White Beam X-ray Research Articles

Surface morphology of naphtacene single crystals grown by the physical vapor transport technique

The surface morphology of naphtacene single crystals grown by the physical vapor transport technique was investigated by atomic force microscopy and white-beam X-ray topography. Locally, two types of line pattern were observed on the basal (001) plane along the [110] and [010] directions, and analyzed from crystallographic viewpoints. Such line patterns are considered in relation to crystallographic periodicities, dislocation lines, and slip-plane phenomena.

In-situ observation on the δ to γ transformation behavior of carbon steels

In recent in-situ observations on the solidification of thin carbon steel experiments, the primary δ-phase appears to be highly undercooled into the single γ phase region of the phase diagram, where the massive δ to γ transformation occurred without the intervention of the liquid phase following a typical peritectic reaction. Whether such a massive transformation is the main mode, even in bulk steel alloys, is of prime importance because it would drastically change our view on the defect generation mechanism during steel solidification in continuous casting. In this research, the in-situ imaging technique using synchrotron white X-ray beam was conducted to investigate the effects of carbon content and the specimen thickness on the δ to γ transformation behaviors in the carbon steel. The moment of δ to γ transformation is identified through synchronized recording of Laue diffraction patterns and in-situ images. The relative average brightness (RAB) change during solidification was evaluated to analyze the transformation. The results revealed that hypo-peritectic steel grades predominantly underwent a massive transformation, while hyper-peritectic steel grades showed a combination of massive and diffusional transformations. The sudden drop in liquid fraction after massive transformation was detected, while diffusional transformation exhibited a gradual decrease in the present study. The findings demonstrated that specimen thickness influenced the transformation mode in hyper-peritectic steel grades. The study provides insights into the behavior of δ to γ transformation and its implications for carbon steel solidification processes.

First white beam on a von Hámos spectrometer at the PolyX beamline of SOLARIS

Synchrotrons are brilliant sources of X-ray radiation used in a variety of methods to study the structure of matter and dynamics of processes on the atomic scale. X-ray spectroscopy methods at synchrotrons are typically combined with the beam monochromatization which guarantees better energy resolution but also reduces orders of magnitude the photon flux incident on the sample and thus wastes the vast majority of the photons produced. Here we report on the commissioning of an X-ray spectrometer at the PolyX beamline of SOLARIS specialized in application of white, broadband and monochromatic X-ray beam in multimodal microimaging, microtomography and microscpectroscopic studies. The spectrometer was used to acquire good quality Fe K-edge X-ray absorption spectrum over about 120 eV-range within seconds. In this work we present the first X-ray absorption spectrum measured using a synchrotron white beam in combination with a von Hámos geometry-based spectrometer.

Dislocation arrangements in 4H-SiC and their influence on the local crystal lattice properties.

Two wafers of one 4H-silicon carbide (4H-SiC) bulk crystal, one cut from a longitudinal position close to the crystal's seed and the other close to the cap, were characterized with synchrotron white-beam X-ray topography (SWXRT) in back-reflection and transmission geometry to investigate the dislocation formation and propagation during growth. For the first time, full wafer mappings were recorded in 00012 back-reflection geometry with a CCD camera system, providing an overview of the dislocation arrangement in terms of dislocation type, density and homogeneous distribution. Furthermore, by having similar resolution to conventional SWXRT photographic film, the method enables identification of individual dislocations, even single threading screw dislocations, which appear as white spots with a diameter in the range of 10 to 30 µm. Both investigated wafers showed a similar dislocation arrangement, suggesting a constant propagation of dislocations during crystal growth. A systematic investigation of crystal lattice strain and tilt at selected wafer areas with different dislocation arrangements was achieved with high-resolution X-ray diffractometry reciprocal-space map (RSM) measurements in the symmetric 0004 reflection. It was shown that the diffracted intensity distribution of the RSM for different dislocation arrangements depends on the locally predominant dislocation type and density. Moreover, the orientation of specific dislocation types along the RSM scanning direction has a strong influence on the local crystal lattice properties.

