Search

The three-wave X-ray diffraction in strongly disordered epitaxial layers of GaN and ZnO is experimentally investigated. The charts of the intensity distribution in the reciprocal space are plotted in coordinates qθ and qϕ for the most intensive three-wave combination (1010)/(1011) by means of subsequent θ- and ϕ-scanning. A nontrivial shape of the θ-sections of these contours at a distance from the ϕ center of reflection is revealed; it is different for different samples. For the θ-curves at the center of reflection, we observed a common peak that may be approximated by the Voigt function with a power-low decrease in the intensity at the wings; the decrease law (from–4.5 to–5.0) is found to be considerably greater than that for the similar curves of two-wave diffraction and not depending on the dislocation density and distribution in layers. In some films we observed a coarse-block structure; in addition, it follows from the distribution in the reciprocal space that these blocks are turned with respect to each other around a normal to the surface, which allows us to suggest the existence of low-angle boundaries between them, consisting exclusively of edge dislocations.

The structural and magnetometric characteristics of a nanoporous carbon with nickel clusters in nanopores have been studied. The main attention is focused on the fact of substantial decrease in the nickel magnetization as nickel has a close contact to carbon as compared to similar parameter of free nickel microparticles with almost the same sizes. A theoretical model of a periodic cluster consisting of 16 carbon atoms and one nickel atom in a micropore surrounded by carbon atoms has been built. The electron density in such a cluster and its magnetism were found by computer calculations. It is shown that the magnetic moment of a nickel atom in the cluster is substantially smaller than that of a free nickel atom. Based on the results, it is concluded that the decrease in the magnetization of nickel clusters in the carbon environment observed experimentally is due to the adsorption interaction of the nickel and carbon electron subsystems.

The integrated intensity of X-ray diffraction reflections has been measured for a series of epitaxial layers of AIII nitrides (GaN, AlN, AlGaN) grown on different substrates (sapphire, SiC) and characterized by different degrees of structural perfection. It has been shown that, despite a high density of dislocations and a significant broadening of the diffraction peaks, the obtained values are not described by the kinematic theory of X-ray diffraction and suggest the existence of extinction. The results have been analyzed on the basis of the Darwin and Zachariasen extinction models. The secondary extinction coefficients and the thicknesses of epitaxial layers have been determined using two orders of reflection both in the Bragg geometry (0002 and 0004) and in the Laue geometry (\(10\bar 10\)) and \(10\bar 20\)). It has been demonstrated that the secondary extinction coefficient is the greater, the smaller is the broadening of the diffraction peaks and, consequently, the dislocation density. It has been found that, for epitaxial layers with a regular system of threading dislocations, the secondary extinction coefficient for the Laue reflections is substantially greater than that for the Bragg reflections.

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al2O3 substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10−2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.

The electric and structural characteristics of silicon-doped GaN and Al0.3Ga0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 × 1020 cm−3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al0.3Ga0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 × 1019 cm−3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al0.3Ga0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.

The three-wave diffraction in epitaxial layers of an AlxGa1 − xN solid solution has been measured using the Renninger scheme. From a comparison with similar diagrams measured earlier for GaN and AlN layers, it follows that the curves for the solid solution have a significantly higher intensity in the range of azimuthal angles outside the three-wave peaks. This indicates the existence of the two-wave Bragg reflection 0001 forbidden for wurtzite structures. It has been shown that the appearance of this reflection is due to a partial ordering of the solid solution in AlxGa1 − xN layers. From the measured structure factor F(0001) of the layers with various Al concentrations, it follows that the degree of ordering (an Al excess in one cation AlGa plane and an Al deficit in another plane) of the given series of samples is almost independent of the solid solution layer composition.

Nanoporous carbon samples with a large specific surface area can be filled with heavier elements or their compounds, which makes it possible to investigate the interaction of their electronic subsystems with carbon. One of the elements convenient for filling pores of carbon materials is bromine. Impregnation of nanoporous carbon samples with bromine causes the occurrence of the processes of micropore filling, monolayer adsorption, and intercalation. It has been found that samples impregnated with bromine substantially change their electrical and galvanomagnetic properties, and these changes depend on the structure of the samples. It has been shown that, if in the skeleton of a porous carbon sample there is a fraction of graphite clusters, the impregnation of the sample with bromine increases the concentration of charged carriers (holes). But when the sample has a quasi-amorphous structure, the injection of bromine into the sample leads to the appearance of a certain concentration of electrons in addition to charged mobile holes of the initial sample; i.e., the electrical conductivity becomes bipolar. In the former case, bromine molecules intercalate graphite clusters and, since bromine is an acceptor during intercalation of graphite, the hole concentration in the carbon skeleton network increases. In the latter case, bromine molecules can only be adsorbed on pore walls. As a result, the adsorption interaction between the electron shells of bromine molecules and the carbon surface leads to the formation of a donor layer near the surface and to the generation of electrons in the carbon skeleton network.

