Rocking Curve Imaging Research Articles

Mono-like silicon ingots grown on low angle misoriented seeds: Defect characterization by synchrotron X-ray diffraction imaging

The present work studies the generation and propagation of sub-grain boundaries and dislocations in mono-like silicon ingots grown on monocrystalline seeds with a very small relative misorientation between them (<0.06° around the x, y and z axes). Special emphasis is put on the region close to the area between the seeds at the bottom of the crucible, which appears to be crucial in determining the crystalline quality of the final ingot. For this investigation, X-ray rocking curve imaging (RCI) in transmission geometry, a directly quantitative version of monochromatic beam Bragg diffraction imaging (“topography”) has been used. This technique has been developed at the European Synchrotron Radiation Facility (ESRF), beamline BM05 and allows us to visualize the spatial distribution of the lattice distortion of a single crystal. It was found that the solidified ingot takes the crystallographic orientation of the seeds without creating any distorted area at the interface. However, dislocation bunches having a strong screw component are generated between the seeds and propagate along the growth direction. Sub-grain boundaries above the top of the seeds, mainly composed of edge dislocations, were also observed. These have a detrimental influence on the minority carrier lifetime of the ingot, since, as is known, sub-grain boundaries show active recombination. In this work taking advantage of the high angular resolution and the sensitivity of the technique it was shown that a relative misorientation between the seeds (δθx, δθy, δθz) as small as 0.02° can produce cascades of dislocations that propagate and multiply higher up in the ingot becoming electrically active.

Three-dimensional rocking curve imaging to measure the effective distortion in the neighbourhood of a defect within a crystal: an ice example

Rocking curve imaging (RCI) is a quantitative version of monochromatic beam diffraction topography that involves using a two-dimensional detector, each pixel of which records its own 'local' rocking curve. From these local rocking curves one can reconstruct maps of particularly relevant quantities (e.g. integrated intensity, angular position of the centre of gravity, FWHM). Up to now RCI images have been exploited in the reflection case, giving a quantitative picture of the features present in a several-micrometre-thick subsurface layer. Recently, a three-dimensional Bragg diffraction imaging technique, which combines RCI with 'pinhole' and 'section' diffraction topography in the transmission case, was implemented. It allows three-dimensional images of defects to be obtained and measurement of three-dimensional distortions within a 50 × 50 × 50 µm elementary volume inside the crystal with angular misorientations down to 10-5-10-6 rad. In the present paper, this three-dimensional-RCI (3D-RCI) technique is used to study one of the grains of a three-grained ice polycrystal. The inception of the deformation process is followed by reconstructing virtual slices in the crystal bulk. 3D-RCI capabilities allow the effective distortion in the bulk of the crystal to be investigated, and the predictions of diffraction theories to be checked, well beyond what has been possible up to now.

Synchrotron X-ray imaging applied to solar photovoltaic silicon

Photovoltaic (PV) cell performance is dictated by the material of the cell, its quality and purity, the type, quantity, size and distribution of defects, as well as surface treatments, deposited layers and contacts. A synchrotron offers unique opportunities for a variety of complementary X-ray techniques, given the brilliance, spectrum, energy tunability and potential for (sub-) micron-sized beams. Material properties are revealed within in the bulk and at surfaces and interfaces. X-ray Diffraction Imaging (X-ray Topography), Rocking Curve Imaging and Section Topography reveal defects such as dislocations, inclusions, misorientations and strain in the bulk and at surfaces. Simultaneous measurement of micro-X-Ray Fluorescence (μ-XRF) and micro-X-ray Beam Induced Current (μ-XBIC) gives direct correlation between impurities and PV performance. Together with techniques such as microscopy and Light Beam Induced Current (LBIC) measurements, the correlation between structural properties and photovoltaic performance can be deduced, as well as the relative influence of parameters such as defect type, size, spatial distribution and density (e.g [1]). Measurements may be applied at different stages of solar cell processing in order to follow the evolution of the material and its properties through the manufacturing process. Various grades of silicon are under study, including electronic and metallurgical grades in mono-crystalline, multi-crystalline and mono-like forms. This paper aims to introduce synchrotron imaging to non-specialists, giving example results on selected solar photovoltaic silicon samples.