The early development of synchrotron white-beam X-ray topography analysis for crystal investigations at Pohang light source-II

The early development of synchrotron white-beam X-ray topography analysis for crystal investigations at Pohang light source-II

白色X線応力測定法の基礎と応用

The X-ray diffraction method has been already established as one of the most effective nondestructive evaluation method of a stress in crystalline materials. Since white X-ray beam has a wide wavelength range, a diffraction angle has to fix during a measurement to obtain a lattice spacing using the Bragg’s low. Therefore, the white X-ray method has some different features from the characteristic X-ray method. For example, the information of lattice spacing of many lattice planes in a material can be obtained simultaneously on the same diffraction angle. Because all equipment are held fixed position during a measurement, it is advantageous to in-situ measurement under the special environment such as high temperature or high pressure. In the present review article, a brief history of development of the white X-ray method is covered and some factors related to the measurement accuracy is explained. Fundamentals of the measurement system using white X-ray and the energy dispersive method are presented. Applications of the method using white X-ray obtained from a rotating target type X-ray generator to the measurement of residual stress of a surface ground material and a coating material are described. Furthermore, applications of the method using high energy white X-ray generated in the synchrotron radiation facility SPring-8 to the measurement of internal strain of a material loaded by four-point bending and strain distribution in the vicinity of crack tip in a material introduced a fatigue crack are also described.

In situ compression of micropillars under coherent X-ray diffraction: a case study of experimental and data-analysis constraints.

Micropillar compression is a method of choice to understand mechanics at small scale. It is mainly studied with electron microscopy or white-beam micro-Laue X-ray diffraction. The aim of the present article is to show the possibilities of the use of diffraction with a coherent X-ray beam. InSb micropillars in epitaxy with their pedestals (i.e. their support) are studied in situ during compression. Firstly, an experiment using a collimated beam matching the pillar size allows determination of when the sample enters the plastic regime, independently of small defects induced by experimental artefacts. A second experiment deals with scanning X-ray diffraction maps with a nano-focused beam; despite the coherence of the beam, the contributions from the pedestal and from the micropillar in the diffraction patterns can be separated, making possible a spatially resolved study of the plastic strain fields. A quantitative measurement of the elastic strain field is nevertheless hampered by the fact that the pillar diffracts at the same angles as the pedestal. Finally, no image reconstructions were possible in these experiments, either in situ due to a blurring of the fringes during loading or post-mortem because the defect density after yielding was too high. However, it is shown how to determine the elastic bending of the pillar in the elastic regime. Bending angles of around 0.3° are found, and a method to estimate the sample's radius of curvature is suggested.

Millisecond-Order Temporal-Resolution Synchrotron X-ray Tomography without Sample Rotation

A multi-beam optics consisting of curved crystals with blades for a white synchrotron X-ray beam from a bending magnet was developed, and millisecond-order temporal-resolution X-ray tomography with a voxel size of 20 μm was successfully realized without sample rotation. The multi-beam optics opens the spatio-temporal frontier of X-ray tomography for unrepeatable and nonequilibrium systems and is expected to be applied to various fields in materials and life sciences.

Using High Energy X-Ray White Beam Tomography to Quantify the Location and Morphology of ZnO Discharge Products in Alkaline Batteries

In alkaline Zn anodes, the active material is discharged to both aqueous zincate ions and solid ZnO. The mobility of zincate in the electrolyte allows considerable transport of the discharge products, which can form directly on the Zn active material or far from it, effectively relocating material long distances inside the cell. Additionally, ZnO can exist in more than one form, namely type I and type II, which differ in their density and passivating character.1 This leads to complex compositions of discharged primary Zn anodes and can cause cell failure due to redistribution of Zn in rechargeable cells. Models of Zn anodes are typically designed for continuous discharge,2 yet real-world use conditions usually involve intermittent discharge. Therefore, the ability of these models to predict ZnO morphology and distribution for cells used in real-world applications is questionable.It has been previously reported that when alkaline AA cells are discharged, ZnO forms passivating shells around undischarged Zn cores. At high discharge rates, this passivation of Zn increases in severity and hinders cell performance. Additionally, higher discharge rates increase the non-uniformity of ZnO distribution within the anode, preferentially precipitating ZnO near the separator which impedes hydroxide transport from the cathode to the inner portion of the anode.3 In this work, we analyze cylindrical cells discharged using intermittent profiles that approximate real battery use. We then compare the ZnO distribution to that found after continuous discharge.Synchrotron X-ray tomography was used to obtain in situ volumetric reconstructions at a spatial resolution of 3 microns. To observe the spatial distribution and morphology of ZnO formation in partially discharged Zn anodes, multiple AAA cells were discharged using distinct protocols. When using a continuous discharge, the reconstructions verified previous findings that core-shell structure is the primary morphology of ZnO and is non-uniformly distributed towards the separator.3 However, a radically different morphology and spatial distribution of ZnO was observed when using an intermittent discharge protocol. Specifically, when providing an 8 hour rest after each hour of discharge, ZnO forms isolated clumps distributed throughout the anode. Using this intermittent discharge protocol avoids passivating active material and potentially enables a higher Zn utilization by minimizing isolation of active material.

