Double-crystal Diffractometry Research Articles

Nanoscale tensile strain in perfect silicon crystals studied by high-resolution X-ray diffraction

A device was designed, built, and tested to apply small tensile strain to perfect single silicon crystals. It was used on the second axis of a double crystal diffractometer to obtain controllable strain fields. The strain field quality was evaluated by double crystal X-ray diffractometry. The dependence of atomic plane distances on applied stress was determined. Stress-strain curves were obtained from fitted rocking curves in the Bragg-Bragg and Bragg-Laue configurations. These results show that it is possible to obtain a tensile strained lattice with quality suitable for X-ray optics.

Distribution of Deformations in the Oscillating X-Ray Acoustic Element Based on the X-Cut Quartz Crystal

The mechanical deformations in the quartz X-ray acoustic element arising by the excitation of resonant thickness ultrasound oscillations are investigated experimentally and theoretically. Experiments were carried out using the X-ray topography and double-crystal diffractometry, and theoretical computations were performed using the mathematical modeling by the finite element method. It follows from the computations that the mechanical deformation arising in the resonator as a result of the excitation of thickness variations, is strongly stratified and represents a series of maxima, the distance between which depends on the crystallographic direction. The large number of nodes and antinodes of the oscillations directly depends on the resonant frequency of the X-ray acoustic element. The obtained experimental results are in good agreement with theoretical computations.

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.

Two-wave approximation in surface effects in asymmetric Laue crystals

This paper is a study of surface effects, e.g. roughness or asymmetrical cut, in the Laue diffraction of X-rays by crystals, based on the Takagi-Taupin equations. By means of Riemann-Green integrals, first a formal solution has been obtained when the entrance and the exit surfaces are arbitrary. Then a coordinate transformation mapping a propagation domain with arbitrary boundaries into a rectangular domain with straight boundaries is given. Potential measurement errors in \gamma-ray wavelength and silicon lattice-parameter measurements by double-crystal diffractometry and X-ray interferometry, respectively, are outlined and anticipated by studying, in the two-wave approximation, the reflection peak shift and extra phase originating from an asymmetrically cut crystal. A relationship between analyser displacement, interferometer-signal phase and relative uncertainty in lattice-parameter measurement is also given.

Structural features of Ge(Ga) single crystals grown by the floating zone method in microgravity

Structural features of the Ge(Ga) single crystal grown by the floating zone (FZ) method in microgravity environment aboard the FOTON-9 spacecraft are investigated by methods of X-ray topography, double-crystal diffractometry, selective chemical etching and spreading resistance measurements. It is established that the crystal structure is characterized by the presence of an incompletely melted region and defects caused by its formation. Growth striations revealed in regrown part of the crystal, testify to development of non-stationary capillary Marangoni convection in melt at the realized parameters of FZ remelting under space conditions. Periodicity of the growth striations is compared to frequency characteristics of heat flux pulsations through the crystallization front, found as a result of numerical simulation of melt hydrodynamics.

Investigation on Large-Sized Sapphire Crystalline Perfection by X-Ray Double-Crystal Diffraction Method

The large-sized sapphire (Ø225×205 mm, 27.5 kg) was grown successfully by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at cooled center). The surface quality of the specimens was characterized by micro-Raman spectroscopy, and double crystal X-ray diffractometry (DCD) was utilized to investigate its crystalline perfection. The measurement of rocking curves was performed on various specimens from different region of large sapphire boule. The experimental results showed that CMP (chemo-mechanical polishing) with subsequent suitable chemically etching can develop the best-quality sapphire crystal surface and the values of FWHM obtained by conventional DCD were in the range from 27” to 58”. The infrared spectral transmission (2.0-4.5 5m) of sapphire crystal exceeded 82%. It is confirmed of SAPMAC growth method characteristics with in-situ annealing, small temperature gradient and low residual stress level by numerical simulation analysis.