The Alx Ga1–xN layers with a wide range of AlN molar fraction (0 < x < 0.6) were grown by ammonia MBE on (0001)-oriented sapphire substrates using silane as the silicon dopant. The structural, electrical and optical properties of the layers were investigated by atomic force microscopy, X-ray diffraction, Hall, Raman, photoluminescence and cathodoluminescence measurements. It was found that doping affects surface morphology, dislocation density and luminescence intensity of the Alx Ga1–xN layers. A sharp decrease of the luminescence intensity for the heavily doped GaN and Al0.3Ga0.7N layers was explained by enhancement of the nonradiative recombination rate in degenerate GaN layers and by an increase in the defect concentration in AlGaN layers. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray multiple diffraction in epitaxial films of III-nitrides has been studied. A series of single layers of GaN, AlN, AlGaN and superlattices AlGaN/GaN of different thickness and structural perfection grown on c-sapphire are under study. Renninger scans for a primary forbidden 0001 reflection was used. The three beam diffraction peaks were measured in both φ-scans and θ-scans and their FWHM was analyzed. In the Renninger pattern all the possible 10 multiple peaks in an angular range (0–30)° are observed. It is shown that the FWHM of the φ-peaks reflects a crystalline perfection level but does not depend markedly on the multiple combination type. The angular width of the θ-scan peaks also increases with increasing of dislocation density and strongly depends on the type of three-wave reflection. The combinations with pure Laue secondary reflection (1–100)/(–1101), (3–1–20)/(–3121) and (3–2–10)/(–3211) are most sensi-tive to the dislocations. Most narrow peaks for the samples with the large dislocation density take place for the multiple combination (01–13)/(0–11–2) with the largest Bragg component of the diffraction vector. For the superlattices, θ–2θ-scan curve measured in the multiple position on the φ-scale contains satellites with angular distance corresponding to the forbidden 0001 reflection. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the first observation of folded acoustic phonons in the Raman spectra of short-period AlN/GaN superlattices (SLs). The values of longitudinal and transverse sound velocities along the [0001] direction of SL were estimated. It is shown that the SL period can be evaluated from Raman data with a good accuracy. Ab initio calculations of the zone-center phonons in (AlN)n(GaN)n SLs with the number of monolayer equal to n = 2,4,6 were performed. The frequencies of phonon modes were calculated and the mode displacement patterns have been established. The theoretical Raman spectra were simulated and compared with experimental ones. A strong sensitivity of A1(LO) phonons confined within the AlN layer to its thickness was revealed both experimentally and in calculations. This finding can be used for quantitative characterization of short-period AlN/GaN SLs.

Extended X-ray absorption fine structure above the Ga–K edge has been used to study the local structure of heterointerfaces in GaN/AlN multiple quantum wells (MQWs) grown by ammonia molecular beam epitaxy. The thickness of AlN and GaN layers in MQWs were evaluated using transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Two sets of MQWs structures with the number of period about of 20 and more than 100 were studied. The Ga-Ga interatomic distance in MQWs is lower than that in bulk GaN due to elastic compression of GaN layers in MQWs. This effect is less pronounced in thick structures because of almost full relaxation of GaN layers. The Ga-Al interatomic distance in thick MQWs is too long, about 0.1 A, longer than typical values for AlGaN alloys. Then number of Ga cations in the second coordination shell of Ga atoms is lower than that predicted for abrupt heterointerfaces that evidences intermixing of heterointerfaces. The intermixing degree depends on the total thickness of MQWs (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescent and structural properties of n-FZ-Si and n-Cz-Si implanted with Si ions at amorphizing doses and annealed at 1100°C in a chlorine-containing atmosphere have been studied. An analysis of proton Rutherford backscattering spectra of implanted samples demonstrated that an amorphous layer is formed, and its position and thickness depend on the implantation dose. An X-ray diffraction analysis revealed that defects of the interstitial type are formed in the samples upon annealing. Photoluminescence spectra measured at 78 K and low excitation levels are dominated by the dislocation-related line D1, which is also observed at 300 K. The peak position of this line, its full width at half-maximum, and intensity depend on the conduction type of Si and implantation dose. As the luminescence excitation power is raised, a continuous band appears in the spectrum. A model is suggested that explains the fundamental aspects of the behavior of the photoluminescence spectra in relation to the experimental conditions.