Recovery of small orientation gradients in a recrystallized grain observed in 3D during ex-situ annealing

We report the observation of a small orientation gradient (~0.5° over a few micrometers) in a recrystallized grain formed at the sample surface in pure Al and the elimination of this orientation gradient during subsequential ex-situ annealing. The observation is in 3D and done using synchrotron white-beam differential-aperture X-ray microscopy with high angular resolution of 0.01°. The elimination of the orientation gradient during subsequent annealing are analyzed and discussed.

Applicability of a Flat-Bed Birefringence Setup for the Determination of Threading Dislocations of Silicon Carbide Wafers

Screw-type dislocations like micropipes (MP) and threading screw dislocations (TSD) are prohibiting the function or at least diminishing the efficiency of electronic devices based on silicon carbide (SiC). Therefore, it is essential to characterize wafers in an efficient and fast manner. Molten potassium hydroxide (KOH) etching or white-beam X-ray topography (SWXRT) are either destructive or not economically viable for an in-depth characterization of every wafer of one SiC crystal. Birefringence microscopy is being utilized as a fast and non-destructive characterization method. Instead of microscopic setups, commercially available flat-bed scanners equipped with crossed polarizer foils can be used for fast large-area scans. This work investigates the feasibility of such a setup regarding the detection rate of MPs and TSDs. The results of a full-wafer mapping are compared with birefringence microscopy and KOH etching. In the investigated sample clusters of MPs caused by a polytype switch in the beginning of the growth could be identified by both birefringence microscopy and the flat-bed scanner setup, as well as small angle grain boundaries and TED arrays. However, the resolution of the scanner was not sufficient to identify TSDs. Nevertheless the setup proves to be an easy-to-setup and cheap characterization method, able to quickly identify defect clusters in 4H-SiC wafers.

High-Throughput Powder Diffraction Using White X-Ray Beam and a Simulated Energy-Dispersive Array Detector

High-throughput powder X-ray diffraction (XRD) with white X-ray beam and an energy-dispersive detector array is demonstrated in this work on a CeO2 powder sample on a bending magnet synchrotron beamline at the Shanghai Synchrotron Radiation Facility (SSRF), using a simulated energy-dispersive array detector consisting of a spatially scanning silicon-drift detector (SDD). Careful analysis and corrections are applied to account for various experimental hardware-related and diffraction angle-related factors. The resulting diffraction patterns show that the relative strength between different diffraction peaks from energy-dispersive XRD (EDXRD) spectra is consistent with that from angle-resolved XRD (ARXRD), which is necessary for analyzing crystal structures for unknown samples. The X-ray fluorescence (XRF) signal is collected simultaneously. XRF counts from all pixels are integrated directly by energy, while the diffraction spectra are integrated by d-spacing, resulting in a much improved peak strength and signal-to-noise (S/N) ratio for the array detector. In comparison with ARXRD, the diffraction signal generated by a white X-ray beam over monochromic light under the experimental conditions is about 104 times higher. The full width at half maximum (FWHM) of the peaks in q-space is found to be dependent on the energy resolution of the detector, the angle span of the detector, and the diffraction angle. It is possible for EDXRD to achieve the same or even smaller FWHM as ARXRD under the energy resolution of the current detector if the experimental parameters are properly chosen.

The inclination of threading dislocation in chemical vapor deposition-grown single-crystal diamond analyzed by synchrotron white beam X-ray topography

The inclination of threading dislocation in chemical vapor deposition-grown single-crystal diamond analyzed by synchrotron white beam X-ray topography

Characterization of Structural Defects in (Cd,Zn)Te Crystals Grown by the Travelling Heater Method

Structural defects and compositional uniformity remain the major problems affecting the performance of (Cd, Zn)Te (CZT) based detector devices. Understanding the mechanism of growth and defect formation is therefore fundamental to improving the crystal quality. In this frame, space experiments for the growth of CZT by the Travelling Heater Method (THM) under microgravity are scheduled. A detailed ground-based program was performed to determine experimental parameters and three CZT crystals were grown by the THM. The structural defects, compositional homogeneity and resistivity of these ground-based crystals were investigated. A ZnTe content variation was observed at the growth interface and a high degree of stress associated with extensive dislocation networks was induced, which propagated into the grown crystal region according to the birefringence and X-ray White Beam Topography (XWBT) results. By adjusting the growth parameters, the ZnTe variations and the resulting stress were efficiently reduced. In addition, it was revealed that large inclusions and grain boundaries can generate a high degree of stress, leading to the formation of dislocation slip bands and subgrain boundaries. The dominant defects, including grain boundaries, dislocation networks and cracks in the interior of crystals, led to the resistivity variation in the crystals. The bulk resistivity of the as-grown crystals ranged from 109 Ωcm to 1010 Ωcm.

