Si grows layer-by-layer. During deposition the growth front roughens as indicated by the damping of the X-ray oscillations and corresponding atomic force micrographs. The X-ray oscillations are modified during growth at substrate temperatures of 180 °C and below.

MnSb deposited on GaSb(0 0 1) by molecular beam epitaxy (MBE) is found to separate into disconnected volumes penetrating into the substrate. The MnSb nanocrystals formed in this way are single crystals having a strongly preferred alignment with the substrate matrix and a flat top coplanar with the substrate surface. This endotaxial growth mode opens up new possibilities for self-assembled fabrication of thermodynamically stable micro- and nanostructures. It demonstrates that atoms deep inside the substrate can become mobile during MBE growth, allowing the investigation and design of new approaches to self-assembled growth that utilize bulk migration.

Using a combination of molecular beam epitaxy and in situ surface X-ray diffraction, we investigate the nucleation and coarsening of monolayer high islands on GaSb(0 0 1) during deposition in real time. We find an activation energy for island nucleation of 1.55 ± 0.16 eV, indicating a stable nucleus size larger than two atoms. For intermediate temperatures where GaSb homoepitaxy is stable, the lateral coarsening of the islands after deposition is described by Ostwald ripening. The average island sizes during coarsening are isotropic, although with different size distributions in different directions. The size distributions do not change during coarsening, implying kinetic scaling.

We present results on fabrication, and structural and electrical properties of single-crystal Fe 3 Si / GaAs ( 0 0 1 ) heterostructures grown by molecular beam epitaxy. The exact stoichiometry of the Heusler alloy films can be achieved for almost lattice matched films. As evidenced by high-resolution X-ray diffraction, transmission electron microscopy, and resistivity measurements, we find an optimum growth temperature of T G = 250 ∘ C , to obtain ferromagnetic layers with high crystal and interface perfection as well as high degree of atomic ordering. .

We study the strain evolution during molecular beam epitaxy growth of MnAs on GaAs(113)A, its change during postgrowth annealing and the interfacial structure by in situ grazing incidence x-ray diffraction. A MnAs peak is detected at a coverage as low as 0.5 monolayer (ML). The film is compressively strained at the onset of growth and relaxes as the thickness increases. The in-plane grain size increases in two distinct stages: a fast increase up to 4 ML nominal thickness is followed by a slow increase. Postgrowth annealing of the layer improves the crystal quality and leads to a further relaxation of the layer. An ordered array of misfit dislocations is found at the interface.

We present an analysis of the relaxation of thin MnAs films on GaAs(001) by x-ray grazing incidence diffraction during molecular beam epitaxy. Separate in-plane peaks of the MnAs layer and the GaAs substrate are detected for average thicknesses starting from ≈1 monolayer indicating the formation of a relaxed MnAs lattice. The variation of position of the MnAs peaks during growth yields the time dependence of relaxation. MnAs domains of different orientations are detected. A line broadening due to size and strain effects is observed. We find a regular arrangement of misfit dislocations at the interface. The range of twist of the domains reduces during growth indicating an improvement of the epitaxy.

We compare the molecular beam epitaxy growth and recovery kinetics under layer-by-layer growth conditions on the (0 0 1) surfaces of GaAs, InAs and GaSb using in situ surface X-ray diffraction. Whereas the growth conditions can be adjusted to yield comparable deposition kinetics, we find distinct differences in the recovery behavior of the three surfaces. We conclude that the mesoscopic structure of the growth front that often determines device performance depends on the detailed kinetics on an atomic scale.

We study the coarsening of two-dimensional crystalline islands on the (001) face of GaAs, InAs and GaSb after deposition at typical growth conditions in molecular beam epitaxy. The time-resolved island/pit size distributions are measured in situ using synchrotron X-ray diffraction and are analyzed together with the diffraction intensity oscillations during deposition. Whereas the deposition kinetics is similar for the three materials, they strongly differ in the coarsening. During coarsening, the mean correlation length grows proportional to t n. GaAs shows coarsening exponents around n = 1 and an exponential island size distribution, in clear contrast to the behavior of InAs and GaSb, where we find coarsening closer to the expected Ostwald ripening behavior (n between 1/3 and 1/2) on InAs and extremely slow kinetics on GaSb.

Grazing incidence X-ray diffraction is used to study the GaAs(0 0 1) surface kinetics after a fractional number of monolayers is deposited by molecular beam epitaxy. We compare submonolayer depositions with the growth of non-integer number of layers. The coarsening exponent n of the mean 2D island size, l ( t ) ∼ t n , is found to be n = 0.93 ± 0.18 . This exponent is notably larger than the Lifshitz–Slyozov exponent n = 1 / 3 expected for Ostwald ripening. A possible origin of this difference is discussed.