X-ray diffraction studies of nanoporous carbon synthesized from silicon carbide have been performed. The coordination shell radii and the coordination number in nanoporous carbon have been calculated using the Finback method. It has been shown that, when the coordination shell radii coincide, the distribution of carbon atoms over the coordination shells differs from that of hexagonal graphite. Based on an analysis of the interference functions and the results of the calculation, a model of atomic arrangement in the region of short-range ordering has been constructed. It has been demonstrated that the model cluster consists of seven graphene networks with different curvature radii.

Surface X-ray diffraction was applied to study structure of the fluorite-silicon interface forming upon epitaxial growth of CaF2 on Si(001) surface kept at 750 degrees C. Samples with CaF2 coverage of 1.5-4 (110)-monolayers were grown and in-situ characterized using synchrotron radiation. The 3 x 1-like surface reconstruction was observed in agreement with the previous studies by electron diffraction. Interestingly, a well pronounced splitting of the fractional x 1/3 reflections was revealed. This splitting was ascribed to the effect of antiphase domain boundaries in the row-like structure of the interface layer. The in-plane integrated intensities were used to reconstruct two-dimensional atomic structure of the high-temperature CaF2/Si(001) interface.

Strains and crystalline perfection of ZnO epitaxial layers on sapphire grown by molecular-beam epitaxy were studied using high-resolution X-ray diffrac tometry. The strain state of samples was determined by curvature measurements. The structural quality of layers was analyzed by diffraction measurements in the Bragg and Laue geometry using θ and (θ-2θ) scanning modes. It was found that the grown layers (Zn/O > 1) are subject to biaxial tensile stresses, while stresses are absent for Zn/O < 1. The density and depth of different dislocations were calculated from the diffraction peak broadening.

To determine the electronic properties of metal-carbon clusters, palladium clusters are introduced into nanoporous carbon samples. The structural and magnetic properties of the nanocomposite thus obtained are investigated. A ferromagnetic resonance (FMR) component is observed in the electron spin resonance (ESR) spectra measured in a wide temperature range. The electron density of states (DOS) of clusters consisting of a palladium atom surrounded by carbon atoms is calculated, and it is shown that, due to the interaction between electronic terms of carbon and metal, such a cluster should exhibit magnetism. The investigation of the structures of the porous skeleton of nanoporous carbon and of palladium clusters has allowed us to reveal how carbon and palladium atoms are brought into close contact so that to give rise to the observed ferromagnetism.

The structural state of GaN epitaxial layers grown on r-plane sapphire through metal-organic vapor phase epitaxy has been investigated using X-ray diffraction. The interplanar spacings in two directions in the (11\( \bar 2 \)0) plane of the interface and in the direction perpendicular to it, as well as the diffraction peaks in the ϑ and ϑ-2ϑ scan modes in the Bragg and Laue geometries, are measured on double- and triple-crystal diffractometers. The intensity distribution maps for asymmetric Bragg reflections are constructed for two azimuthal positions of the sample. An analysis of the data obtained has demonstrated that the elastic strain is anisotropic and that the X-ray diffraction pattern parallel to the interface plane is broadened. The layers are contracted in the [1\( \bar 1 \)00] direction and unstrained in the [0001] direction. The broadening of the Bragg reflections in the [1\( \bar 1 \)00] direction is considerably larger than that in the [0001] direction. It is shown using the Williamson-Hall plots for the Bragg and Laue reflections that these broadenings are not related to different degrees of mosaicity but are determined by the local dilatations and misorientations around defects. The data obtained are analyzed, and the conclusions regarding the dislocation structure of the samples are drawn.

Multilayer structures based on the InxGa1 − xN/GaN compounds grown by gas-phase epitaxy from organometallic compounds are studied using photoluminescence spectroscopy and high-resolution X-ray diffraction. A method for analyzing the experimental rocking curves of multilayer structures in terms of the Parratt-Speriosu model is developed. This method permits one to determine the thickness, period, and average composition of InxGa1 − xN/GaN layers, as well as the deformation of the active region in the samples under study. The local indium content is determined using the theoretical model which describes the radiation energy as a function of the thicknesses of the InGaN layers taking into account the energy of quantum confinement, the energies of the spontaneous polarization and piezoelectric polarization, and the parameters determined from high-resolution X-ray diffraction data.

