Double Crystal X-ray Topography Research Articles

Double-Crystal X-Ray Diffractometry and Topography Methods in the Analysis of the Real Structure of Crystals

The high efficiency of double-crystal X-ray diffractometry and topography methods in the characterization of crystals during refinement of the crystal-growth technology is demonstrated for the example of diamond bulk crystals and films obtained by the high pressure, high temperature method and by chemical vapor deposition, respectively. Techniques and study schemes for analysis of the real crystal structure, assessment of the chemical composition and the period of the crystal lattice, and analysis of the distinctive features of the deformation and thickness of thin films are described. Major structural defects (dislocations, stacking defects, inclusions of a different phase, and others) that emerge during the production of synthetic diamond crystals are identified.

Optimization of the Growth Technology for Crystals with Garnet Structure Based on X-Ray Diffraction Data

High efficiency of the methods of double-crystal X-ray diffractometry (DCXRD) and topography for improving the growth technology of highly homogeneous crystals has been demonstrated on the example of gadolinium gallium garnet (GGG) single crystals. The main types of structural defects observed in Czochralski-grown GGG crystals are found to be macroscopic inhomogeneity of composition distribution, caused by the facet effect manifestation; microinhomogeneous distribution of impurity and main components of the composition in striations; dislocations; and second-phase inclusions. The relationship between the type and density of newly formed defects and the technological conditions for crystal growth are considered. Optimization of the composition of crystals and their growth technology made it possible to obtain high-quality dislocation-free crystals of GGG and complex-substituted garnets on its basis for magneto-optical and microwave devices, elements of solid-state lasers, and other applications.

Mapping of Lattice Strain in 4H-SiC Crystals by Synchrotron Double-Crystal X-ray Topography

The presence of lattice strain in n-doped 4H-SiC substrate crystals grown by a physical vapor transport method can strongly influence the performance of related power devices that are fabricated on them. Information on the level and the variation of lattice strain in these wafer crystals is thus important. In this study, a non-destructive method is developed based on synchrotron double-crystal x-ray topography to map lattice strains in 4H-SiC wafers. Measurements are made on two 4H-SiC substrate crystals—one is an unprocessed commercial wafer while the other was subject to a post-growth high-temperature heat treatment. Maps of different strain components are generated from the equi-misorientation contour maps recorded using synchrotron monochromatic radiation. The technique is demonstrated to be a powerful tool in estimating strain fields in 4H-SiC crystals. Analysis of the strain maps also shows that the normal strain components vary much more significantly than do the shear/rotation components, indicating that lattice dilation/compression rather than lattice tilt is the major type of deformation caused by both the incorporation of nitrogen dopants and the nucleation of basal plane dislocations.

X-ray diffraction characterization of epitaxial CVD diamond films with natural and isotopically modified compositions

Comparative investigations of homoepitaxial diamond films with natural and modified isotopic compositions, grown by chemical vapor deposition (CVD) on type-Ib diamond substrates, are carried out using double-crystal X-ray diffractometry and topography. The lattice mismatch between the substrate and film is precisely measured. A decrease in the lattice constant on the order of (Δa/a)relax ∼ (1.1–1.2) × 10–4 is recorded in isotopically modified 13С (99.96%) films. The critical thicknesses of pseudomorphic diamond films is calculated. A significant increase in the dislocation density due to the elastic stress relaxation is revealed by X-ray topography.

X-ray diffraction characterization of synthetic garnet, diamond and sapphire crystals

The structural features of synthetic garnet, diamond, and sapphire single crystals are investigated via the nondestructive methods of double-crystal X-ray diffractometry and topography. The conditions of diffraction studies into these crystals performed with the help of Ge monochromators are optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) of crystals arising from their generation are revealed. It is demonstrated that the experimental techniques for studying the real structure turn out to be highly efficient in optimizing crystal-growth technology.

All-diamond optical assemblies for a beam-multiplexing X-ray monochromator at the Linac Coherent Light Source.

A double-crystal diamond (111) monochromator recently implemented at the Linac Coherent Light Source (LCLS) enables splitting of the primary X-ray beam into a pink (transmitted) and a monochromatic (reflected) branch. The first monochromator crystal, with a thickness of ∼100 µm, provides sufficient X-ray transmittance to enable simultaneous operation of two beamlines. This article reports the design, fabrication and X-ray characterization of the first and second (300 µm-thick) crystals utilized in the monochromator and the optical assemblies holding these crystals. Each crystal plate has a region of about 5× 2 mm with low defect concentration, sufficient for use in X-ray optics at the LCLS. The optical assemblies holding the crystals were designed to provide mounting on a rigid substrate and to minimize mounting-induced crystal strain. The induced strain was evaluated using double-crystal X-ray topography and was found to be small over the 5× 2 mm working regions of the crystals.

