Tamzin Lafford

Characterization of defects in mono-like silicon for photovoltaic applications using X-ray Bragg diffraction imaging

Rocking curve imaging (projection and section X-ray topography) has been used to study different kinds of defects such as precipitates, dislocations and twins in directionally solidified mono-like silicon ingots. The qualitative and quantitative information extracted from the reconstructed integrated intensity, FWHM and peak position maps provides clues about the initial stages of silicon growth.

Multiscale 3D virtual dissections of 100-million-year-old flowers using X-Ray Synchrotron micro- and nanotomography

A multiscale approach combining phase-contrast X-ray micro- and nanotomography is applied for imaging a Cretaceous fossil inflorescence in the resolution range from 0.75 μm to 50 nm. The wide range of scale views provides three-dimensional reconstructions from the external gross morphology of the inflorescence fragment to the finest exine sculptures of in situ pollen. This approach enables most of the characteristics usually observed under light microscopy, or with low magnification under scanning and transmission electron microscopy, to be obtained nondestructively. In contrast to previous tomography studies of fossil and extant flowers that used resolutions down to the micron range, we used voxels with a 50 nm side in local tomography scans. This high level of resolution enables systematic affinities of fossil flowers to be established without breaking or slicing specimens.

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.

Synchrotron Bragg diffraction imaging characterization of synthetic diamond crystals for optical and electronic power device applications

Bragg diffraction imaging techniques are described, and their capabilities for studying the quality of diamond substrates and overgrown layers, and the surface damage caused by mechanical polishing, are illustrated by examples.

Investigation by coherent X-ray section topography of ferroelectric domain behaviour as a function of temperature in periodically poled Rb:KTP

The behaviour of ferroelectric domains at high temperatures near the Curie temperature in a periodically poled rubidium-doped potassium titanyl phosphate crystal (Rb:KTP) has been studied by Bragg-Fresnel X-ray diffraction imaging in situ using a compact coherence-preserving furnace. The development and partial disappearance of the inverted domain structure as the temperature increases has been successfully modelled, and is explained by invoking a built-in electric field produced under heating in a low vacuum by out-diffusion of atoms from the sample.

Ultra-smooth single crystal diamond surfaces resulting from implantation and lift-off processes

A method for obtaining a smooth, single crystal diamond surface is presented, whereby a sacrificial defective layer is created by implantation and graphitized by annealing before being selectively etched. We have used O+ at 240 keV, the main process variables being the ion fluence (ranging from 3x10^15 cm^-2 to 3x10^17 cm^-2) and the final etching process (wet etch, H2 plasma and annealing in air). The substrates were characterized by atomic force microscopy, optical profilometry and white beam X-ray topography. The influence of the various process parameters on the resulting lift-off efficiency and final surface roughness is discussed. An O+ fluence of 2x10^17 cm^-2 was found to result in sub-nanometre roughness over tens of um^2.

Observation of the initiation and propagation of solidification cracks by means of in situ synchrotron X-ray radiography

We report on the in situ, time resolved, observation of solidification cracking in a thin sample of an Al-15wt%Sn alloy at ESRF BM05. During the experiment, solidification cracking was seen to occur during natural cooling of the sample at a solid fraction of ~95%, between directionally agglomerated dendritic networks. Through detailed analysis, three stages of crack growth were observed: crack initiation, which typically occurs asymmetrically with the liquid film separating from one side of the dendritic network prior to full detachment; crack propagation, where the liquid film generally detaches from both sides simultaneously; and crack coalescence. We correlate our observations using scanning electron microscopy, which shows that voids between grains, and also spikes, can appear asymmetrically on either side of the crack surface.

X-ray topography of diamond using forbidden reflections: which defects do we really see?

Natural and synthetic diamonds with various concentrations and types of point and extended defect were investigated using X-ray topography employing allowed (111, 004) and forbidden (222) reflections. On the topographs of the forbidden reflections, weak stress fields from lattice imperfections and extended defects are readily observed. Comparison of the topographs with IR maps of the distribution of point defects suggests that certain types of point defect may increase the structure factors of the forbidden reflections.

The ESRF: A Synchrotron In Europe's Silicon Valley

Grenoble, France, is home to one of the four large R&D and production sites in Europe for microelectronics chips. What sets it apart is that Grenoble is also one of the best sites in the world for academic research when it comes to analyzing and characterizing atomic structures and materials using synchrotron X-rays and neutrons. The European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL) are co-hosted next to the French technology research organizations CEA-LETI center and the ST Microelectronics R&D laboratories. However, it is not only for this reason that the challenge of harnessing the power of the ESRF and the ILL for industrial R&D been taken up by the ESRF and the ILL ever since they opened for business [1, 2].

In Situ and Real-Time Analysis of TEM Forces Induced by a Permanent Magnetic Field during Solidification of Al-4wt%Cu

It is well known that the application of a magnetic field during the growth process can have pronounced effects on cast material structures and their properties, so that magnetic fields have been widely applied since the 1950s. In the case of a permanent magnetic field, some recent results revealed a dual effect on the liquid metal flow. 1: the magnetic field has a selective damping action on the flow at the scale of the crucible, due to the Lorentz force; 2: the interaction of thermo-electro-magnetic (TEM) currents in the close vicinity of the solid-liquid interface with the applied magnetic field leads to the generation of electromagnetic forces, which act both on the liquid and on the solid at the scale of the microstructure. We present an experimental investigation of the TEM forces induced by a permanent magnetic field during columnar and equiaxed solidification of Al-4wt%Cu. In situ visualization was carried out by means of synchrotron X-ray radiography, which is a method of choice for studying dynamic phenomena. It was shown that the TEM forces were at the origin of a motion of dendritic particles, perpendicular to the direction of gravity. A heuristic analysis allowed us to estimate the fluid velocities and the velocities of the solid particles, and a good agreement was achieved with the experimental data. Similar observations were also made during equiaxed growth in a temperature gradient. The in situ observation of the grain trajectories for various values of the temperature gradient demonstrated that gravity and TEM forces were the driving forces which controlled the grain motion.