Structural state of nonpolar a-plane GaN layers grown by MOVPE on r-plane sapphire is investigated by X-ray diffraction method. Interplanar spacings were measured in three directions and corresponding strains were determined. A crystalline perfection was studied by measurement of diffraction peaks of θ- and θ-2θ-scanning in Bragg- and Laue-diffraction geometry. Their FWHM was analyzed by Williamson-Hall plots. Anisotropy of the elastic strains and the peak broadening was revealed. It is shown that a different broadening of diffraction pattern in two in-plane directions is not caused by mosaicity and can be explained by a specific distribution of dislocations.

The structure of nanoporous carbon samples (prepared from silicon and boron carbides) with incorporated palladium clusters was studied. X-ray and electron diffraction studies show that most Pd clusters have an fcc lattice. Using small-angle X-ray scattering measurements, the metal cluster sizes are determined under certain assumptions. The sizes are not very close to the cluster sizes (4–14 nm) that are found from photomicrographs obtained using a transmission electron microscope (TEM). The difference is likely due to the local character of the TEM measurements. Apart from relatively coarse clusters of the sizes indicated above, the samples contain very small clusters, which are smaller in size than a micropore. Such small clusters are particularly large in number in C(SiC)B: Pd, where they are 0.5–0.7 nm in size. The content of small clusters in C(B4C)B: Pd is substantially lower, and they are somewhat larger in size (1.2–1.6 nm). The possible reasons of the ferromagnetism observed in these samples are discussed. It is assumed that the magnetism may be due to the small clusters, which do not have cubic symmetry.

The structure of Si:Er layers grown by MBE on Si(001) substrates was investigated by transmission electron microscopy. We studied the dependence of the layer structure on the erbium concentration [Er] (8 × 1018 – 4 × 1019 cm–3) as well as the epitaxial growth temperature (400–700 °C). In general, ErSi2 platelets and spherical metallic precipitates are formed during the epitaxial growth through the layer and in the near-interface region, respectively. For [Er] ≥2 × 1019 cm–3, an oscillation of platelet density with periodicity approximately 200–250 nm was observed. A new type of complex structural defect was observed in a specimen with [Er] = 4 × 1019 cm–3. In the layers grown at 400 °C, complexes of extended defects consisting of partial dislocations, stacking faults and twins are generated. The formation of silicides and Er precipitates is accompanied by emission of vacancies, which leads to the formation of pores in the layer. A high concentration of vacancies should also result in formation of V–V and V–Er complexes. The presence of these point-defect complexes can explain the appearance of deep acceptor levels with an activation energy of 360 meV, which can be responsible for nonradiative paths. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and surface morphology studies of MnF2 epitaxial layers have been performed by x-ray diffraction and atomic force microscopy (AFM). MnF2 layers of different thicknesses (0.12–1.25 µm) were grown by MBE on Si(0 0 1) substrates with the deposited CaF2-buffer layer having a 1 1 0 orientation and a grooved and ridged surface. X-ray diffraction patterns in the 10°–60° angular range have been measured. Profiles of diffraction peaks in two directions—parallel and normal to the diffraction vector—as well as the intensity distribution around 2 2 0 reciprocal lattice points were obtained. It was found that the metastable orthorhombic phase of α-PbO2 type with [1 1 0] growth direction dominates in the films; a much smaller portion of the stable rutile type phase was also detected, though it increases with the film thickness. As follows from the x-ray diffraction data, the films have a mosaic block structure. Parameters of the crystallites were determined using the Williamson–Hall analysis of angular width of the diffraction peaks. It was obtained that the crystallites of the orthorhombic phase have an elongated shape oriented along the direction of the CaF2 ridges. The AFM study confirms a regular distribution of the orthorhombic crystallites and shows the appearance of some irregular shaped crystallites on the top of the thick MnF2 films.