Analysis of synthetic diamond single crystals by X-ray topography and double-crystal diffractometry

Structural features of diamond single crystals synthesized under high pressure and homoepitaxial films grown by chemical vapor deposition (CVD) have been analyzed by double-crystal X-ray diffractometry and topography. The conditions of a diffraction analysis of diamond crystals using Ge monochromators have been optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) formed during crystal growth have been revealed. The nitrogen concentration in high-pressure/high-temperature (HPHT) diamond substrates is estimated based on X-ray diffraction data. The formation of dislocation bundles at the film-substrate interface in the epitaxial structures has been revealed by plane-wave topography; these dislocations are likely due to the relaxation of elastic macroscopic stresses caused by the lattice mismatch between the substrate and film. The critical thicknesses of plastic relaxation onset in CVD diamond films are calculated. The experimental techniques for studying the real diamond structure in optimizing crystal-growth technology are proven to be highly efficient.

Threading Dislocations in 4H-SiC Observed by Double-Crystal X-Ray Topography

Threading dislocations in 4H-SiC wafer were observed by using a double crystal X-ray topography technique. The incident beam was collimated by the first crystal so that the angular divergence was reduced to sufficiently smaller than the rocking curve of a diffraction of the specimen (the second crystal). At off-Bragg angles, cross-sectional projection of threading dislocations appeared on the topographic images. From these results, we found that outcrops of the screw dislocation at the wafer surfaces suffered from significant strain, giving strong contrast on the topograph even under the condition, $\vec{g}\cdot\vec{b}=0$. In addition, the edge and screw components in the mixed dislocations were clearly distinguished.

Ｘ線トポグラフィの進展 平行ビームを使ったＸ線トポグラフィによるＳｉＣの転位観察

Dislocations in silicon carbide (SiC) have been analyzed by means of double-crystal X-ray topography. Strain caused by a dislocation is sensitively observed by using the X-rays with small angular divergence. In this article, the authors describe the principle and method of the double-crystal X-ray topography, introduce SiC as a material developing for power electronics devices, and show practical observations of dislocations with high resolution.

On the role of nitrogen in stiffening the diamond structure

Three large (4–7 mm) natural diamonds, each half brown and half white, from the Argyle Diamond Mine in Western Australia, have been studied for growth history and crystal perfection. Scanning electron microscopy and X-ray single-crystal topography showed the diamonds to have good octahedral morphologies but poor internal perfection. X-ray double-crystal topography quantified this lack of perfection, rocking-curve widths taking unusually large values of 300 and 75 s of arc for the brown and white regions, respectively, in a diffraction geometry for which 3 s of arc would be expected for a perfect diamond. Fourier transform infrared spectroscopy revealed significant differences in nitrogen concentration between the brown and white regions of the diamonds. The white regions, with 400 to 600 p.p.m. nitrogen, contained over 1.6 times more nitrogen than the brown regions. It is concluded that the extra nitrogen (in A and B forms) in the white regions stiffens the lattice against distortion by natural plastic deformation.

Wafer Bonding of CdZnTe / Si Structures

In this work, we demonstrate the transfer of Cd0.96Zn0.04Te layers to Si substrates by both bond and etch back and by ion exfoliation techniques. The (211)A face was employed for the bonding face to expose the pre-ferred (211)B face for subsequent HgCdTe growth. SiN intermediate lay-ers were used to facilitate the bond process. For the bond-and-etch proc-ess, 10 mm CdZnTe layers were transferred to silicon substrates. For ion-exfoliated layers, layers of 0.3 - 1 μm were transferred. The low level of implant-induced strain in the rpe-exfoliated layer is consistent with results from other materials. After transfer, these structures exhibited no strain and exhibited high crystal quality (as determined by double crystal x-ray topography and triple axis x-ray diffraction).

