Projection Topographs Research Articles

Role of threading dislocations on the growth of HgCdTe epilayers investigated using monochromatic X-ray Bragg diffraction imaging.

High-quality Hg1-xCdxTe (MCT) single crystals are essential for two-dimensional infrared detector arrays. Crystal quality plays an important role on the performance of these devices. Here, the dislocations present at the interface of CdZnTe (CZT) substrates and liquid-phase epitaxy grown MCT epilayers are investigated using X-ray Bragg diffraction imaging (XBDI). The diffraction contributions coming from the threading dislocations (TDs) of theCZT substrate and the MCT epilayers are separated using weak-beam conditions in projection topographs. The results clearly suggest that the lattice parameter of the growing MCT epilayer is, at the growth inception, very close tothat of the CZT substrate and gradually departs from the substrate's lattice parameter as the growth advances. Moreover, the relative growth velocity of theMCT epilayer around the TDs is found to be faster by a factor of two to fourcompared with the matrix. In addition, a fast alternative method to the conventional characterization methods for probing crystals with low dislocation density such as atomic force microscopy and optical interferometry is introduced. A 1.5mm × 1.5 mm area map of the epilayer defects with sub-micrometre spatial resolution is generated, using section XBDI, by blocking the diffraction contribution of the substrate and scanning the sample spatially.

Three-dimensional X-ray topographic characterization of synthesized diamond crystals

Synthesized diamond crystals grown at high temperatures and high pressures were characterized by three-dimensional X-ray topography. The diffraction planes and wavelength of the monochromatic X-rays used were (004), {111}, and , and 0.0521 nm, respectively. Images of lattice defects in diamond crystals were reconstructed from a stack of about 300 X-ray limited projection topographs using the image processing software Image-J. The three-dimensional structures and nature of lattice defects were identified on the basis of the reconstructed topographs. The pyramidal shape of the four-part stacking fault generated from the seed crystal was identified using the dependence of visible or invisible defect images with diffraction planes. The sector boundary between the [001] and [111] growth directions was also first observed by three-dimensional topography. The image of the sector boundary did not show any dependence on diffraction planes.

X-ray white beam topography of self-organized domains in flux-grownBaTiO3single crystals

The phenomenon of self-organization of domains into a ``square-net pattern'' in single-crystal, flux-grown $\mathrm{BaTi}{\mathrm{O}}_{3}$ several degrees below the ferroelectric to paraelectric phase transition was investigated using in situ synchrotron x-ray topography. The tetragonal distortion of the crystal was determined by measuring the angular separation between the diffraction images received from 90\ifmmode^\circ\else\textdegree\fi{} $a$ and $c$ domains in the projection topographs, and shows a rapid decrease towards 110 \ifmmode^\circ\else\textdegree\fi{}C, the onset temperature for self-organization. The onset of self-organization is accompanied by bending of the ${100}$ lattice planes parallel to the crystal surface, which produces a strain that persists up to and beyond the Curie temperature, where the crystal becomes cubic and the self-organized domains disappear. At the Curie point, the bending angle ${\ensuremath{\alpha}}_{100}=8.1(\ifmmode\pm\else\textpm\fi{}0.3)\phantom{\rule{0.16em}{0ex}}\mathrm{mrad}$ is at a maximum and corresponds to the radius of curvature of the surface being 16.3(\ifmmode\pm\else\textpm\fi{}0.6) mm.

High‐resolution X‐ray diffractometric, topographic and reflectometric studies of epitaxial layers on porous silicon destined for exfoliation

The presently studied substrates for manufacturing freestanding silicon epitaxial layers contained two porous layers with different porosities. The upper layers of significantly lower porosity after annealing provided smooth surfaces for deposition of epitaxial layers and the deeper layers of greater porosity enabling easier exfoliation epitaxial foil. The layered structures have been studied by means of synchrotron diffraction topography and by recording of rocking curves from small local area. The most effective method of revealing defects in freestanding exfoliated layers were Bragg‐case projection topographs and ‘zebra’ pattern monochromatic beam topographs providing a map of sample curvature. The samples have been additionally examined by means of high‐resolution diffractometry and reflectometry.The porous layers without epitaxial deposition provided strong narrow maximum at low angle side of the substrate maximum common for both of the porous layers. The most probable effect of the deposition of epitaxial layer is the decrease of lattice parameter in the upper layer with lower porosity, which provided the maxima in the diffraction curves located on low angle side of the substrate maximum. The topographic investigations revealed some defects in deposited epitaxial layers, connected with some irregularities of the annealed porous layer. Very characteristic common defects were large caverns, overgrown in the epitaxial process with the diameters of several tenths of micrometers and defects (possibly the overgrown submicron caverns), associated with some bundles of dislocations resolved in the topographs. In layers of large thickness, the characteristic long black and white stripes along <110> have also been observed, which most probably were stacking faults with dislocation loops. Copyright © 2015 John Wiley & Sons, Ltd.