Crystal Defect Investigation in PVT-Grown ZnSe Under Different Seeding Conditions and Growth Configurations Using Synchrotron X-Ray Topography

Zinc Selenide (ZnSe) as a prominent II-VI semiconductor is notable for its excellent performance on high efficiency electronic and optoelectronic devices including blue light-emitting diodes, lasers, and blue/ultraviolet-light-sensitive photodetectors, which are of considerable interest for applications in optical storage, lighting and illumination, biomedical sensing, etc. The effective utilization of ZnSe is owing to its superior properties such as wide bandgap, low optical absorption in the visible and infrared spectral region, and intense edge luminescence in the short-wavelength spectral region. The preliminary requisite to satisfy all those essential properties is the crystalline quality. Therefore, the realization of high-performance devices requires high-quality ZnSe bulk crystals with minimized structural defects. In order to overcome limitations and continuously fulfill the requirement of modern device applications, obtaining a comprehensive understanding on ZnSe crystallographic structure and defect formation mechanism is crucial. In this study, characterization techniques including synchrotron white beam X-ray topography (SWBXT), optical microscopy and high-resolution triple X-ray diffraction (HRTXD) were conducted on seeded and self-seeded ZnSe crystals grown by physical vapor transport (PVT) method in horizontal and vertical growth configurations. The effect of seeding condition and gravity vector orientation with respect to growth geometry on crystalline quality and defect structures is investigated. The presence of grains and the occurrence of twinning is revealed and analyzed. This study provides a clear understanding of factors that causes ZnSe crystal quality variation during the growth process, which contributes to achieving the routine production of high-quality ZnSe crystal and modern device enhancement. Figure 1

Crystal Defect Investigation in PVT-Grown ZnSe Under Different Seeding Conditions and Growth Configurations Using Synchrotron X-Ray Topography

Zinc selenide (ZnSe) is a widely utilized II-VI semiconducting material for fabricating high-performance optoelectronics devices owing to its unique electrical and optical properties. As those properties highly depend on the native point defects and impurity distribution, achieving routine production of high-quality ZnSe crystal is essential. This study conducted synchrotron white beam X-ray topography (SWBXT), optical microscopy and high-resolution triple X-ray diffraction (HRTXD) characterization on five as-grown ZnSe boules grown in physical vapor transport (PVT) method with different seeding conditions and growth configurations. Study indicated that nucleation of multiple grains is induced during seeded growth, while self-seeded boules tend to obtain single crystals. ZnSe boules grown in vertical configuration are found to have relatively poor crystalline quality comparing to horizontally grown boules that are characterized by low energy facets. The twinning mechanism is also discussed. This study successfully investigated factors that induce crystal quality variation during the growth process.

Recrystallization boundary migration in the 3D heterogeneous microstructure near a hardness indent

Boundary migration during recrystallization in the heterogeneous microstructure near a hardness indent in lightly rolled pure Al was followed in 3D. The microstructure after multiple steps of ex-situ annealing was examined using synchrotron white-beam differential-aperture X-ray microscopy, supplemented by scanning electron microscopy. Heterogeneous recrystallization boundary migration was observed and analyzed in terms of driving force and boundary characteristics. The results reveal very similar local stored energies and boundary misorientations for the migrating and stationary boundary segments, whereas the grain boundary normals differ significantly. Effects of grain boundary mobility and deformation microstructure morphology on the migration are discussed.

Impact of selenium addition to the cadmium-zinc-telluride matrix for producing high energy resolution X-and gamma-ray detectors

Both material quality and detector performance have been steadily improving over the past few years for the leading room temperature radiation detector material cadmium-zinc-telluride (CdZnTe). However, although tremendous progress being made, CdZnTe still suffers from high concentrations of performance-limiting defects, such as Te inclusions, networks of sub-grain boundaries and compositional inhomogeneity due to the higher segregation coefficient of Zn. Adding as low as 2% (atomic) Se into CdZnTe matrix was found to successfully mitigate many performance-limiting defects and provide improved compositional homogeneity. Here we report record-high performance of Virtual Frisch Grid (VFG) detector fabricated from as-grown Cd0.9Zn0.1Te0.98Se0.02 ingot grown by the Traveling Heater Method (THM). Benefiting from superior material quality, we achieved superb energy resolution of 0.77% at 662 keV (as-measured without charge-loss correction algorithms) registered at room temperature. The absence of residual thermal stress in the detector was revealed from white beam X-ray topographic images, which was also confirmed by Infra-Red (IR) transmission imaging under cross polarizers. Furthermore, neither sub-grain boundaries nor their networks were observed from the X-ray topographic image. However, large concentrations of extrinsic impurities were revealed in as-grown materials, suggesting a high likelihood for further reduction in the energy resolution after improved purification of the starting material.