Using the electron spin resonance (ESR) method, conductance measurements, and small-angle X-ray scattering (SAXS), the properties of nanoporous carbon (NPC) containing atoms of Ni in the pores are studied in comparison with the nickel-free NPC studied before. Although the amount of Ni introduced into the nanopores is small, the NPC properties change significantly. The ESR spectra show that, at low temperatures, the charge carriers are captured at the localized states. The bulk ferromagnetism is not observed at temperatures as low as 3.2 K. Magnetic properties are strongly temperature dependent and are not limited by the paramagnetism of free charge carriers. The temperature dependence of the ESR resonance field indicates the influence of ferromagnetic inclusions. The ferromagnetic shift of the resonance fields is observed up to a temperature of 80 K. The temperature dependence of the conductance reveals an exponential growth with the activation energy close to those obtained from the ESR measurements. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The results of x-ray diffraction, x-ray photoelectron spectroscopy, electrical, and galvanomagnetic studies of nanoporous carbon with sulfur and selenium nanoclusters introduced into pores are analyzed. Conclusions are drawn concerning the short-range order in the introduced clusters and their interaction with carbon.

The defect structure of AlGaN/GaN superlattices and GaN layers grown through vapor-phase epitaxy from organometallic compounds is investigated using x-ray diffraction analysis before and after implantation with erbium ions at an energy of 1 MeV and a dose of 3 × 1015 cm−2, as well as after annealing. For a superlattice with a total thickness larger than the implantation depth, the satellites of the superlattice region strained under the action of ions disappear in the x-ray diffraction pattern after annealing at temperatures higher than 900°C. This suggests that the radiation-induced defects responsible for the positive deformation in the layer are annealed at these temperatures. However, annealing even at a temperature of 1050°C does not lead to complete recovery of the initial state and the positive deformation in the remaining regions is caused by residual defects. An analysis of the x-ray diffraction patterns demonstrates that, in samples with thin superlattices located at the depth corresponding to maximum radiation damage, the periodic structure that disappears after implantation at a dose of 3 × 1015 cm−2 is not recovered even after annealing at a temperature of 1050°C. This inference is confirmed by the results of examinations with an electron microscope.

High-resolution x-ray diffractometry and electron microscopy are used to study the defect structure and relaxation mechanism of elastic stresses in AlGaN/GaN superlattices grown by the MOCVD method on sapphire covered with a preliminarily deposited GaN and AlGaN buffer layer. Based on an analysis of the half-widths of three-crystal scan modes of x-ray reflections measured in different diffraction geometries, the density of different dislocation families is determined. For all the dislocation families, the density is shown to increase with the Al concentration in the solid-solution layers and depend only weakly on the superlattice period. From the electron-microscopic patterns of planar and cross sections, the types of dislocations and their distribution in depth are determined. It is shown that, in addition to high-density vertical edge and screw dislocations, which nucleate in the buffer layer and propagate through the superlattice layers, there are sloped intergrowing dislocations with a large horizontal projection and bent mixed dislocations with a Burgers vector $$\left\langle {11\overline 2 3} \right\rangle $$ at the interface between individual superlattice layers. The former dislocations form at the interface between the buffer layer and the superlattice and remove misfit stresses between the buffer and the superlattice as a whole, and the latter dislocations favor partial relaxation of stresses between individual superlattice layers. In samples with a high Al concentration (greater than 0.4) in AlGaN layers, there are cracks surrounded by high-density chaotic horizontal dislocations.

A combination of x-ray diffraction methods and electron microscopy was used for the structural study of Czochralski silicon crystals highly doped by As. For as-grown crystals a weak strain field and probable clustering of As-impurity atoms follow from section topography and a reduction in the Borrmann effect intensity. For annealed crystals, it is shown that precipitates and dislocation loops are formed during annealing. The defects are located in highly distorted stripes lying nearly parallel to the crystal surface with period 200?300??m. The concentration of the defects drastically decreases along the crystal radius from the centre to the periphery and along the growth axis from the seed. The average size of the defects obtained from the diffuse scattering increases with increasing distance from the centre to the periphery and along the growth axis from the seed. One concludes that the dependence of the defect parameters observed is associated with the change in oxygen concentration along the crystal radius and growth axis.

Paper presents an overview of recent results on novel CdSe quantum dot (QD) fabrication techniques with the main impact on a stressor-controlled self-organization process resulting in nucleation of real CdSe QDs optically active up to room temperature, with a narrower size distribution, a higher density and a high Cd content. The main idea of the stressor-controlled epitaxy is an intentional introduction of a super-strained fractional monolayer of the much higher lattice-mismatch compound - stressor - to create strong local stress fields on the growth surface which govern self-assembling of the main QD material. The studies are performed on a type-II non-common atom CdSe/BeTe system, where CdTe (Δa/a ∼ +14%) and BeSe (Δa/a ∼ -10%) interface bonds play a role of intrinsic stressors. Both experimental data (growth, structural and optical characterization) and Monte Carlo simulation of the growth process are presented.