X-ray diffraction projection topography: Possibilities of the quantitative analysis of images of defects

We propose methods for obtaining quantitative data on the parameters of defects that are observed on projection topographs and determining the main parameters (the displacement-field value and character, the depth of the occurrence of defects under the input surface of the crystal, their spatial orientation, and other parameters) of defects from the characteristics of a diffraction image that is recorded on projection topographs. The possibility of determining the main characteristics of defects through the analysis of diffraction images on projection topographs is shown using the example of rectilinear dislocations introduced into a silicon monocrystal upon plastic bending.

X-ray topography studies of dislocations in single crystal CVD diamond

X-ray topography has been used to study single crystal diamond samples homoepitaxially grown by microwave plasma-assisted chemical vapour deposition (CVD) on high pressure high temperature (HPHT) and CVD synthetic diamond substrates. Clusters of dislocations in the CVD diamond layers emanated from points at or near the interface with the substrate. The Burgers vectors of observed dislocations have been determined from sets of {111} projection topographs. Dislocations have line directions close to the [001] growth direction and are either edge or 45° mixed dislocations. Where groups of dislocations originated at isolated points they tended to be of the edge variety. Where the substrate surface was deliberately damaged before growth, two sets of dislocations were observed to have propagated from each line of damage and there was a tendency for dislocations to be of the 45° mixed variety with a component of their Burgers vector parallel to the polishing direction. It is demonstrated that X-ray topography can be used to deduce the growth history of CVD synthetic diamond samples produced in multiple growth stages.

Synchrotron white beam topography studies of SrLaGaO 4 crystals

Strontium lantanum gallate SrLaGaO 4 tetragonal single crystal was investigated by white beam synchrotron radiation topography. Projection and section topographs were taken in back reflection and transmission geometry. The central “core” crystal region was practically free of defects; only one extended “oval” defect with strong boundary contrast was observed. The strong white–black contrasts connected with elongated volume defects and cracks were observed in surrounding the “core” region.

Synchrotron X‐ray investigation of La0.3Sr0.7Al0.65Ta0.35O3 crystals

AbstractThe La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrates were studied with a number of X‐ray diffraction methods exploring both synchrotron white and monochromatic beam. The observation of selective etching patterns was performed in neighbouring samples. White beam back‐reflection projection topographs revealed only the segregation fringes around the central core and a delicate grain‐like structure. The most important for verification of dislocation presence were the transmission projection and section topographs. These topographs also did not reveal any images of extended dislocation but only the growth bands and tiny grain‐like structure. The section topographs provided some details of paroboloidal growth surface inside the crystal. It may be concluded that the investigated crystal probably contains small coherent inclusions responsible for the etch pits and the mentioned grain‐like structure. The synchrotron rocking curves revealed a number of maxima, which might be attributed to the regions with different lattice parameter, connected with segregation fringes. The sequence of maxima was different for different regions of the sample. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of Structural Imperfections in Natural Type I Diamonds by High-Resolution X-ray Diffraction Techniques