Low-temperature growth and doping of InP:S single crystals from melt-solution in microgravity

The effect of microgravity on the growth of bulk InP:S single crystals from a melt with an initial equilibrium composition (84 at % In, 16 at % P, and ∼2.2 × 1018 at cm−3 of S) on board the Foton-11 satellite was investigated. The growth of crystals on board the satellite and on Earth (a reference crystal) was carried out by the traveling heater method. The samples of the grown crystals were investigated by metallography, double-crystal X-ray diffractometry, single-and double-crystal X-ray topography, and secondary-ion mass spectrometry. It is shown that the mass transfer in the melt in microgravity is similar to the diffusion mode. Hence, the mass transfer in the melt results in the following: the formation of a nonstationary boundary layer, depleted in phosphorus; the constitutional supercooling at the crystallization front accompanied with the development of a cellular substructure in the early growth stage; and the hypothetical phase structurization of the transition layer with the formation of In-based associates (clusters), which were found in the grown crystals in the form of spherical defects 10–20 μm in diameter. The coefficients of sulfur distribution k0 = 0.274 and keff = 0.43, the sulfur diffusivity in the melt DS = 4.2 × 10−7 cm2/s, and the effective thickness of the transition layer δ = 0.07 cm in terrestrial gravity are determined. The data obtained are necessary to develop a mathematical model of crystallization in zero gravity.

Observations of Lattice Distortions near Silicon Surfaces Implanted with Low-Energy Nitrogen Ions by Reflection X-ray Topography

Mirror-polished Czochralski-grown wafers with near-(100)-oriented surfaces were implanted with N+ at 8 keV energy at a dose of 1 ×1015 cm-2. Lattice distortions produced by the implantation process were observed by X-ray double-crystal topography using extremely asymmetric reflection. The intensity contrast caused by the lattice extensions in thin layers was clearly visualized. By annealing at 700°C for more than 90 min, the imperfect crystal in the ion-implanted region evidently recovers to a more perfect one, except for the boundary of the implanted region. The lattice distortions at the boundary, consisting of a narrow striated region, are thought to be due primarily to variations in the lattice spacing that has been deformed nonelastically. From a series of topographs taken by changing the glancing angle from the low-angle to the high-angle side of the Bragg peak, the differences in lattice spacing between the striated portions and the unimplanted regions were estimated.

Measurements of Minute Lattice Distortions in Silicon Crystals by X-Ray Double-Crystal Topography Using Extremely Asymmetric Reflection

Reflection X-ray topography using double-crystal arrangement has been applied to the observation and the evaluation of lattice distortions in the near-(100)-oriented surface of silicon crystals. A series of X-ray topographs were taken with extremely asymmetric 311 reflection of CuKα1 radiation at a glancing angle of approximately 0.20° near the critical angle of total reflection. The experimental condition is realized using wafers with the (100) lattice plane inclined to the surface at an angle Δα (approximately 2.6°), which usually are used as substrates for epitaxially grown films. Both the local lattice misorientation induced at the boundary of oxide-deposited films of 4–5 nm thickness on a silicon wafer and the surface distortions arising through the processes of mechanochemical polishing and epitaxial growth are clearly visualized as a result of low background noise from the bulk layer due to a shallow X-ray penetration depth of several tens nanometers. From a series of topographs taken by changing the glancing angle from a low angle to a high angle of the Bragg peak at intervals of 5 arcsec, the average tilt angle of lattice planes localized at the oxide film edges was estimated.

Stoichiometry related defects in CdTe crystals

In this work, we report on the structural analyses of undoped CdTe samples grown by the vapor phase and the Bridgman methods. Different techniques were used for determining the structural defects: wet etching, high resolution X-ray diffraction, double crystal X-ray topography and monochromatic SEM-cathodoluminescence mapping. The density and the nature of the structural defects were found to be correlated to the stoichiometry of the samples, as determined by a detailed analysis of the temperature dependence of the partial pressure of the vapors in equilibrium with the solid. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SiC substrate defects and III-N heteroepitaxy

This study addressed how defects in SiC substrates influence the crystallographic properties of AlGaN/GaN layers deposited by metallorganic vapour phase epitaxy and by molecular beam epitaxy. We employed double crystal reflection x-ray topography using symmetric (0008) and (00012) reflections with CuKα radiation (λ = 1.54 Å) to image dislocations, micropipes, and low angle boundaries in SiC substrates. Lattice strain near the core of a micropipe defect was estimated to be of the order of 10−7. The substrates investigated exhibited radial patterns of strain and, primarily, of tilt of the order of tens of arcsec.After deposition of the AlGaN and GaN layers, DCXRT images were generated from the substrate (0008) or (00012) and GaN epitaxial layer (0004) reflections. Full-width at half-maximum values ranging from ∼100 to 300 arcsec were typical of the GaN reflections, while those of the 4H–SiC reflections were ∼20–70 arcsec. Micropipes, tilt boundaries, and inclusions in the SiC were shown to produce structural defects in the GaN layers. A clear correlation between SiC substrate defects and GaN defects has been established.