Three-crystal x-ray diffractometry is used for structural studies of nitride AlGaN/GaN superlattices (SLs) grown by metal-organic chemical vapor deposition on sapphire with GaN and AlGaN buffer layers with widely varied SL period (from 50 to 3500 A), Al content in Alx Ga1−x N layers (0.1≤x≤0.5), and buffer layer composition. Satellite peaks characteristic of SLs are well pronounced up to the third order in θ-2θ scans of symmetric Bragg reflections and θ scans of the symmetric Laue geometry. The corresponding curves are well modeled by kinematic formulas. The average SL parameters, as well as the thickness, composition, and strain of individual layers, are determined using a combination of symmetric Bragg and Laue reflections. It is shown that all the samples under study are partially relaxed structures in which the elastic stresses between the entire SL and the buffer layer, as well as between individual layers, are relaxed. The AlGaN layers are stretched and the GaN layers are compressed. The GaN layer compression is larger in magnitude than the AlGaN layer tension because of thermoelastic stresses.

We suggest on approach including lattice dynamics calculations and Raman measurements of individual phonon modes in hexagonal GaN/AlN and GaN/AlGaN superlattices, which can prove to be a valuable tool in quantitative characterization of these nanostructures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X-raying is used to study the structure of nanoporous carbon particles produced from carbides of various materials (SiC, TiC, Mo2C, and B4C) by chemical removal of carbide-forming elements. The X-ray diffraction patterns exhibit reflections in the double reflection angle ranges 2θ=23°–26° and 42°–44°. An analysis of the angular position, half-widths, and intensities of these peaks shows that the former peak is a reflection from basal planes of distorted graphite-like crystalline fragments, and the latter reflection can be caused by two-dimensional diffraction $$(10\bar 1)$$ from graphene plane fragments in the sample bulk. The amount of graphite-like crystallites depends on the sample preparation method. Their size varies within 40–100 A, and the volume fraction is from 5 to 12% for various samples.

Structural study of CdSe/BeTe superlattices (SLs) grown by molecular beam epitaxy on GaAs substrate was performed by using double and triple crystal x-ray diffractometry. The period of the studied structures was about 5 nm, while the thickness of thin CdSe insertions varied from 0.4 to 1.5 monolayer. It is shown that new Be-Se bonds arise at the BeTe-CdSe interfaces in addition to the Be-Se bonds expected at the CdSe-BeTe interfaces. From the analysis of the diffraction curves of 002-reflection the complex composition of interfaces and thin insertions has been determined and contribution of all types of bonds in each SL period calculated. The diffraction curves of 004-reflection were used for the specification of the fine structure of the interfaces.

A new nanocomposite material containing approximately 50 vol % S is prepared by filling pores of bulk nanoporous carbon samples with sulfur. The initial nanoporous carbon samples are synthesized from polycrystalline α-SiC through the chemical reaction. A comparative investigation of small-angle x-ray scattering (SAXS) is performed for the prepared nanocomposite and the initial material. The possible changes in the scattering power of the initial material upon filling of its pores with sulfur are considered in the framework of a simple model. The regularities revealed are used to interpret the experimentally observed changes in the scattering power. The size distribution functions of incorporated sulfur nanoclusters in the nanocomposite (or filled nanopores in the initial material) are determined within the Guinier approximation. It is demonstrated that the smallest sized pores (8–16 A) remain unfilled, whereas the filling factor for larger sized pores can reach several ten percent by volume. The conditions favorable for small-angle x-ray scattering upon filling of the nanopores are analyzed.

Molecular beam epitaxy was used for the first time to grow novel GaAs/GaSb heterostructures with ultrathin (0.8–3 monolayers) GaAs layers embedded in GaSb. These structures were studied by X-ray diffraction, transmission electron microscopy, and photoluminescence. By contrast to known structures with self-assembled quantum dots, GaAs layers in GaAs/GaSb structures are subject to elastic tensile stresses due to 7% lattice mismatch. The structures exhibit intense photoluminescence in the 2 µm region at low temperatures. Quantum-dimensional islands are formed in the structure at a nominal GaAs layer thickness exceeding 1.5 monolayers. The band alignment of the structures is of type II.

The data on the structural organization of bulk nanoporous carbon materials produced from a number of carbides by selective etching reaction with chlorine are reported. The studies were carried out by the techniques of small angle X-ray scattering, X-ray diffraction, and high resolution transmission electron microscopy. It is found that the typical nanopore size is 1–2 nm depending on the carbide precursor type. Differently organized nanodomains are identified in the highly disordered carbon skeleton of the materials.

