Gilbert A. Chahine

Imaging of strain and lattice orientation by quick scanning X-ray microscopy combined with three-dimensional reciprocal space mapping

Numerous imaging methods have been developed over recent years in order to study materials at the nanoscale. Within this context, scanning X-ray diffraction microscopy has become a routine technique, giving access to structural properties with sub-micrometre resolution. This article presents an optimized technique and an associated software package which have been implemented at the ID01 beamline (ESRF, Grenoble). A structural scanning probe microscope with intriguing imaging qualities is obtained. The technique consists in a two-dimensional quick continuous mapping with sub-micrometre resolution of a sample at a given reciprocal space position. These real space maps are made by continuously moving the sample while recording scattering images with a fast two-dimensional detector for every point along a rocking curve. Five-dimensional data sets are then produced, consisting of millions of detector images. The images are processed by the user-friendly X-ray strain orientation calculation software (XSOCS), which has been developed at ID01 for automatic analysis. It separates tilt and strain and generates two-dimensional maps of these parameters. At spatial resolutions of typically 200–800 nm, this quick imaging technique achieves strain sensitivity below Δa/a = 10−5 and a resolution of tilt variations down to 10−3° over a field of view of 100 × 100 µm.

Strain and lattice orientation distribution in SiN/Ge complementary metal–oxide–semiconductor compatible light emitting microstructures by quick x-ray nano-diffraction microscopy

This paper presents a study of the spatial distribution of strain and lattice orientation in CMOS-fabricated strained Ge microstripes using high resolution x-ray micro-diffraction. The recently developed model-free characterization tool, based on a quick scanning x-ray diffraction microscopy technique can image strain down to levels of 10−5 (Δa/a) with a spatial resolution of ∼0.5 μm. Strain and lattice tilt are extracted using the strain and orientation calculation software package X-SOCS. The obtained results are compared with the biaxial strain distribution obtained by lattice parameter-sensitive μ-Raman and μ-photoluminescence measurements. The experimental data are interpreted with the help of finite element modeling of the strain relaxation dynamics in the investigated structures.

Imaging Structure and Composition Homogeneity of 300 mm SiGe Virtual Substrates for Advanced CMOS Applications by Scanning X-ray Diffraction Microscopy

Advanced semiconductor heterostructures are at the very heart of many modern technologies, including aggressively scaled complementary metal oxide semiconductor transistors for high performance computing and laser diodes for low power solid state lighting applications. The control of structural and compositional homogeneity of these semiconductor heterostructures is the key to success to further develop these state-of-the-art technologies. In this article, we report on the lateral distribution of tilt, composition, and strain across step-graded SiGe strain relaxed buffer layers on 300 mm Si(001) wafers treated with and without chemical-mechanical polishing. By using the advanced synchrotron based scanning X-ray diffraction microscopy technique K-Map together with micro-Raman spectroscopy and Atomic Force Microscopy, we are able to establish a partial correlation between real space morphology and structural properties of the sample resolved at the micrometer scale. In particular, we demonstrate that the lattice plane bending of the commonly observed cross-hatch pattern is caused by dislocations. Our results show a strong local correlation between the strain field and composition distribution, indicating that the adatom surface diffusion during growth is driven by strain field fluctuations induced by the underlying dislocation network. Finally, it is revealed that a superficial chemical-mechanical polishing of cross-hatched surfaces does not lead to any significant change of tilt, composition, and strain variation compared to that of as-grown samples.

X-ray Excited Optical Fluorescence and Diffraction Imaging of Reactivity and Crystallinity in a Zeolite Crystal : Crystallography and Molecular Spectroscopy in One

Abstract Structure–activity relationships in heterogeneous catalysis are challenging to be measured on a single‐particle level. For the first time, one X‐ray beam is used to determine the crystallographic structure and reactivity of a single zeolite crystal. The method generates μm‐resolved X‐ray diffraction (μ‐XRD) and X‐ray excited optical fluorescence (μ‐XEOF) maps of the crystallinity and Brønsted reactivity of a zeolite crystal previously reacted with a styrene probe molecule. The local gradients in chemical reactivity (derived from μ‐XEOF) were correlated with local crystallinity and framework Al content, determined by μ‐XRD. Two distinctly different types of fluorescent species formed selectively, depending on the local zeolite crystallinity. The results illustrate the potential of this approach to resolve the crystallographic structure of a porous material and its reactivity in one experiment via X‐ray induced fluorescence of organic molecules formed at the reactive centers.