Results of characterization of defects in natural diamond crystals by high-resolution X-ray diffractometry, absolute integrated intensity (ρ) measurements, topography and diffuse X-ray scattering (DXS) techniques are reported. The samples are 〈111〉 natural diamond platelets of dimensions ~2 × 2 mm with thickness in the range ~0.5–1 mm. A five-crystal X-ray diffractometer in three-crystal configuration as well as a four-crystal diffractometer were employed with Mo Kα1 as the exploring radiation. Infrared absorption measurements showed that the samples belong to type Ia variety and contained varying concentrations of A and B forms of nitrogen aggregates as well as platelets. There were large variations in the values of diffraction curve half-widths (18 to 550′′) and values of ρ (2.4 × 10−5–76 × 10−5 rad) for 111 reflection, showing a wide variation in crystalline perfection. Projection and composite stationary X-ray topographs recorded with different diffraction vectors showed the presence of defects like low-angle boundaries. From the analysis of the observed distribution of diffuse X-ray scattering (DXS), point defect clusters were characterized. The clusters were of interstitial as well as vacancy type. The sizes of the defect clusters (Rcl) were determined to be in the range 40 to 190 nm and the volumes of the defect clusters (Acl) were in the range ~1 × 10−25–28 × 10−25 m3. The defect clusters with sizes reported here could be investigated as the measurements were made close to the reciprocal-lattice points. The platelet size was found to vary inversely with the total concentration of nitrogen.

Hollow-core screw dislocations in 6H-SiC single crystals: A test of Frank’s theory

Hollow-core screw dislocations, also known as “micropipes”, along the [0001] axis in 6H-SiC single crystals, have been studied by synchrotron white beam x-ray topography (SWBXT), scanning electron microscopy (SEM), and Nomarski optical microscopy (NOM). Using SWBXT, the magnitude of the Burgers vector of screw dislocations has been determined by measuring the following four parameters: (1) the diameter of dislocation images in back-reflection topographs; (2) the width of bimodal dislocation images in transmission topographs; (3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and (4) the magnitude of the tilt of lattice planes in section topographs. The four methods show good agreement. SEM results reveal that micropipes emerge as holes on the as-grown surface, with their diameters ranging from about 0.1 to a few micrometers. Correlation between topographic images and SEM micrographs shows that micropipes are hollow-core screw dislocations with Burgers vector magnitudes from 2c to 7c (c is the lattice parameter along the [0001] axis). There is no empirical evidence that 1c dislocations have hollow cores. The Burgers vector magnitude of screw disloca-tions, b, and the diameter of associated micropipes, D, were fitted to Frank’s prediction for hollow-core screw dislocations: D = μb2/4π2γ, where μ is shear modulus, and γ is specific surface energy. Statistical analysis of the relationship between D and b2 shows that it is approximately linear, and the constant γ/μ ranges from 1.1 × 10−3 to 1.6 × l0−3 nm.

Quantitative analysis of screw dislocations in 6H−SiC single crystals

Screw dislocations along the [0001] axis in 6H−SiC single crystals have been studied extensively by Synchrotron White-Beam X-ray Topography (SWBXT), Scanning Electron Microscopy (SEM), and Nomarski Optical Microscopy (NOM). Using SWBXT, the magnitude of the Burgers vector of screw dislocations has been determined by measuring the following four parameters: 1) the diameter of dislocation images in back-reflection topographs; 2) the width of bimodal dislocation images in transmission topographs; 3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and 4) the magnitude of the tilt of lattice planes in section topographs. The four methods show good agreement. SEM results reveal that micropipes in the form of hollow tubes run through the crystal emerging as holes on the as-grown surface, with their diameters ranging from about 0.1 to a few micrometers. Correlation between topographic images and SEM micrographs shows that micropipes are screw dislocations with Burgers vector magnitudes from 2c to 7c (c is the lattice constant along the [0001] axis). There isno were fitted to Frank’s prediction for hollow-core screw dislocations:D =μb2/4π 2 γ, where μ is the shear modulus, and γ is the specific surface energy. Statistical analysis of the relationship betweenD and b2 shows that it is approximately linear, and the constant, γ/μ, obtained from the slope, ranges from 1.1×10−3 to 1.6×10−3 nm.

Applications of Synchrotron Radiation to Defect Characterization and Pendellösung Fringe-Spacing Measurement in a Natural Diamond

A small polished rectangular block of natural diamond unusually free from impurity zoning was examined by birefringence and synchrotron X-ray topography to map internal defects and assess effects of some accidental mechanical damage. ;Orthogonal-view' projection topographs facilitated comparisons between optical and X-ray images. Speedy and precise positioning of crystal sections cut in synchrotron X-ray section topographic studies was aided by radiographic and fluoroscopic procedures, which are described. Several X-ray reflection and refraction phenomena are illustrated, including high-contrast refraction images of crystal edges, chipping and crack outcrops. It was demonstrated that by combining optical and X-ray goniometry it was possible to record Pendellösung fringe systems employing precisely selected X-ray wavelengths in well defined crystal volumes. Valuable gain in fringe visibility resulted from the use of pure sigma-mode polarized synchrotron radiation. Absolute values of the diamond 004 structure factor were derived from five fringe systems measured, the findings (in upward order) being 11.7, 11.7, 12.0, 12.5, 12.6. Their scatter greatly exceeds the uncertainty in the measurement procedure (believed to be within +/-1%) and is attributed to different strain distributions within the five volumes sampled.