We report on optical and structural studies of the interface symmetry in CdSe/BeTe multiple-layer structures containing self-assembled quantum dots. Temperature and decay behavior of the broad photoluminescence (PL) band is consistent with the type-II transitions involving deeply localized electron states. Large linear in-plane polarization of the PL (up to 80%) is observed, implying the C-2v (or lower) symmetry of the individual places of the electron localization.

The X-Ray diffraction (XRD) reflections from epitaxial layers with various microstructures have been measured in the Bragg and Laue geometries. The shapes of XRD peaks measured in the θ and θ–2θ scan modes have been analyzed using the approximation by the Voigt function. It is shown that, for the structures with more regular dislocation systems, the Gaussian component predominates, whereas a more chaotic distribution of dislocations leads to an increase in the Lorentzian component. Far on the wings of XRD peaks, the rate of intensity decrease exceeds that predicted by the Voigt function.

A set of AlGaN/GaN superlattices (SLs) with various periods (5-40 nm) and various composition of barriers and buffer layers was grown by MOCVD on sapphire substrates. The aluminum incorporation depending on growth rate was investigated. It was observed that for growth of AlGaN layers with AlN mole fraction above 15-20% the growth rate must be significantly reduced to avoid aluminum incorporation saturation. Structures were studied by X-ray diffraction, Raman scattering, and optical absorption spectroscopy. A significant red-shift of absorption edge was observed for SLs with period of 10-40 nm.

The dislocation structure of hydride vapour phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tensor components. Symmetrical reflections (including reflections from planes forming large angles with the basal plane) with two modes of scanning (θ and θ-2θ) in two geometries (Bragg and Laue) are used to obtain the tensor components. The instant connections between the tensor components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaN films grown on sapphire without and with undoped and Si-doped metal-organic chemical vapour deposited templates.

The dislocation structure of hydride vapor phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tensor components. Symmetrical reflections (including reflections from planes forming a large angle with the basal plane) with two modes of scanning (θ and θ–2θ) in two geometries (Bragg and Laue) are used to obtain the tensor components. The instant connections between the tensor components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaNfilmsgrown on sapphire with and without undoped and Si-doped metalorganic chemical vapor deposited templates. Transmission electron microscopy was used to confirm the x-ray results by direct visualization of defect rearrangements.

The method of two-and three-crystal x-ray diffractometry (TCD) is used for studying the dislocation structure of thick GaN layers grown by chloride gaseous-phase epitaxy (CGE) on sapphire, as well as on a thin GaN layer, which is grown by the metalloorganic synthesis (MOS) method. Five components of the microdistortion tensor 〈ɛij〉 and the sizes of the coherent scattering regions along the sample surface and along the normal to it are obtained from the measurements of diffracted intensity in the Bragg and Laue geometries. These quantities are used to analyze the type and geometry of the dislocation arrangement and to calculate the density of the main types of dislocations. The density of the vertical screw dislocations, as well as of the edge dislocations, decreases (by a factor of 1.5 to 3) during growth on a thin GaN layer. The diffraction parameters of the thick layer on the MOS-GaN substrate suggest that it has a monocrystalline structure with inclusions of microcrystalline regions.

We present studies of the excitonic spectrum in superlattices (SLs) of CdSe insertions in a ZnSe matrix aimed at elucidating the CdSe/ZnSe interface morphology. The experimental photoluminescence excitation spectra are compared with the results of variational exciton calculations performed within the effective mass approximation. The shape of the average vertical (along the SL growth axis) distribution of CdSe within each insertion, used in the calculations, was obtained from a theoretical simulation of X-ray diffraction (XRD) rocking curves measured in the same samples. The results indicate that the thinnest layers are graded composition ZnCdSe quantum wells (QWs), generally homogeneous in the layer planes, whereas flat islands enriched by Cd appear at the CdSe nominal thickness larger than 0.5-0.6 monolayer (ML).

Double and triple X-ray diffractomery was used for a structural characterization of the short-period superlattices (SLs) CdSe/ZnSe with submonolayer sheets of CdSe. The multilayer structures containing SLs were grown by molecular beam epitaxy (MBE) on (001) GaAs substrates. The period of SLs was in range (30–60 A), the nominal thickness of CdSe sheets varied from 0.2 to 1.2 monolayer. The parameters of SLs have been determined. Broadening of CdSe sheets in direction normal to interface has been revealed. Full width at half maximum (FWHM) of this broadening is equal to about 4.5 monolayers and does not depend on the period and content of Cd in SLs.

