Influence of an Overshoot Layer on the Morphological, Structural, Strain, and Transport Properties of InAs Quantum Wells

InAs quantum wells (QWs) are promising material systems due to their small effective mass, narrow bandgap, strong spin-orbit coupling, large g-factor, and transparent interface to superconductors. Therefore, they are promising candidates for the implementation of topological superconducting states. Despite this potential, the growth of InAs QWs with high crystal quality and well-controlled morphology remains challenging. Adding an overshoot layer at the end of the metamorphic buffer layer, i.e., a layer with a slightly larger lattice constant than the active region of the device, helps to overcome the residual strain and provides optimally relaxed lattice parameters for the QW. In this work, we systematically investigated the influence of overshoot layer thickness on the morphological, structural, strain, and transport properties of undoped InAs QWs on GaAs(100) substrates. Transmission electron microscopy reveals that the metamorphic buffer layer, which includes the overshoot layer, provides a misfit dislocation-free InAs QW active region. Moreover, the residual strain in the active region is compressive in the sample with a 200 nm-thick overshoot layer but tensile in samples with an overshoot layer thicker than 200 nm, and it saturates to a constant value for overshoot layer thicknesses above 350 nm. We found that electron mobility does not depend on the crystallographic directions. A maximum electron mobility of 6.07 × 105 cm2/Vs at 2.6 K with a carrier concentration of 2.31 × 1011 cm-2 in the sample with a 400 nm-thick overshoot layer has been obtained.

Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE

The fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs_1-xP_x/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80-150 nm) GaAs_1-xP_x epilayers appear partially relaxed (within 1-12% of the initial misfit) through a network of misfit dislocations along the sample [011] and [011-] in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews-Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs_1-xP_x composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy E_A = 1.41 ± 0.04 eV over the entire alloy compositional range.

Lattice Strain and Defects Analysis in Nanostructured Semiconductor Materials and Devices by High-Resolution X-Ray Diffraction: Theoretical and Practical Aspects

The reliability of semiconductor materials with electrical and optical properties are connected to their structures. The elastic strain field and tilt analysis of the crystal lattice, detectable by the variation in position and shape of the diffraction peaks, is used to quantify defects and investigate their mobility. The exploitation of high-resolution X-ray diffraction-based methods for the evaluation of structural defects in semiconductor materials and devices is reviewed. An efficient and non-destructive characterization is possible for structural parameters such as, lattice strain and tilt, layer composition and thickness, lattice mismatch, and dislocation density. The description of specific experimental diffraction geometries and scanning methods is provided. Today's X-ray diffraction based methods are evaluated and compared, also with respect to their applicability limits. The goal is to understand the close relationship between lattice strain and structural defects. For different material systems, the appropriate analytical methods are highlighted.

Recent structural insights into the mechanism of lysozyme hydrolysis

The complex of lysozyme with an N-acetylglucosamine tetramer shows a relatively strong hydrogen-bond network around a catalytic residue via high-resolution X-ray structural analysis. This indicates a potentially different hydrolysis mechanism to that through a glycosyl intermediate, and this is expected to be proved using neutron experiments.

Lysozyme hydrolyzes the glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycans located in the bacterial cell wall. The mechanism of the hydrolysis reaction of lysozyme was first studied more than 50 years ago; however, it has not yet been fully elucidated and various mechanisms are still being investigated. One reaction system that has commonly been proposed is that the lysozyme intermediate undergoes covalent ligand binding during hydrolysis. However, these findings resulted from experiments performed under laboratory conditions using fluorine-based ligands, which facilitate the formation of covalent bonds between the ligands and the catalytic side chain of lysozyme. More recently, high-resolution X-ray structural analysis was used to study the complex of lysozyme with an N-acetylglucosamine tetramer. As a result, the carboxyl group of Asp52 was found to form a relatively strong hydrogen-bond network and had difficulty binding covalently to C1 of the carbohydrate ring. To confirm this hydrogen-bond network, neutron test measurements were successfully performed to a resolution of better than 1.9 Å.

Effect of germanium auto-diffusion on the bond lengths of Ga and P atoms in GaP/Ge(111) investigated by using X-ray absorption spectroscopy

The germanium auto-diffusion effects on the inter-atomic distance between the nearest neighbors of the Ga atom in GaP epilayers are investigated using high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy. The GaP layers grown on Ge (111) are structurally coherent and relaxed but they show the presence of residual strain which is attributed to the auto-diffusion of Ge from the results of secondary ion mass spectrometry and electrochemical capacitance voltage measurements. Subsequently, the inter-atomic distances between the nearest neighbors of Ga atom in GaP are determined from X-ray absorption fine-structure spectra performed at the Ga K-edge. The estimated local bond lengths of Ga with its first and second nearest neighbors show asymmetric variation for the in-plane and out-of-plane direction of GaP/Ge(111). The magnitude and direction of in-plane and out-of-plane microscopic residual strain present in the GaP/Ge are calculated from the difference in bond lengths which explains the presence of macroscopic residual tensile strain estimated from HRXRD. Modified nearest neighbor configurations of Ga in the auto-diffused GaP epilayer are proposed for new possibilities within the GaP/Ge hetero-structure, such as the conversion from indirect to direct band structures and engineering the tensile strain quantum dot structures on (111) surfaces.