Evolution of the microdefect structure in silicon on isothermal annealing as determined by X‐ray diffractometry

AbstractThe dynamical theory of X‐ray diffraction by imperfect crystals with randomly distributed microdefects, applicable to double‐crystal diffractometry (DCD), has been extended to account for the simultaneous presence of several types of microdefects. The obtained formulae for coherent and diffuse components of the rocking curve (RC) have been applied to the quantitative characterization of the complex microdefect structure in silicon samples annealed at 750 °C for various time intervals by using high‐resolution double‐crystal X‐ray diffraction measurements. Time dependencies of concentrations and average sizes of oxygen precipitates and dislocation loops have been determined by using characterization results. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved quality of InGaN/GaN multiple quantum wells by a strain relief layer

Different InGaN/GaN multi quantum wells (MQWs) structures were grown by metalorganic chemical vapor deposition (MOCVD). Samples were investigated by photoluminescence (PL), atom force microscopy (AFM) and double crystal X-ray diffractometry (DCXRD) to character their optical, morphological and crystal properties. By inserting the strain relief layer, the PL intensity was increased more than two times. The surface morphology was improved and the density of V-pits was reduced from 16–18×10 8 to 6–7×10 8/cm 2. Further, the interface abruptness was also improved. We attributed the improvements of the quality of InGaN/GaN MQWs to the relief of strain in the InGaN/GaN MQWs.

Thickness dependent formation and properties of GdSi2/Si(100) interfaces

Epitaxial and polycrystalline orthorhombic GdSi2 films were grown on Si(100) substrates by solid phase reaction between Si and Gd films at different thicknesses of the Gd film . The most important property of these GdSi2/Si interfaces was defect formation. This was investigated by studying the properties of the Schottky barriers by means of current voltage and capacitance–voltage characteristics, deep level transient spectroscopy by double crystal X-ray diffractometry, and transmission electron microscopy. Epitaxial growth of the silicide layer ensured a relatively low interface defect density (about 1010 cm-2), while the non-epitaxial growth induced defects of a much higher density (about 1012 cm-2). The defects generated during the silicide formation are located within a depth of about 10 nm from the GdSi2/Si interface.

Lattice strain effects in the measurement of the Si lattice parameter by Laue-case double-crystal diffractometry

The measurement of the (220) Bragg-plane spacing of Si by Laue-case double-crystal diffractometry with an uncertainty lower than 10−8requires a detailed theoretical framework that includes the study of lattice strain. In the present paper, the propagation of X-rays through a deformed crystal is re-examined and the influence of a constant strain gradient on the centre of the reflection domain is studied by means of Takagi's equations. Their analytical and numerical solutions indicate that the measured lattice spacing refers to the crystal entrance surface.

Structural, electrical, and optical analysis of ion implanted semi-insulating InP

Semi-insulating InP was implanted with MeV P, As, Ga, and In ions, and the resulting evolution of structural properties with increased annealing temperature was analyzed using double crystal x-ray diffractometry and cross sectional transmission electron microscopy. The types of damage identified are correlated with scanning spreading resistance and scanning capacitance measurements, as well as with previously measured Hall effect and time resolved photoluminescence results. We have identified multiple layers of conductivity in the samples which occur due to the nonuniform damage profile of a single implant. Our structural studies have shown that the amount and type of damage caused by implantation does not scale with implant ion atomic mass.

The influence of Al xGa 1 −xN intermediate buffer layer on the characteristics of GaN/Si(1 1 1) epitaxy

The characteristics of GaN/Si(1 1 1) epitaxial layers grown by metalorganic vapor phase epitaxy(MOVPE) with Al x Ga 1 −x N/AlN composite buffer layer(CBL) where x varies from 0.07 to 0.15 in Al x Ga 1 −x N were investigated. The x values of Al x Ga 1 −x N grown with various trimethyl-gallium(TMG) flow rates while feeding trimethyl-aluminum(TMA) flow constant were determined by triple crystal X-ray diffractrometry(TCXRD). The surface images of GaN/Si(1 1 1) epitaxy observed by SEM show some cracks, which are due to thermal mismatch between GaN and Si. According to double crystal X-ray diffractometry (DCXRD) rocking curves of GaN(0002), the crystal quality of GaN/Si(1 1 1) grown on Al 0.11Ga 0.89N/AlN CBL exhibits the best result with FWHM of 886 arcsec. PL spectrum at RT of the GaN/Si(1 1 1) epitaxy grown on the CBL shows a sharp band edge emission peak at 365 nm with average FWHM of 45 meV, which is compared with the best result achieved so far for GaN/Si. Therefore, it can be concluded that the Al x Ga 1− x N/AlN CBL having suitable Al mole fraction of Al x Ga 1− x N layer plays an important role in improving the quality of GaN/Si(1 1 1) epitaxy by reducing the lattice and thermal mismatch between GaN and Si(1 1 1).