Structural Mapping of Functional Ge Layers Grown on Graded SiGe Buffers for sub-10 nm CMOS Applications Using Advanced X-ray Nanodiffraction

We report a detailed advanced materials characterization study on 40 nm thick strained germanium (Ge) layers integrated on 300 mm Si(001) wafers via strain-relaxed silicon-germanium (SiGe) buffer layers. Fast-scanning X-ray microscopy is used to directly image structural inhomogeneities, lattice tilt, thickness, and strain of a functional Ge layer down to the sub-micrometer scale with a real space step size of 750 μm. The structural study shows that the metastable Ge layer, pseudomorphically grown on Si(0.3)Ge(0.7), exhibits an average compressive biaxial strain of -1.27%. By applying a scan area of 100 × 100 μm(2), we observe microfluctuations of strain, lattice tilt, and thickness of ca. ±0.03%, ±0.05°, and ±0.8 nm, respectively. This study confirms the high materials homogeneity of the compressively strained Ge layer realized by the step-graded SiGe buffer approach on 300 mm Si wafers. This presents thus a promising materials science approach for advanced sub-10 nm complementary metal oxide-semiconductor applications based on strain-engineered Ge transistors to outperform current Si channel technologies.

Microstrain distributions in polycrystalline thin films measured by X-ray microdiffraction

Microstrain distributions were acquired in functional thin films by high-resolution X-ray microdiffraction measurements, using polycrystalline CuInSe2 thin films as a model system. This technique not only provides spatial resolutions at the submicrometre scale but also allows for analysis of thin films buried within a complete solar-cell stack. The microstrain values within individual CuInSe2 grains were determined to be of the order of 10−4. These values confirmed corresponding microstrain distribution maps obtained on the same CuInSe2 layer by electron backscatter diffraction and Raman microspectroscopy.

Strain release management in SiGe/Si films by substrate patterning

The nucleation and the evolution of dislocations in SiGe/Si(001) films can be controlled and confined along stripes aligned along pits carved in the substrate, leaving micrometric coherent areas free of dislocations. In this work, we have addressed the stability of such metastable areas versus, film thickness, different Ge contents (xGe = 10%–30%) and larger pit-pattern periods, revealing the flexibility and effectiveness of this method even for coherent areas of about 64 μm2. The thermal stability of such configuration has been finally verified by post-growth annealing treatment, in order to simulate device processing. Finally, μRaman spectroscopy and X-ray nanodiffraction have been used to characterize the periodic strain variations across the pattern.

Microstrain distribution mapping on CuInSe2 thin films by means of electron backscatter diffraction, X-ray diffraction, and Raman microspectroscopy

The investigation of the microstructure in functional, polycrystalline thin films is an important contribution to the enhanced understanding of structure-property relationships in corresponding devices. Linear and planar defects within individual grains may affect substantially the performance of the device. These defects are closely related to strain distributions. The present work compares electron and X-ray diffraction as well as Raman microspectroscopy, which provide access to microstrain distributions within individual grains. CuInSe2 thin films for solar cells are used as a model system. High-resolution electron backscatter diffraction and X-ray microdiffraction as well as Raman microspectroscopy were applied for this comparison. Consistently, microstrain values were determined of the order of 10(-4) by these three techniques. However, only electron backscatter diffraction, X-ray microdiffraction exhibit sensitivities appropriate for mapping local strain changes at the submicrometer level within individual grains in polycrystalline materials.

Effect of dimensionality on sliding charge density waves: The quasi-two-dimensional TbTe 3 system probed by coherent x-ray diffraction