We study the kinetics of the GaAs(0 0 1) surface during and after homoepitaxial deposition of GaAs by in situ surface X-ray diffraction. Two stages of recovery are recognized and quantitatively described with kinematical scattering theory. In the first, fast stage of recovery, adatom islands and advacancy islands (pits) on adjacent layers annihilate, giving rise to a surface with only one incompletely filled layer. The second, slow stage of recovery is the coarsening of the remaining islands or pits. The experimental data are compared with Monte Carlo simulations of the deposition and recovery kinetics.

Periodic elastic domains in epitaxial films of MnAs(11̅00)/GaAs(001) are studied near the ferromagnetic phase transition. We obtain the period of the domain structure from the observed the X-ray diffraction satellites arising from the lateral periodicity. The application of the X-ray methods provides a high level of accuracy and gives information about average values over areas of several mm2. The observed periods are in a good agreement with those obtained by calculation and minimization of the elastic energy.

A combination of a molecular beam epitaxy (MBE) machine and a six circle diffractometer has been constructed at a dedicated wiggler beamline at the storage ring BESSY II for in situ investigations of III–V compound crystal growth. The growth conditions in our system reach a high MBE standard with a noncooled base pressure of 2×10−10 mbar. A fast entry load lock is available for sample exchange. Large-area Be windows in the ultrahigh vacuum chamber allow us to measure reflections at entrance and exit angles up to 45°, i.e., large perpendicular momentum transfers are possible. In situ reflection high energy electron diffraction and x-ray fluorescence measurements can be performed simultaneously with x-ray scattering. A GaAs(001) surface prepared and examined in our system reveals terrace widths of 450 nm and β(2×4) reconstruction domain sizes of 210 nm. The possibility of time-resolved x-ray diffraction studies is demonstrated by observation of intensity oscillations during layer-by-layer homoepitaxial growth on the GaAs(001)β(2×4) surface. The resolution functions of our experiment are determined.

We investigate the time-dependent surface evolution during molecular beam epitaxy of GaAs(0 0 1) using synchrotron X-ray diffraction at a newly-built dedicated beamline at the synchrotron BESSY II. The crystal truncation rods analyzed at growth temperature agree with the room-temperature bð2 � 4Þ reconstruction published in the literature. The layer coverage evolution during growth is analyzed by fitting the oscillating intensity along crystal truncation rods. Our results show that the structure of the reconstructed surface unit cell does not change during growth. Using numerical simulations,we determine the terrace size distribution on the surface at growth temperature and verify the validity of our analysis for multi-level initial surfaces,as long as the mean terrace size is larger than the nucleation distance during growth. 2002 Elsevier Science B.V. All rights reserved.

We present experimental evidence for the equilibrium coexistence between crystalline phases in thin MnAs layers grown epitaxially on GaAs. The phases, which can coexist in the bulk system only at one temperature point, are simultaneously present in the heterostructures over a temperature interval of more than 20 °C, which varies with the thickness of the MnAs layer. This phase coexistence is explained by the constraint on the lateral expansion of epitaxial layers which gives rise to strain in the layer. For the thicker MnAs layer a hysteresis of the temperature dependence of the phase composition is observed. The curvature of the samples is strongly anisotropic, with the larger curvature along the c-axis of the MnAs layer, and originates from thermal contraction of the film on cooling from growth temperature.

The experimental x-ray diffraction patterns of a Si$_{0.4}$Ge$_{0.6}$/Si(001) epitaxial film with a low density of misfit dislocations are modeled by the Monte Carlo method. It is shown that an inhomogeneous distribution of 60$^\circ$ dislocations with dislocations arranged in bunches is needed to explain the experiment correctly. As a result of the dislocation bunching, the positions of the x-ray diffraction peaks do not correspond to the average dislocation density but reveal less than a half of the actual relaxation.

We present a systematic theoretical study of the influence of elastic strain relaxation on the built-in electrostatic potentials and the electronic properties of axial [Formula: see text] nanowire (NW) heterostructures. Our simulations reveal that for a sufficiently large ratio between the thickness of the [Formula: see text] disk and the diameter of the NW, the elastic relaxation leads to a significant reduction of the built-in electrostatic potential in comparison to a planar system of similar layer thickness and In content. In this case, the ground state transition energies approach constant values with increasing thickness of the disk and only depend on the In content, a behavior usually associated to that of a quantum well free of built-in electrostatic potentials. We show that the structures under consideration are by no means field-free, and the built-in potentials continue to play an important role even for ultrathin NWs. In particular, strain and the resulting polarization potentials induce complex confinement features of electrons and holes, which depend on the In content, shape, and dimensions of the heterostructure.