Three‐dimensional distortion measurements by section rocking curve imaging: Application to ice crystals

AbstractWe present a new method to measure distortions in the entire crystal volume, down to a scale of 50 × 50 × 50 µm3 elementary volume. The method combines rocking curve imaging (RCI), section‐ and pinhole diffraction topography. As results we obtain three‐dimensional (3D) maps providing the integrated diffracted beam intensity and the fields of angular lattice misorientation. Angular misorientations down to 10−5–10−6 have been measured. In this paper the method is applied to one of the grains of a three‐grained ice polycrystal (tri‐crystal).

Rocking curve imaging for diamond radiator crystal selection

The combination of low atomic number, high crystal packing density, and very high Debye temperature makes diamond the best material for use as a bremsstrahlung radiator in the coherent bremsstrahlung (CB) process, a process that is uniquely suited for generating highly polarized high-energy photon beams for photonuclear experiments. The crystal quality of the diamond radiator has a vital effect on the polarization and other properties of the photon beam and the best large-area diamond monocrystals currently available, both natural and synthetic, contain many defects that can degrade their performance as CB radiators. The diamonds used for this study were synthetic type Ib samples produced through the HPHT process by the firm Element Six. They were examined using the double crystal rocking curve imaging method in a synchrotron X-ray beam. Dislocation densities were calculated from the measured rocking curve peak position maps in the way proposed by Ferrari et al. [1]. It is shown that dislocation is one major defect that affects the rocking curve width in local regions. The most significant contribution to the whole-crystal rocking curve width for thin crystals is the systematic variation of the peak position across its surface. This is interpreted in terms of a large-scale bending of the entire crystal. Data supporting this interpretation are presented, and possible explanations for the bending and methods for its mitigation are discussed.

Rocking Curve Imaging Studies of Laterally Overgrown GaAs and GaSb Epitaxial Layers

X-ray rocking curve imaging technique was used to study crystallographic perfection of laterally overgrown epitaxial structures. We focus on rocking curve imaging studies of Si-doped GaAs and GaSb laterally overgrown layers grown by liquid phase epitaxy on SiO2 masked GaAs and GaSb/GaAs substrates, respectively. High spatial resolution offered by rocking curve imaging technique allows studying the effect of laterally overgrown epitaxial wing tilt towards the mask. Distribution of tilt magnitude over the area of laterally overgrown epitaxial stripes is easily determined. In heteroepitaxial GaSb/GaAs laterally overgrown epitaxial structures local mosaicity in the wing area was detected. Since individual grains are clearly visible on rocking curve imaging maps, their size and relative misorientation can be determined.

Correctness of a particular solution of inverse problem in rocking curve imaging

AbstractLocal lattice misorientations on crystalline substrates can be visualized by rocking curve imaging. Local deviations from Bragg peak positions are extracted from a series of digital topographs recorded by a CCD detector under different azimuths. Bragg peaks from surface regions such as crystallites with a larger local misorientation overlap on the detector, which requires a back‐projection method in order to reconstruct the misorientation components on the sample surface from the measured angular position on the detector planes. From mathematical point of view, the reconstruction problem is an inverse problem. In this paper, we formulate the forward and back‐projection problems and we prove the correctness of a particular solution. The usability of the method is demonstrated on a phantom data set.

Analysis of epitaxial laterally overgrown silicon structures by high resolution x‐ray rocking curve imaging

AbstractSpatially resolved rocking curve imaging has been used to analyze laterally overgrown silicon layers grown by liquid phase epitaxy. We were able to study both the overgrown layer as well as the strain fluctuations of the Si substrate underneath by means of a tabletop x‐ray topographic setup. The strain‐field analysis reveals relative changes of the lattice parameter up to 3.5×10‐6 in the silicon substrate underneath the overgrown layer in particular regions and a down bending of both wings of the epitaxial overgrown layers. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spatially Resolved X-ray Diffraction Technique for Crystallographic Quality Inspection of Semiconductor Microstructures

