M. Klaus

Exploiting the features of energy-dispersive synchrotron diffraction for advanced residual stress and texture analysis:

Responding to a growing interest from the materials science community for residual stress, texture, and microstructure analysis, strong efforts are made to enhance existing and develop novel methods that allow for fast in-situ studies at elevated temperature, measurements under external load, or residual strain, and stress scanning with high spatial resolution. In the paper, energy-dispersive diffraction using high-energy white synchrotron radiation is shown to provide some distinct advantages concerning residual stress and texture analysis, which mainly arise from the fact that the energy-dispersive diffraction mode allows for the measurement of complete diffraction patterns under fixed but arbitrary scattering angles, 2θ. A new two-detector set-up for simultaneous in- and out-of-plane diffraction analysis, which has been put into operation recently at the energy-dispersive materials science beamline EDDI at BESSY II, is introduced by using the examples of real-space residual stress and texture depth profiling on mechanically treated polycrystalline materials as well as of the in-situ study of (residual) stress evolution in a thin film at elevated temperature. It will be demonstrated that the individual measuring problems require the application of different geometrical slit configurations to define the pathways of the diffracted beams.

Analysis of the Deformation Behavior of Magnesium-Rare Earth Alloys Mg-2 pct Mn-1 pct Rare Earth and Mg-5 pct Y-4 pct Rare Earth by In Situ Energy-Dispersive X-ray Synchrotron Diffraction and Elasto-Plastic Self-Consistent Modeling

The deformation behavior of the Mg-RE alloys ME21 and WE54 was investigated. Although both alloys contain rare earth elements, which alter and weaken the texture, the flow curves of the alloys deviate significantly, especially in uniaxial compression test. Apart from the higher strength of the WE54 alloy, the compression flow curve does not exhibit the typical sigmoidal shape, which is associated with tension twinning. However, optical microscopy, X-ray texture measurements, and EBSD analysis reveal the activity of tension twinning. The combination of in situ energy-dispersive X-ray synchrotron diffraction and EPSC modeling was used to analyze these differences. The investigation reveals that twin propagation is decelerated in the WE54 alloy, which requires a change of the twinning scheme from the ‘finite initial fraction’ to the ‘continuity’ assumption. Furthermore, an enhanced activity of the 〈c+a〉 pyramidal slip system was observed in case of the WE54 alloy.

Energy‐dispersive diffraction stress analysis under laboratory and synchrotron conditions: a comparative study

For a feasibility study of energy-dispersive residual stress analysis under laboratory conditions, an X-ray diffractometer that has been operated so far in the angle dispersive diffraction mode was equipped with a commercial tungsten tube and an energy-dispersive solid-state germanium detector. Starting from systematic investigations to find the optimum configuration regarding geometrical resolution, measuring time and stability of the applied detector system, different materials were characterized with respect to the near-surface residual stress state. The results achieved with the modified laboratory equipment within reasonable measuring times are in good agreement with synchrotron measurements performed on the same samples. With the example of a shot-peened Al2O3 ceramic with a highly non-uniform near-surface residual stress distribution it is furthermore shown that the different size and shape of the diffracting gauge volume used for the laboratory and synchrotron measurements might have a significant influence on the experimentally obtained Laplace-space residual stress depth profiles σ||(τ).

Sin2ψ-based residual stress gradient analysis by energy-dispersive synchrotron diffraction constrained by small gauge volumes. II. Experimental implementation

On the basis of the theoretical concept for the use of small gauge volumes to study near-surface residual stress fields with high spatial resolution [Meixner, Klaus & Genzel (2013). J. Appl. Cryst. 46, 610–618], the experimental implementation of the approach is demonstrated. It is shown that specifically designed slit systems are required to avoid effects such as diffuse scattering at the slit blades and total external reflection, both giving rise to a reduced resolution. Starting from the characterization of the small gauge volume, practical guidance on how to control the alignment of the sample relative to the gauge volume for different geometrical conditions of energy-dispersive diffraction is given. The narrow-slit configuration as well as the formalism for data evaluation introduced in the first part of this series is applied to the analysis of a very steep in-plane residual stress gradient in a shot-peened Al2O3 ceramic sample. The results are compared with those obtained by means of a conventional wide-slit setup using the classical universal plot method for residual stress analysis on the one hand, and with the simulations performed in the first part on the other hand.

Sb-doping of ZnO: Phase segregation and its impact on p-type doping

The incorporation of antimony (Sb) in pulsed-laser deposited ZnO thin-films was investigated employing scanning electron microscopy, Raman spectroscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction (XRD) measurements. It is shown that an increase in the Sb concentration in the target leads to a significant deterioration of the sample structure which is accompanied by a decrease in the deposition rate. Furthermore, the dopant transfer factor depends strongly on the deposition temperature and exhibits a steplike behavior above 600 °C. XRD measurements clearly show that significant Sb–O phase precipitations occur. The implications of our data on p-type doping of ZnO are discussed.

Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells

In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron–hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of – partially electronically active – planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu–Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu–Se at the surface of the Cu(In,Ga)Se2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.

The role of interparticle heterogeneities in the selenization pathway of Cu–Zn–Sn–S nanoparticle thin films: a real-time study

Real-time energy dispersive X-ray diffraction (EDXRD) analysis has been utilized to observe the selenization of Cu–Zn–Sn–S nanoparticle films coated from three nanoparticle populations: Cu- and Sn-rich particles roughly 5 nm in size, Zn-rich nanoparticles ranging from 10 to 20 nm in diameter, and a mixture of both types of nanoparticles (roughly 1 : 1 by mass), which corresponds to a synthesis recipe yielding CZTSSe solar cells with reported total-area efficiencies as high as 7.9%. The EDXRD studies presented herein show that the formation of copper selenide intermediates during the selenization of mixed-particle films can be primarily attributed to the small, Cu- and Sn-rich particles. Moreover, the formation of these copper selenide phases represents the first stage of the CZTSSe grain growth mechanism. The large, Zn-rich particles subsequently contribute their composition to form micrometer-sized CZTSSe grains. These findings enable further development of a previously proposed selenization pathway to account for the roles of interparticle heterogeneities, which in turn provides a valuable guide for future optimization of processes to synthesize high quality CZTSSe absorber layers.

Fast Cu(In, Ga)Se 2 formation by processing Cu-In-Ga precursors in selenium atmosphere

The selenization of metallic precursors is a widely used and investigated technique for the fabrication of Cu(In, Ga)Se2 films on large areas. A vacuum process with Se supply from the gas phase can be a suitable way to achieve a homogeneous, fast and controllable selenization reaction. In this study in situ XRD measurements are employed to investigate the reaction path for this type of process. The experimental setup is based on a reaction box mounted at a white light beamline of the synchrotron facility BESSY. Diffraction signals as well as Kα fluorescence lines of Mo, In and Se can be measured with high time resolution via energy dispersive detection. To elucidate the influence of selenium on the metallic precursors upon heating a comparison of experiments with and without Se exposure is presented. For the Se-free process the phases In and a Cux(In, Ga)y-phase are detected at room temperature. The solid In phase melts according to its melting point at approximately 150°C, the remaining metallic phase melts at significantly higher temperatures of approximately 600°C. In the selenization process the metallic phases behave similar to the Se-free annealing process. The first detectable Se-containing phases are indium selenides. The indium selenides and the metallic diffraction signals vanish when chalcopyrite is formed. The Se fluorescence intensity was utilized to evaluate Se incorporation in the layers. Solar cells made out of absorbers from this kind of process exhibit fill factors of up to 72% and efficiencies up to 14%. The open circuit voltage was comparatively low with 535 mV and QE measurements confirmed a low band gap of approximately 1.0 eV. Energy-dispersive X-ray spectroscopy (EDS) measurements on the cross section showed a significant Ga enrichment at the back of the film.

Diffraction analysis of strongly inhomogeneous residual stress depth distributions by modification of the stress scanning method. II. Experimental implementation

The modified stress scanning method [Meixner, Fuss, Klaus & Genzel (2015). J. Appl. Cryst. 48, 1451–1461] is experimentally implemented for the analysis of near-surface residual stress depth distributions that are strongly inhomogeneous. The suggested procedure is validated by analyzing the very steep in-plane residual stress depth profile of a shot-peened Al2O3 ceramic specimen and comparing the results with those that were obtained by well established X-ray diffraction-based gradient methods. In addition, the evaluation formalism is adapted to the depth-dependent determination of the residual stresses inside of multilayer thin-film systems. The applicability for this purpose is demonstrated by investigating the residual stress depth distribution within the individual sublayers of a multilayered coating that consists of alternating Al2O3 and TiCN thin films. In this connection, the specific diffraction geometry that was used for the implementation of the stress scanning method at the energy-dispersive materials science beamline EDDI@BESSYII is presented, and experimental issues as well as limitations of the method are discussed.

Residual stress analysis of diamond‐coated WC–Co cutting tools: separation of film and substrate information by grazing X‐ray diffraction

Chemical vapour deposition (CVD) of diamond surface layers is an effective way of improving the properties of cemented carbide cutting tools. Inadequate coating adhesion is one of the main issues and it may be affected by the residual stresses of the CVD diamond films. The most common methods for nondestructive residual stress analysis are based on X-ray diffraction. The present paper deals with the particular case of determining the residual stress state of thin CVD diamond layers deposited on cobalt cemented tungsten carbide (WC–Co) substrates. It will be shown that the application of the conventional sin2ψ method might lead to erroneous results, as a result of superimposing diffraction lines originating from cobalt and the diamond coating. An approach to separating information on the substrate and film, based on grazing conditions in the symmetrical Ψ mode of diffraction, is presented. The results, revealing large compressive stresses within the coating, are compared with those obtained by supplementary micro-Raman spectroscopy investigations.