Basal Plane Bending of Homoepitaxial MPCVD Single-Crystal Diamond

We report herein high-resolution X-ray diffraction measurements of basal plane bending of homoepitaxial single-crystal diamond (SCD). We define SCD (100) as the base plane. The results revealed that growth parameters such as temperature, growth time, and basal plane bending of the substrate all affect the basal plane bending of SCD. First, the basal plane bending of SCD depends mainly on the substrate and becomes severe with increasing basal plane bending of the substrate. The SCD growth experiments show that the basal plane bending increases with elevated growth temperature and increased growth time. Finally, to understand the mechanism, we investigated the substrate-surface temperature distribution as a function of basal plane bending of SCD fabricated by chemical vapor deposition (CVD). This allowed us to propose a model and understand the origin of basal plane bending. The results indicate that an uneven temperature distribution on the substrate surface is the main cause of the base-plane bending of CVD diamond.

Topological Analysis of Electron Density in Large Biomolecular Systems

BACKGROUND

A great step toward describing the structure of the molecular electron was made in the era of quantum chemical methods. Methods play a very important role in the prediction of molecular properties and in the description of the reactivity of compounds, which cannot be overestimated. There are many works, books, and articles on quantum methods, their applications, and comparisons. At the same time, quantum methods of a high level of theory, which give the most accurate results, are time-consuming, which makes them almost impossible to describe large complex molecular systems, such as macromolecules, enzymes, supramolecular compounds, crystal fragments, and so on.

OBJECTIVES

To propose an approach that allows real-time estimation of electron density in large systems, such as macromolecules, nanosystems, proteins.

METHODS

AlteQ approach was applied to the tolopogical analysis of electron density for "substrate - cytochrome" complexes. The approach is based on the use of Slater's type atomic contributions. Parameters of the atomic contributions were found using high resolution X-ray diffraction data for organic and inorganic molecules. Relationships of the parameters with atomic number, ionization potentials and electronegativities were determined. The sufficient quality of the molecular electron structure representation was shown under comparison of AlteQ predicted and observed electron densities. AlteQ algorithm was applied for evaluation of electron structure of "CYP3A4 - substrate" complexes modeled using BiS/MC restricted docking procedure. Topological analysis (similar to Atoms In Molecules (AIM) theory suggested by Richard F.W. Bader) of the AlteQ molecular electron density was carried out for each complex. The determination of (3,-1) bond, (3,+1) ring, (3,+3) cage critical points of electron density in the intermolecular "CYP3A4 - substrate" space was performed.

RESULTS

Different characteristics such as electron density, Laplacian eigen values, etc. at the critical points were computed. Relationship of pKM (KM is Michaelis constant) with the maximal value of the second Laplacian eigen value of electron density at the critical points and energy of complex formation computed using MM3 force field was determined.

CONCLUSION

It was shown that significant number of (3,-1) bond critical points are located in the intermolecular space between the enzyme site and groups of substrate atoms eliminating during metabolism processes.

Inflexible stoichiometry in bulk pyrite FeS2 as viewed by in situ and high-resolution X-ray diffraction

Non-stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS₂, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS_2-δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line-compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS₂ even at different Fe/S ratios. By properly standardizing in situ gas-flow X-ray diffraction measurements, no significant changes in the lattice parameter of FeS₂ can be resolved, which portrays iron pyrite as prone to forming sulfur-deficient compounds, but not intrinsic defects in the manner of NiS_2-δ.

Introduction to the special issue on high-resolution X-ray diffraction and imaging

The latest virtual special issue of Journal of Applied Crystallography features some highlights of the 13th Biennial Conference on High-Resolution X-ray Diffraction and Imaging (XTOP 2016), held in Brno, Czech Republic, in September 2016.

Effect of Sr Content and Strain on Sr Surface Segregation of La1-xSrxCo0.2Fe0.8O3-δ as Cathode Material for Solid Oxide Fuel Cells

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La_1-x Sr_xCo_0.2Fe_0.8O_3-δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO₃, SrTiO₃, and GdScO₃ substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD), and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Lead-Free α-La₂WO₆ Ferroelectric Thin Films

(001)-Epitaxial La2WO6 (LWO) thin films are grown by pulsed laser deposition on (001)-oriented SrTiO3 (STO) substrates. The α-phase (high-temperature phase in bulk) is successfully stabilized with an orthorhombic structure (a = 16.585(1) Å, b = 5.717(2) Å, c = 8.865(5) Å). X-ray-diffraction pole-figure measurements suggest that crystallographic relationships between the film and substrate are [100]LWO ∥ [110]STO, [010]LWO ∥ [11̅0]STO and [001]LWO ∥ [001]STO. From optical properties, investigated by spectroscopic ellipsometry, we extract a refractive-index value around 2 (at 500 nm) along with the presence of two absorption bands situated, respectively at 3.07 and 6.32 eV. Ferroelectricity is evidenced as well on macroscale (standard polarization measurements) as on nanoscale, calling for experiments based on piezo-response force-microscopy, and confirmed with in situ scanning-and-tunneling measurements performed with a transmission electron microscope. This work highlights the ferroelectric behavior, at room temperature, in high-temperature LWO phase when stabilized in thin film and opens the way to new functional oxide thin films dedicated to advanced electronic devices.