There is an increasing demand for techniques that allow detection and visualization of crystalline lattice microdefects. From the many techniques available the X-ray Rocking Curve Imaging (RCI), based on synchrotron X-ray diffraction, has gained much attention recently as a method of wafer defect analysis [1]. However, for most crystal growers laboratory techniques are preferred. Therefore, instead of applying RCI we have developed a microimaging technique based on spatially resolved X-ray diffraction (SRXRD) that makes use of conventional high resolution X-ray diffractometer. Briefly, the size of incident X-ray beam is reduced by a set of slits. Then the sample is moved in small steps while the rest of the system stays fixed. At each sample position X-ray diffraction curve is measured from precisely defined area. Finally, all diffraction curves are collected to construct the X-ray diffraction image of the sample. Both the ω and 2θ/ω scans are performed allowing independent mapping of crystallographic misorientation and lattice parameter distributions across the sample. Spatial resolution offered by the SRXRD technique is smaller than that obtainable with the use of synchrotron radiation based methods. This is due to a limited radiation intensity of conventional X-ray sources. We have checked that in our experimental setup measurable signals can be obtained with the X-ray beam as small as 10/sinθ × 100 μm 2 , where θ is the Bragg angle for the reflection studied. This corresponds to the beam spot size of ~18 × 100 μm 2 for 004 reflections of GaAs and Si. The aim of this report is to present advantages of SRXRD over a standard X-ray diffraction when applied for analysis of crystalline microstructures. First, we focus on SRXRD studies of epitaxial laterally overgrown (ELO) GaAs layers grown by liquid phase epitaxy on SiO2-masked GaAs substrates. ELO layer consists of a few hundreds micrometers wide monocrystalline stripes regularly arranged on a substrate. Since each stripe is strained such samples contain a periodic distribution of strain and/or defects. Thus, they are very suitable to demonstrate potential of the technique. We show that high spatial and angular resolutions of SRXRD allow studying a strong effect of ELO wings tilt towards the mask as well as very fine residual strain caused by dopant concentration inhomogeneity. Direction of the tilt and the distribution of tilt magnitude across width of each wing can be easily determined. In fully overgrown GaAs ELO layers additional strain was observed at a coalescence front where ELO wings grown from adjacent seeds merged. With the use of SRXRD a local structure of the crystal lattice around this coalescence front was studied. In heteroepitaxial GaSb/GaAs ELO layers local mosaicity in the wing area was found. By SRXRD the size of microblocks and their relative misorientation were analyzed. As the final example, we report on strain distribution in thermally oxidized Si wafers. Microscopic curvature of lattice planes confined between two neighboring slip bands was measured allowing its correlation with the dislocation structure. It is noting worthy that all the phenomena presented are difficult to study by standard X-ray diffraction. On the contrary, due to its high spatial resolution the SRXRD technique allows deeper insight into a localized strain fields present in semiconductor microstructures.

Visrock: a program for digital topography and X-ray microdiffraction imaging

Visrockis a program for interactive analysis of sequences of digital X-ray images.Visrockwas developed in the context of the rocking-curve imaging method of full-field X-ray microdiffraction imaging. Its functionality is based on parallel profile analysis of millions of local diffraction profiles. Options for subsequent visualization of the spatial distribution of extracted parameters include automatic contrast enhancement, noise reduction and multi-peak analysis. In addition to microdiffraction imaging, further useful applications of the program lie particularly in computed tomography, sequential radiography and analyser-based imaging.

Crystallite misorientation analysis in semiconductor wafers and ELO samples by rocking curve imaging

Rocking curve imaging is based on measuring a series of Bragg-reflection digital topographs by monochromatic parallel-beam synchrotron radiation in order to quantify local crystal lattice rotations within a large surface area with high angular and high spatial resolution. In this paper we apply the method to map local lattice tilts in two distinct semiconductor sample types with lattice misorientations up to 0.5 ° and with spatial resolution from 30 μ m down to 1 μ m . We analyse the measured surface-tilt data volumes for samples with almost smoothly varying specific misoriented defect formation in GaAs wafers and for an inherent subsurface grain structure of epitaxial lateral overgrowth wings in GaN. Back-projected tilt maps and histograms provide both local and global characteristics of the microcrystallinity.