Defect-induced luminescence in high-resistivity high-purity undoped CdTecrystals

This work focuses on the influence of stoichiometry deviation (Cd/Te excess in thecrystal) on the type and density of crystalline defects. In particular, we report astudy on the evaluation of extended defects and precipitates, carried out byspecific preferential etching, high-resolution x-ray diffraction, double-crystal x-raytopography and cathodoluminescence (CL) analyses. A remarkable differencebetween the crystals grown from the vapour phase and from the melt has beenfound. Dislocations were found to arrange in cellular structures 2–300 μm in diameter while precipitates were homogeneouslydistributed on the growth plane. The CL spectra show an intensenear-band-edge emission through the whole range of temperatures andtwo large emission bands centred at 1.2 and 1.4 eV. It has beenobserved that the luminescence intensity of the 1.4 eV band increasesclose to crystal defects and simultaneously the near-band-edgeintensity decreases. The disappearance of this emission both in theCL spectra and in the CL images on increasing the temperature up to300 K suggests a donor–acceptor pair nature for the transitioninvolved.

Effect of Si doping on the relaxation mechanism of InGaAs on GaAs

We report measurements on the initial stages of relaxation of Si-doped In0.04Ga0.96As epitaxial layers on (001) GaAs using in situ high-resolution double-crystal x-ray topography during molecular beam epitaxial growth. For Si concentrations up to 5×1018 cm−3, the critical thickness for formation of the first B(g) misfit dislocations is modeled accurately by the Matthews–Blakeslee model, extended to include a lattice friction force varying linearly with the dopant concentration. Below a Si concentration of 2×1018 cm−3, the model can be used to predict the critical thickness for generation of the orthogonal A(g) dislocation set, with the x-ray topographs showing that the B(g) misfit multiplication occurs at damaged edges of the wafer. However, above 2×1018 cm−3 Si concentration, the critical thickness for A(g) nucleation becomes almost independent of concentration and the x-ray topographs show that cross slip becomes important in the multiplication process. For most of the Si concentrations examined, the critical thickness for nucleation of the slow A(g) misfit dislocations corresponded to that of multiplication of the fast B(g) set.

Stroboscopic topographies on iron borate crystal in 9.6 MHz rf magnetic field

The influence of magnetoacoustic wave on the crystal deformation was studied by stroboscopic double crystal X-ray topography. The acoustic wave was excited by the rf magnetic field, which was synchronized with synchrotron radiation X-ray pulse. In measured rocking curves of FeBO 3(4 4 4) reflection, we observed, for the first time, that the application of rf magnetic field (| H rf| max>8.4 Oe) brought about the extreme narrowing of full width at half maximum (FWHM). Recorded topographs showed that the narrowing of FWHM was due to the magnetoacoustic standing wave which is excited in FeBO 3 crystal. In our experiments, the influence of additional static magnetic field on the magnetoacoustic standing wave of FeBO 3 crystal was investigated too.

Misfit dislocation and threading dislocation distributions in InGaAs and GeSi/Si partially relaxed heterostructures

Single and compositionally graded InGaAs/GaAs and SiGe/Ge partially relaxed heterostructures of equivalent lattice mismatch and thickness have been characterized by high resolution X-ray diffraction and by X-ray double crystal topography. X-ray topography evidences a rapid decrease of the average length of misfit dislocation segments with the increase of the density of misfit dislocations both in InGaAs and GeSi/Si structures. For an equivalent amount of strain release, SiGe/Si heterostructures exhibit shorter misfit dislocation lengths with respect to InGaAs/GaAs heterostructures. Compositionally graded InGaAs/GaAs heterostructures show much longer misfit dislocation segments with respect to equivalent single heterostructures, thus confirming the effectiveness of composition grading to reduce the dislocation interaction. The threading dislocation density evaluated from the lattice mismatch and the average length of the misfit dislocations on the basis of a simple formula increases more rapidly as a function of strain release in GeSi/Si heterostructures with respect to InGaAs/GaAs ones.