Determination of Burgers Vector of Screw Dislocations in 6H-SiC Single Crystals by Synchrotron White Beam X-Ray Topography

AbstractIndividual screw dislocations along the [0001] axis in 6H-SiC single crystals have been characterized by means of Synchrotron White Beam X-ray Topography (SWBXT). The magnitude of the Burgers vector was determined from: (1) the diameter of circular diffraction-contrast images of dislocations in back-reflection topographs, (2) the width of bi-modal images associated with screw dislocations in transmission topographs, (3) the magnitude of the tilt of the lattice planes on both sides of dislocation core in projection topographs, and (4) also the magnitude of the tilt of the lattice planes in section topographs. All of the four methods showed reasonable consistency. The sense of the Burgers vector can also be deduced from the abovementioned tilt of the lattice planes. Results revealed that in 6H-SiC a variety of screw dislocations can be found with Burgers vector magnitude ranging from 1c to 7c (c is the lattice constant along [0001] axis). This work demonstrates that SWBXT can be used as a quantitative technique for detailed analyses of line defect configurations.

The peculiarities of the local thermal diffusivity of large synthetic diamonds

A series of large diamond single crystals was prepared using FeNi solvent-catalyst by the temperature gradient method. Local thermal diffusivity was measured by an original experimental technique and the results obtained were: 2.2 to 8.0 cm 2/s for nitrogen-containing samples and ∼ 11 cm 2/s for nitrogen-free samples (grown with Ti getter). To explain the observed peculiarities of the distribution of the local thermal diffusivity, visible-IR absorption spectra, photo- and cathodoluminescence in the visible range and X-ray projection topographs were studied. The body of the results obtained enabled us to suggest that, in synthetic diamonds of the type studied, the main contribution to the thermal resistance is made by phonon scattering on defects including Ni ions in their structure.

Dislocation images in X-ray section topographs of curved crystals

The contrast of dislocation images in X-ray section topographs of curved crystals has been investigated both experimentally and by computer simulation. Good agreement is found between experiment and simulation in both symmetric and asymmetric Laue geometries. Very little change is observed in the symmetric Laue images for radii of curvature as small as 18 m. In the asymmetric geometry, the background intensity rises and the Kato fringe visibility decreases as the curvature increases. The dynamical image becomes more localized and dominates the image as the direct image visibility falls. Our results explain the reversal of dislocation contrast in Lang projection topographs of curved crystals taken in asymmetric geometry.

X-ray topographic observations of 180° magnetic domain walls in (001) plates of nearly perfect iron-silicon alloy single crystals

Abstract Contrary to generally-held views, 180° magnetic domain walls lying in the cube planes normal to (001)-orientation single-crystal plates of Fe + 3wt%Si can be detected by diffraction contrast on X-ray projection topographs by use of conventional experimental arrangements. Conditions for good observations are explained with illustrations. The dominant contrast appears at intersections of the 180° Bloch walls with the X-ray exit surface of the specimen and is generated by small localized deformations at these outcrops. The contrast is at best very weak, being only about 10−2 of that generated by 90° domain walls.

Sophisticated equipment developed for growth and evaluation of perfection of nearly perfect crystals