X-ray diffraction determination of the interface structure of CdSe/BeTe superlattices

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

High-quality GaN/Si(1 1 1) epitaxial layers grown with various Al 0.3Ga 0.7N/GaN superlattices as intermediate layer by MOCVD

We have grown and characterized GaN/Si(1 1 1) epitaxial layers having different total thicknesses and periods of Al 0.3Ga 0.7N/GaN superlattices using metalorganic chemical vapor depositon. When the total thickness of Al 0.3Ga 0.7N/GaN superlattice is 60 nm, GaN/Si(1 1 1) epitaxy shows the best surface and crystal quality. Under the same total thickness of Al 0.3Ga 0.7N/GaN superlattice of 60 nm, as the number of periods of Al 0.3Ga 0.7N/GaN superlattices increases, the quality of GaN/Si(1 1 1) epitaxy is also improved. GaN/Si(1 1 1) epitaxy grown with optimized Al 0.3Ga 0.7N/GaN superlattice of total thickness of 60 nm and 20 periods, shows crack distance of about 120 nm between cracks, FWHM of double crystal X-ray diffractometry rocking curve for GaN(0 0 0 2) of 690 arcsec and FWHM of photoluminescence at RT of 34.49 MeV, which is compared with the best results reported for GaN/Si(1 1 1), so far. Therefore, we obtained a high-quality and nearly crack-free GaN/Si(1 1 1) epitaxy by optimizing Al 0.3Ga 0.7N/GaN superlattice, possibly for the applications of LED or other photon devices, and, we found that the optimized total thickness and periods of Al 0.3Ga 0.7N/GaN superlattice play a very important role in the improvement of quality and reducing cracks in the growth of GaN/Si(1 1 1) epitaxy.

Floating zone growth of high-quality SrTiO 3 single crystals

Strontium titanate single crystals 15–20 mm in diameter and 40–80 mm in length were grown by a floating zone method with radiation heating. Additional crystal heating just below the molten zone by an in-growth annealing furnace was applied in order to lower the temperature gradients and to achieve slower cooling of the grown crystal. The crystal perfection was studied with X-ray topography and double-crystal diffractometry. The most perfect crystals were grown in [0 0 1] direction with single grain rocking curve widths of about 30″ and subgrain misorientations of 1′–3′ over 10×10 mm 2 areas of the boule cross-section for both (0 0 1)-, (1 1 0)- and (1 1 1)-oriented slices. Such high-quality crystal can be grown reproducibly with starting materials of 4N grade quality.

Elastic deformation in a crystal plate where lattice–parameter mismatch is present between adjacent growth sectors. II. Measurement of lattice tilts by synchrotron X–ray reticulography

Synchrotron X–ray reticulography measures lattice–plane orientation variations point by point on the crystal with microradian resolution capability. A fine–scale X–ray–absorbing mesh placed close to the crystal in the back–reflected diffracted beam of ‘white’ X–rays splits the beam into an array of microbeams whose direction differences are measured from the mesh–bar shadow shifts that result from controlled changes of the distance between the mesh and the recording high–resolution photographic plate. In the present work, lattice tilts were measured on either side of a growth–sector boundary outcropping at the surface of a single–crystal synthetic diamond cut and polished parallel to (110) in which the displacements and strains due to lattice–parameter mismatch at coherent growth–sector boundaries have been analysed in part I. Attention was concentrated on a crystallographically well–defined planar segment of a boundary separating two sectors, each of uniform, but significantly differing, concentration of substitutional nitrogen impurity, and for which the anisotropic elasticity theory of part I provided quantitative relations between the lattice–parameter relative difference, Δa/ā, and the lattice tilts due to coherency strains at the boundary. This enabled a novel method for determining Δa/ā to be realized, using reticulographic measurements of tilts of near–surface lattice planes alone. The paper explains the reticulographic technique fully. Factors affecting the quality and reliability of reticulographic records are examined in detail. For the growth sectors concerned, the reticulographic measurement Δa/ā = (1.10 ± 0.10) × 10−5 is in accord with an earlier finding by synchrotron X–ray double–crystal diffractometry, slightly revised as recommended in part I, Δa/ā = (1.20 ± 0.07) × 10−5. The closeness of the findings by two quite different methods confirms the conclusion from X–ray topography that the growth–sector boundary was free from misfit dislocations compensating the lattice–parameter mismatch. On mismatch boundaries in crystals too highly absorbing for transmission X–ray topography, such comparisons of reticulographic and diffractometric measurements would reveal information otherwise inaccessible by non–destructive methods.