We report on sliding Charge Density Wave (CDW) in the quasi two-dimensional TbTe3 system probed by coherent x-ray diffraction combined with in-situ transport measurements. We show that the non-Ohmic conductivity in TbTe3 is made possible thanks to a strong distortion of the CDW. Our diffraction experiment versus current shows first that the CDW remains undeformed below the threshold current IS and then suddenly rotates and reorders by motion above threshold. Contrary to quasi-one dimensional systems, the CDW in TbTe3 does not display any phase shifts below IS and tolerates only slow spatial variations of the phase above. This is a first observation of CDW behavior in the bulk in a quasi-two dimensional system allowing collective transport of charges at room temperature. Interaction between pairs of quasiparticles often leads to broken-symmetry ground states in solids. Typical examples are the formation of Cooper pairs in supercon-ductors, charge-density waves (CDWs) and spin-density waves driven by electron-phonon or electron-electron interactions[1]. The CDW ground state is characterized by a spatial modulation η cos(2k F x + φ) of the electron density and a concomitant periodic lattice distortion with the same 2k F wave vector leading to a gap opening in the electron spectrum. The first CDW systems were discovered in the beginning of the 70s in two-dimensional transition metal dichalcogenides MX 2 [2]. CDW state was then discovered in quasi-one dimensional systems like NbSe 3 , TaS 3 , the blue bronze K 0.3 MoO 3 and in organic compounds like TTF-TNCQ. However, the most remarkable property of a CDW has been discovered a few years later in quasi one-dimensional systems: a CDW may slide carrying correlated charges[3]. The sliding mode is achieved when an electric field applied to the sample is larger than a threshold value, manifesting then collective Frohlich-type transport. This sliding phenomenon is clearly observed by transport measurements. The differential resistance remains constant up to a threshold current and then decreases for larger currents in addition to the generation of an ac voltage, the frequency of which increases with the applied current[3]. In spite of numerous studies, the physical mechanism leading to the sliding phenomenon is still far to be fully understood. One of the difficulties comes from the fact that the sliding mode displays two different aspects. On the one hand, the CDW is a classical state, similar to an elastic object in presence of disorder[4], displaying creep, memory effects and hysteresis[5, 6]. On the other hand, a CDW is a macroscopic quantum state[7], carrying charges by tunneling through disorder[8] and displaying Aharonov-Bohm effects[9] over microscopic distances[10]. Recently a new class of quasi-two dimensional CDW compounds, rare-earth tritellurides RTe 3 , have raised an intense research activity thanks to their peculiar properties[11-13]. RTe 3 structures are orthorhombic (Cmcm) but the a and c lattice parameters lying in the Te planes are almost equal (c-a=0.002A002A with a=4.307A307A for TbTe 3 at T=300K) and the double Te-layers are linked together by a c-glide plane. The almost square Te sheets lead to nearly isotropic properties in the (a,c) plane. The resistance measured along a and c differs by only 10% at 300K in TbTe 3 [14] and the Fermi surface displays an almost square-closed shape in the (a*,c*) plane[15]. These quasi-two dimensional systems exhibit a unidirectional CDW wave vector along c* (2k F ∼ 2/7 c* in TbTe 3) and a surprisingly large Peierls transition temperature, around 300 K, through the whole R-series and above for lighter rare-earth elements. The stabilization of the CDW in TbTe 3 over the almost square underlying atomic lattice is reminiscent of copper-oxide planes in high temperature superconductors in which a CDW state was also recently observed[16]. However, the most surprising property of TbTe 3 is its ability to displays non-linear transport[17] despite the two-dimensional character of the atomic structure. The aim of the present work is to show that, despite similar resistivity curves, the depining process in quasi one and two-dimensional systems are quite different. For that purpose, coherent x-ray diffraction has been used to study the behavior of the 2k F satellite reflection upon application of an external current. As the sliding state of a CDW mainly involves fluctuations of the CDW phase to overcome pinning centers , coherent x-ray diffraction is a suitable technique thanks to its high sensitivity to the phase of any modulation. The extreme case of a single phase shift, such

Through-silicon via-induced strain distribution in silicon interposer

Strain in silicon induced by Through-Silicon Via (TSV) integration is of particular interest in the frame of the integration of active devices in silicon interposer. Nano-focused X-ray beam diffraction experiments were conducted using synchrotron radiation to investigate the thermally induced strain field in silicon around copper filled TSVs. Measurements were performed on thinned samples at room temperature and during in situ annealing at 400 °C. In order to correlate the 2D strain maps with finite elements analysis, an analytical model was developed, which takes into account beam absorption in the sample for a given diffraction geometry. The strain field along the [335] direction is found to be in the 10−5 range at room temperature and around 10−4 at 400 °C. Simulations support the expected plastification in some regions of the TSV during the annealing step.