X‐ray microdiffraction imaging investigations of wing tilt in epitaxially overgrown GaN

AbstractThe crystalline quality in epitaxially laterally overgrown (ELO) GaN and the amount of wing tilt is characterized on a local basis, with high spatial and angular resolution. A method of full‐field X‐ray microdiffraction imaging, termed rocking curve imaging, is used to record simultaneously a large set of local X‐ray diffraction profiles originating from sample surface areas of micrometer size. x –ω maps of diffracted intensity allow to quantify the amount of wing tilt in individual lateral ELO periods as well as to monitor the fluctuations of tilt between adjacent periods. Automated shape analysis of the full set of local rocking curves provides a means to quantitatively characterize the local crystalline perfection of GaN. The ELO window and wing regions can be clearly separated; comparison indicates an average improvement of crystal quality by a factor 3–4 due to the lateral overgrowth process. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Local wing tilt analysis of laterally overgrown GaN by x-ray rocking curve imaging

We report on recent advances in spatially resolved x-ray diffraction, extending the technique known as rocking curve imaging down to 1–2 µm spatial resolution. Application to a set of gallium nitride samples grown by epitaxial lateral overgrowth (ELO) shows the potential of the technique. Quantitative information on crystallographic misorientations and lattice quality can be obtained by direct imaging with high lateral resolution. Results from two samples of ELO-GaN grown on different substrates are compared. Tilt in individual lateral periods of the ELO structure can be quantified. Local tilt fluctuations are distinguished from macroscopic variations (curvature). The local lattice quality can be investigated via the peak width of diffraction profiles recorded in individual camera pixels. The peak broadening previously observed in laboratory x-ray diffraction measurements is found to have (at least) two different reasons. In both cases, peak broadening does not indicate a degradation in local crystalline quality.

Distribution and Burgers vectors of dislocations in semiconductor wafers investigated by rocking-curve imaging

The method called `rocking-curve imaging' (RCI) has recently been developed to visualize lattice imperfections in large crystals such as semiconductor wafers with high spatial resolution. The method is based on a combination of X-ray rocking-curve analysis and digital X-ray diffraction topography. In this article, an extension of the method is proposed by which dislocation densities in large-scale samples (semiconductor wafer crystals) can be quantified and their variation across the sample surface determined in an instrumentally simple way. Results from a nearly dislocation-free S-doped InP crystal and a semi-insulating GaAs are presented; both display a clearly non-random distribution of dislocations.

Spatial micrometer-resolution three-dimensional local lattice misorientation determination of GaAs wafers by synchrotron radiation rocking-curve imaging

Spatial micrometer-resolution three-dimensional local lattice misorientation determination of GaAs wafers by synchrotron radiation rocking-curve imaging

μm-resolved high resolution X-ray diffraction imaging for semiconductor quality control

We present a method and an equipment for performing μm-resolved X-ray diffraction area maps, and apply it to wafer defect analysis and industrial wafer quality inspection. The method determines simultaneously the macroscopic warpage, the mesoscopic curvature and the microscopic defect structure of semiconductor wagers. It is based on X-ray diffraction rocking curve imaging of the whole wafer with down to 1 μm 2 resolution. The new wafer testing equipment determines the maximal and integral peak intensities, the peak position and the half width of the rocking curves with a microscopic resolution, thus imaging simultaneously the macroscopic quality parameters and the microscopic defect structure. This permits to establish a direct one-to-one correlation between the microscopic defects and the resulting macroscopic effects. As an example, wafers of different materials, fabrication technology and resulting perfection are studied. When investigating layered samples, the technique allows furthermore to determine the influence of the wafer quality on the layer properties. The method can be applied generally to characterize non-destructively the quality of all kinds of crystalline structures, like, e.g. microelectronic and optoelectronic devices.

Single-crystal x-ray diffraction study of the InGaAs-GaAsP/GaAs superlattice system

Single-crystal x-ray rocking curve and transmission imaging topography techniques have been used for the characterization of InGaAs-GaAsP strained-layer superlattices (SLS’s), which are lattice matched to GaAs substrates. The thicknesses of the SLS’s and strains, 〈ε⊥〉 and 〈ε∥〉, were readily determined nondestructively by analyzing the rocking curves obtained with Cu Kβ1 radiation. These quantitative data were combined with the qualitative defect imaging results of x-ray topography to determine the critical intrinsic SLS thickness tISc (=PSL〈ε̄∥〉c) that dominates the generation of misfit dislocations. It was estimated for the InGaAs-GaAsP SLS on a GaAs substrate as 1.05×10−3 Å&amp;lt;tISc&amp;lt;1.56×10−3 Å.