Etching of diamonds for x-ray monochromators

Results of a study of etching of synthetic diamond single crystals are reported. The studies were done to examine the efficacy for improvements in the performance of synchrotron based x-ray monochromators. The three diamonds that we studied were all type Ib and were all (111) oriented. Synthetic diamonds are commonly type Ib and are yellow in color due to nitrogen impurities. Such diamonds are good candidate crystals for use as synchrotron-based x-ray monochromators because reasonably low dislocation densities can result. X-ray topography and x-ray double-crystal diffractometry with the diamond (111) reflection in a slightly dispersive geometry and Cu Kα (8 keV) radiation were used to assess the diffraction properties. Two separate etching procedures were studied. In the first, 1 keV oxygen+argon ion bombardment was used to sputter clean the surface and to introduce oxygen, followed by chemical oxidation at 720–730 °C. The oxidation was performed with KOH and Na2O2. Rocking curve full width at half maximum (FWHM) was improved from 7.3 to 6.4 arc sec and from 8.1 to 6.7 arc sec for two sides of the same slab that were treated separately. The theoretical ideal value is 5.8 arc sec. For the second technique, a patented and commercially available procedure that involves plasma deposition of SiOk compounds on a platter followed by abrasion of the diamond against this platter was studied. Two separate diamonds were treated with this second process, and, for one, the FWHM was slightly improved from 7.2 to 6.6 arc sec, but, for the other, the FWHM values were found to be increased from 8.0 to 8.5 arc sec.

X-ray diffractometry of Si epilayers grown on porous silicon

X-ray double-crystal diffractometry was used to measure lattice deformation of porous silicon (PS) and Si epitaxial layers grown on PS. PS layers 1–10 μm in thickness and 15-65% in porosity were formed by anodization of n +-type Sb doped Si wafers in a 12% HF aqueous solution. Lattice deformations of both PS and epitaxial layers are shown to strongly depend on PS porosity. Grown on uniform PS 40–60% in porosity, the epilayers, single-crystal as they are, display high lattice deformation and defect density. Epilayers grown on two-layer PS are comparable with the films grown on the n +-type single-crystal Si substrate.

Effects of thermal convection of NH 3 during growth of GaN epitaxial layers by horizontal MOCVD reactor

We have studied the effects of thermal convection during the growth of GaN epitaxial layers by horizontal MOCVD reactor. The epilayers were grown with the variation of reaction pressures in order to change the total gas velocity in the horizontal MOCVD reactor. The van der Pauw technique, SEM, double crystal X-ray diffractometry and photoluminescence were used in the characterization of their crystallographic, electrical and optical properties for investigating thermal convection effect of NH 3. The GaN epilayer which was grown under thermal convection of NH 3 related to low gas velocity shows rough surface having large islands. However, the quasi-mirror-like surface without any defect such as hillocks and islands can be seen in the sample which was grown free from thermal convection of NH 3 due to suitable gas velocity. The growth rate is also changed, which results from different thermal convections of NH 3 related to various gas velocities in the reactor during their growth. Judging from various data, it is possible to conclude that thermal convection of NH 3 is taking place when the epilayer is grown in the reactor at a gas velocity below 0.32 m/s. And, the generated thermal convection of NH 3 seriously affects the quality of GaN epilayers such as crystallinity, electrical characteristics and optoelectronic properties.

Investigation of weak damage in Al 0.25Ga 0.75As/GaAs by using RBS/C and Raman spectroscopy

The weak damage induced by 0.8 MeV Si ion implantation in the Al 0.25Ga 0.75As films epitaxially grown on GaAs substrates was studied by using Rutherford backscattering spectrometry/channeling (RBS/C) and Raman spectroscopy. RBS/C spectra measured from the implanted samples showed rather low damage level induced by the ion implantation with ion dose from 1×10 14 to 5×10 15 cm −2. The Raman spectra were measured on these samples. Two kinds of phonon modes, GaAs-like and AlAs-like, are observed, which indicate the existence of multiple phonon vibrational modes in the epitaxial Al 0.25Ga 0.75As films on the GaAs substrate. Compared with the unimplanted sample, the Raman photon peaks for the implanted sample shift gradually to lower energy with the increase of the implantation dose. The strains induced in the implanted layer were also evaluated from the Raman spectra. The result from high resolution double crystal X-ray diffractometry (HRXRD) also verified the evolution of the strains in the implanted layers.