We investigate the origin of the fast recombination dynamics of bound and free excitons in GaN nanowire ensembles by temperature-dependent photoluminescence spectroscopy using both continuous-wave and pulsed excitation. The exciton recombination in the present GaN nanowires is dominated by a nonradiative channel between 10 and 300 K. Furthermore, bound and free excitons in GaN NWs are strongly coupled even at low temperatures resulting in a common lifetime of these states. By solving the rate equations for a coupled two-level system, we show that one cannot, in practice, distinguish whether the nonradiative decay occurs directly via the bound or indirectly via the free state. The nanowire surface and coalescence-induced dislocations appear to be the most obvious candidates for nonradiative defects, and we thus compare the exciton decay times measured for a variety of GaN nanowire ensembles with different surface-to-volume ratio and coalescence degrees. The data are found to exhibit no correlation with either of these parameters, i.e., the dominating nonradiative channel in the GaN nanowires under investigation is neither related to the nanowire surface, nor to coalescence-induced defects. Hence we conclude that nonradiative point defects are the origin of the fast recombination dynamics of excitons in GaN nanowires.

We investigate the structural and optical properties of spontaneously formed GaN nanowires with different degrees of coalescence. This quantity is determined by an analysis of the cross-sectional area and perimeter of the nanowires obtained by plan-view scanning electron microscopy. X-ray diffraction experiments are used to measure the inhomogeneous strain in the nanowire ensembles as well as the orientational distribution of the nanowires. The comparison of the results obtained for GaN nanowire ensembles prepared on bare Si(111) and AlN buffered 6H-SiC(000-1) reveals that the main source of the inhomogeneous strain is the random distortions caused by the coalescence of adjacent nanowires. The magnitude of the strain inhomogeneity induced by nanowire coalescence is found not to be determined solely by the coalescence degree, but also by the mutual misorientation of the coalesced nanowires. The linewidth of the donor-bound exciton transition in photoluminescence spectra does not exhibit a monotonic increase with the coalescence degree. In contrast, the comparison of the root mean square strain with the linewidth of the donor-bound exciton transition reveals a clear correlation: the higher the strain inhomogeneity, the larger the linewidth., 18 pages, 7 figures

We simulate the surface shapes during homoepitaxial overgrowth of patterned GaAs substrates in the sector (0 0 1)–(1 1 1)A. The model for the shape evolution is based on a diffusion equation in which the orientation-dependent parameters vary according to a simple model of the surface free energy of the zincblende crystal structure. Metastable pseudofacets as well as the initial shape of the step are demonstrated to play a major role in the evolution of the surface during overgrowth.

Deformations in heteroepitaxial layer stacks of AlGaAs/GaAs grown on GaAs are measured by triple crystal diffractometry before epitaxial liftoff and after subsequent wafer bonding on various substrates (GaAs, glass, Si, and LiNbO3). The tetragonal deformation present in the as-grown layer stack partially relaxes during epitaxial liftoff. The roughness of the as-grown layer stack gives rise to a bending of the atomic planes after wafer bonding. The widths of the x-ray diffraction peaks are used to estimate the misorientation of the lattice planes and compared with the atomic force microscopy measurements of the surface roughness.

We describe wave propagation from a partially coherent x-ray source taking into account the free-space wave propagation and the Bragg diffraction on a collimating crystal. We find conditions when the free-space propagation becomes irrelevant and the coherence length is given by the extinction length of the corresponding Bragg reflection. We show that a wide wavelength range can be retained if the collimating crystal is illuminated by a convergent beam. The increase of the coherence length by Bragg diffraction is demonstrated in a laboratory experiment on periodic surface gratings with different periods.

We show that the phases of surface reflections can be determined by simultaneously exciting a surface and a bulk reflection at grazing incidence. The interference between the surface reflections excited by the incident and the bulk-diffracted waves has been observed for the Ge(113)-(3 × 1) reconstructed surface.

Lipid monolayers on the surface of water have been studied for over a hundred years, but in the last decade there has been a dramatic evolution in our understanding of the structures and phase transitions of these systems, driven by new experimental techniques and theoretical advances. In this review, dense monolayers of simple lipids are described in detail, including structures revealed by x-ray-diffraction experiments, computer simulations, molecular models, and a phenomenological theory of phase transitions. The effects of chirality and the structures of phospholipid monolayers are considered. Open questions and possible approaches to finding answers are discussed.

Physical review / E 58(2), 2172 - 2178 (1998). doi:10.1103/PhysRevE.58.2172, Structural changes in monolayers of the enantiomer and the racemic mixture of 1-hexadecyl-glycerol with temperature and surface pressure variations are compared. On compression, both monolayers exhibit a variation of the tilt azimuth from the direction to the nearest neighbor to the next nearest neighbor. In the monolayer of the racemate, this variation occurs as a first order transition. In the monolayer of the enantiomer, the unit cell is oblique, and continuously passes from a state close to the low-pressure state of the racemate to a state close to its high-pressure state. The azimuths of the unit-cell distortion and that of the tilt remain almost equal to each other. The effect of chirality decreases when the temperature is increased. Structural changes are explained in detail within the framework of the Landau theory of phase transitions., Published by Inst., Woodbury, NY