Double-crystal x-ray diffraction rocking curves and two-dimensional reciprocal space mapping (2D-RSM) are utilized to characterize the degree of strain relaxation, lattice parameters and assessment of defect propagation in two growth approaches to yield relaxed germanium buffer layers on silicon substrates. Two schemes are investigated: direct epitaxy of a single relaxed buffer layer (SE-RBL) and step-graded multiple relaxed buffer layers (GM-RBLs). The characteristics of these two growth schemes offer prospects of a much thinner grown layer compared with previously reported approaches. Two-dimensional reciprocal space mapping shows that an SE-RBL with a thickness of less than 0.35??m has a superior quality over the GM-RBLs. A high relaxation factor (R = 0.986?0.002) is obtained from the asymmetric (113) 2D-RSM of the SE-RBL with 100% Ge content. Further, the ratios of full width at half maximum of the layer to substrate FWHM (L/S) of nearly unity for both ? and ?/2? scan directions imply a very high-quality crystalline relaxed buffer layer is realized. The 2D-RSM of the material deposited using the GM-RBL scheme, the first of its kind regarding the total grown thickness (approximately 6??m), also show a mosaic final Ge buffer layer with an indication of reduction of dislocation density.

The curves describing small-angle x-ray scattering at npor-C nanoporous carbon samples obtained from polycrystalline α-SiC, TiC, and Mo2C and a 6H-SiC single crystal have been analyzed. An algorithm is developed for taking into account the corrections to experimental curves for the intensity of the primary beam transmitted through the sample and the height of the inlet slit in these measurements. Two systems of nanoclusters observed in the npor-C structure differ in the type of stacking of structural elements: small-scale mass fractals of a dimension 1550 A. In most samples, large-scale nanoclusters can be regarded as objects with a fractal surface and a dimension 22. Peculiarities of the structure formation of nanoporous carbon obtained from various carbides are discussed.

An x-ray small-angle scattering study is reported of the structure of nanoporous carbon prepared by chlorinating carbide compounds having different crystal structures (SiC, TiC, Mo2C). The measurements were carried out both in reflection and transmission. The angular dependences of the scattering intensity obtained are treated as a result of scattering from nanoparticles of different size. By unfolding the experimental curves into components corresponding to particles with different gyration radii Rg, scatterer distribution functions in gyration radius m(Rg) were found. It is shown that, irrespective of the type of the starting carbide, particles with Rg∼5 A make up the largest fraction in porous carbon. Samples prepared from different carbides differ in the degree of nanoparticle uniformity in size. The most uniform in size are nanoparticles in the samples prepared from SiC, in which the average value Rgav

New pseudomorphic superlattices (SLs) based on calcium and cadmium fluorides have been grown from molecular beams and structurally characterized by high-resolution X-ray diffractometry. It was found that positions of the CdF 2 and CaF 2 emission bands in the SLs with the shortest periods shifted to the blue and red parts of the optical spectra, respectively. It was revealed that the Eu 2+ luminescence intensity under continuous optical excitation drastically decreases in a few seconds after the beginning of the excitation. This effect was accounted for by the photoionization of Eu 2+ ions with charge transfer from their 4f 6 5d excited state to the neighbouring CdF 2 layers.

Selective excitation photoluminescence measurements performed in CdSe/ZnSe superlattices with an embedded ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Se}$ quantum well reveal the limitation of the mobility edge concept to describe the dynamics of localized excitons in this system. Both localized and extended excitonic states are found to coexist in a certain energy range. We suggest that this behavior is governed by a macroscopic band-gap modulation imposed on the microscopic random localization potential.

Intrinsic strain along the c-axis of the lattice, zz = c/c, rms fluctuation zz of this strain (microstrain), rms axis-tilt fluctuations zx and xz for the a- and c-axes, block size in the directions parallel and normal to the sample surface and other structural characteristics have been studied on epitaxial GaN films grown by molecular beam epitaxy on Al2O3 and GaAs substrates. The intensities of bound-exciton (3.45 eV) and donor-acceptor luminescence (blue (3.26 eV) and yellow (2.30-2.67 eV)) have been measured as functions of excitation density and correlated to the structural characteristics. A recombination and defect-nucleation model is proposed to explain the fall in the luminescence intensity with increasing xz, observed for the first time.