This paper reviews four major equipment developed at the National Physical Laboratory for growth and perfection evaluation of single crystals, namely (i) a crystal puller for growth of nearly perfect crystals by the Czochralski method; (ii) a microfocus x-ray generator; (iii) an x-ray diffraction topography camera; and (iv) a triple crystal x-ray diffractometer. The crystal puller can provide smooth, uniform and variable pulling rates. The maximum length of pull is nearly 60cm. Efforts have been made to isolate vibrations. Nearly perfect single crystals of KCl, KBr and NaCl with maximum diameter of ∼ 60 mm have been grown. The crystals give diffraction curves with half width in the range of 10–30 sec of arc. In the projection topographs, dislocations can be resolved and characterized. The microfocus x-ray generator is a demountable continuously evacuated system with specially designed electron gun and anode assembly. The vacuum is continuously monitored for ease of maintenance. In the point focus mode the spot size is 40 µm on the anode. X-ray topography system is a versatile equipment used for projection and section topography. It can provide 360° rotations to the specimen disc around an axis perpendicular to it. Rotations of a few sec of arc can be given to the specimen around a vertical axis. Typical diffraction curves of a dislocation-free crystal and a crystal with boundaries are shown. Well-resolved images of dislocations are shown in a topograph as an illustration. In the triple crystal x-ray diffractometer a highly collimated and monochromated Kα1 exploring x-ray beam is obtained by combining microfocus source, a special collimator and crystal monochromators of Bonse-Hart type. With this beam very narrow diffraction curves with half width of about 5 sec of arc can be recorded. Typical results of measurement of diffuse x-ray scattering (dxs) on nearly perfect silicon single crystals are discussed. It has been observed that the contribution of phonons to thedxs is negligible. Thedxs is mainly due to point defect aggregates.

X-ray topography of natural beryl using synchrotron and conventional sources

Continuous-spectrum synchrotron-radiation Laue transmission topographs of natural beryl have been compared with the corresponding projection topographs taken with conventional apparatus and Cu Kα 1 and Mo Kα 1 radiations in an analysis of configurations of grown-in lattice defects and of the diffraction contrast they produce. The natural defects studied included grown-in dislocations, impurity zoning and fault surfaces. A growth-sector map was constructed for the (0001) specimen slice examined. Differences of contrast among the various reflections appearing on the synchrotron-radiation topographs are discussed and it is demonstrated that the contrast characteristics of the synchrotron-radiation images which consist of a superimposition of 3{\bar 2\bar 1}1 and 6{\bar 4 \bar 2}2 reflections can be satisfactorily synthesized by a weighted superimposition of the 3{\bar 21}1 image recorded with Cu Kα 1 and the 6{\bar 4\bar 2}2 with Mo Kα 1. Synchrotron-radiation damage to the crystal manifested itself on the X-ray topographs both as area contrast and as contrast at the periphery of the irradiated area, yet no visual absorption or birefringence due to this damage was detected. Annealing at 770 K for 3 h completely removed the contrast due to radiation damage.

X-ray topographic analysis of dislocation line defects in calcium fluoride crystals

Calcium fluoride crystals have been cleaved along {111} faces and the freshly cleaved faces have been chemically etched in 2.6 N nitric acid solution and studied optically. One-to-one correspondence of dislocation etch pits has been established on the matched cleavage faces and on the opposite sides of thin flakes. The same crystal has been studied using an X-ray topographic technique. By selecting 0 2 2 and 02 2 reflections using Mo Kα 1 radiation, stereo pair projection topographs were studied and critically compared with the optical micrographs. The close resemblance between dislocation etch pits and dislocation outcrop images has been established. The orientations of the Burgers vectors of the dislocation lines have been identified using Ag Kα 1 radiation at 11 1 , 1 1 1 and 1 11 reflections. By using the dislocation contrast condition it has been confirmed that Burgers vectors of dislocation lines lie parallel to 〈110〉 directions. The growth history of the stratigraphical pattern has been studied using the X-ray topographic technique and the implications are discussed.

Optical and X‐ray Topographic Studies of Barium Fluoride Single Crystals

AbstractSingle crystals of synthetic barium fluoride have been cleaved along {111} planes and the freshly cleaved faces have been etched in (1 : 250) concentration of nitric acid solution. One to one correspondence of dislocation etch pits have been observed on matched pairs. The same crystal has been investigated using X‐ray topographic technique. By selecting 0222 and 0222 reflections and MoKα1 radiation, stereopair projection topographs have been obtained and critically compared with the optical micrograph. One‐to‐one correspondence between dislocation out crop images and dislccation etch pits have been established. Due to dislocation contrast, the outcrop of dislocation images have been observed with the associated dislocation etch pits. The orientation of the Burgers vectors of the dislocation lines have been identified using AgKα1 radiation with 1111, 1111 and 1111 reflections and the implications have been discussed.