High-Speed X-Ray Projection Microscopy at Beamline 9D of the Pohang Light Source-II

We constructed a high-speed X-ray projection microscope at beamline 9D of the Pohang Light Source-II (PLS-II). The microscope was able to take X-ray projection micrographs with a submil-lisecond exposure time by using whitebeam X-rays from the PLS-II bending-magnet synchrotron radiation. It also takes advantage of edge enhancement in phase-contrast imaging, which significantly improves the visibility of soft materials and microfluids. Using the microscope, we captured the spreading of a water droplet on a hydrophilic and oleophilic nonwoven membrane at 1,264 frames per second. The computed tomography (CT), which visualized a three-dimensional wooden structure at a spatial resolution of 2.7 µm, took 2.5 s for scanning.

Analysis of Dislocations in PVT-Grown 6H-SiC through Grazing-Incidence X-Ray Topographic Images and Ray-Tracing Simulation

The superior electronic and physical properties of silicon carbide (SiC), such as wide band-gap, high saturated electron velocity, high breakdown electric field strength, and high thermal conductivity, enable successful applications in modern power electronic devices with high power, high frequency, and high temperature operation requirements. [1,2] Therefore, understanding the nature and behavior of different dislocations is of great significance towards crystal quality enhancement. Synchrotron X-ray has been widely adopted for generating topographic images of threading screw dislocations (TSDs) as well as threading edge dislocations (TEDs) and basal plane dislocations (BPDs). Besides, recent study in 4H-SiC revealed the presence of threading mixed dislocations (TMDs) with Burgers vectors nc+ma (where m and n are integers). [3] Since direct TMDs observation methods such as transmission electron microscopy (TEM) and synchrotron white-beam X-ray topography (SWBXT) are not suitable for commercially preferred c-plane-oriented wafers, ray-tracing simulated images of possible dislocation types are compared with X-ray topographs in order to identify the TMDs non-destructively. Except 4H-SiC, ray-tracing simulation has also been proved as an effective approach for dislocation characterization in various materials such as GaN [4], ZnO [5], and AlN [6]. The same principle is implemented in this study for the investigation of dislocations in 6H-SiC wafer.Synchrotron monochromatic beam X-ray topography (SMBXT) analysis was carried out on a PVT grown 6H-SiC wafer for recording its grazing-incidence images of all six different 11-212 reflections (Fig. 1). The distribution of TSDs, TEDs, TMDs, and BPDs are characterized, and the determination of their Burgers vectors is achieved by correlating the simulated results obtained through ray-tracing simulation with real X-ray topographic observations (Fig. 2). This study provides a clear understanding of dislocations in 6H-SiC, which is crucial for quality improvement of the crystal.Reference:[1]: C. Codreanu, et al, Comparison of 3C-SiC, 6H-SiC and 4H-SiC MESFETs Performances, Materials Science in Semiconductor Processing 3 137 (2000).[2]: J. W. Sun, et al, Shockley-Frank Stacking Faults in 6H-SiC, Journal of Applied Physics 111, 113527 (2012).[3]: J. Q. Guo, et al, Direct Determination of Burgers Vectors of Threading MixedDislocations in 4H-SiC Grown by PVT Method, Journal of Electronic Materials 45, 4 (2016).[4]: B. Raghothamachar, et al, X-ray Topography Characterization of Gallium Nitride Substrates for Power Device Applications, submitted for publication.[5]: T. Y. Zhou, “Threading Dislocation Characterization and Stress Mapping Depth Profiling via Ray Tracing Technique,” Ph.D. Thesis, SUNY Stony Brook, Advisor: Michael Dudley (2015).[6]: T. Y. Zhou, et, al, Characterization of Threading Dislocations in PVT-Grown AlN Substrates via x-Ray Topography and Ray Tracing Simulation, Journal of Electronic Materials volume 43, 838 (2014). Figure 1