Topology and temperature dependence of the diffuse X-ray scattering in Na0.5Bi0.5TiO3 ferroelectric single crystals

The results of high-resolution measurements of the diffuse X-ray scattering produced by a perovskite-based Na0.5Bi0.5TiO3 ferroelectric single crystal between 40 and 620 K are reported. The study was designed as an attempt to resolve numerous controversies regarding the average structure of Na0.5Bi0.5TiO3, such as the mechanism of the phase transitions between the tetragonal, P4bm, and rhombohedral | monoclinic, R3c | Cc, space groups and the correlation between structural changes and macroscopic physical properties. The starting point was to search for any transformations of structural disorder in the temperature range of thermal depoling (420-480 K), where the average structure is known to remain unchanged. The intensity distribution around the {032} pseudocubic reflection was collected using a PILATUS 100K detector at the I16 beamline of the Diamond Light Source (UK). The data revealed previously unknown features of the diffuse scattering, including a system of dual asymmetric L-shaped diffuse scattering streaks. The topology, temperature dependence, and relationship between Bragg and diffuse intensities suggest the presence of complex microstructure in the low-temperature R3c | Cc phase. This microstructure may be formed by the persistence of the higher-temperature P4bm phase, built into a lower-temperature R3c | Cc matrix, accompanied by the related long-range strain fields. Finally, it is shown that a correlation between the temperature dependence of the X-ray scattering features and the temperature regime of thermal depoling is present.

Ab initio simulation of diffractometer instrumental function for high-resolution X-ray diffraction

A method for the simulation of the diffractometer instrumental function for high-resolution X-ray diffraction, applicable for coplanar and noncoplanar measurement geometry and for any combination of X-ray optical elements, is proposed. Good agreement is demonstrated between the measured and the simulated reciprocal-space maps, which account for the instrumental function.

Modeling of the X-ray diffractometer instrumental function for a given optics configuration is important both for planning experiments and for the analysis of measured data. A fast and universal method for instrumental function simulation, suitable for fully automated computer realization and describing both coplanar and noncoplanar measurement geometries for any combination of X-ray optical elements, is proposed. The method can be identified as semi-analytical backward ray tracing and is based on the calculation of a detected signal as an integral of X-ray intensities for all the rays reaching the detector. The high speed of calculation is provided by the expressions for analytical integration over the spatial coordinates that describe the detection point. Consideration of the three-dimensional propagation of rays without restriction to the diffraction plane provides the applicability of the method for noncoplanar geometry and the accuracy for characterization of the signal from a two-dimensional detector. The correctness of the simulation algorithm is checked in the following two ways: by verifying the consistency of the calculated data with the patterns expected for certain simple limiting cases and by comparing measured reciprocal-space maps with the corresponding maps simulated by the proposed method for the same diffractometer configurations. Both kinds of tests demonstrate the agreement of the simulated instrumental function shape with the measured data.

XTOP: high-resolution X-ray diffraction and imaging

The latest virtual special issue of Journal of Applied Crystallography includes some highlights of the 12th Conference on High-Resolution X-ray Diffraction and Imaging (XTOP), which took place in Villard-de-Lans and Grenoble in September 2014.

On atom-atom 'short contact' bonding interactions in crystals

Professor Dunitz questions the usefulness of ascribing crystalline structural stability to individual atom-atom intermolecular interactions viewed as bonding (hence stabilizing) whenever linked by a bond path. An alternative view is expressed in the present essay that articulates the validity and usefulness of the bond path concept in a crystallographic and crystal engineering context.

Covariant description of X-ray diffraction from anisotropically relaxed epitaxial structures

A general theoretical approach to the description of epitaxial layers with essentially different cell parameters and in-plane relaxation anisotropy is presented.

A general theoretical approach to the description of epitaxial layers with essentially different cell parameters and in-plane relaxation anisotropy has been developed. A covariant description of relaxation in such structures has been introduced. An iteration method for evaluation of these parameters on the basis of the diffraction data set has been worked out together with error analysis and reliability checking. The validity of the presented theoretical approaches has been proved with a-ZnO on r-sapphire samples grown in the temperature range from 573 K up to 1073 K. A covariant description of relaxation anisotropy for these samples has been estimated with data measured for different directions of the diffraction plane relative to the sample surface.

High-resolution X-ray diffraction and imaging

This issue of Journal of Applied Crystallography includes some highlights of the 11th Biennial Conference on High-Resolution X-ray Diffraction and Imaging (XTOP), held in St Petersburg in 2012.