Multimodal X-ray imaging of grain-level properties and performance in a polycrystalline solar cell.

The factors limiting the performance of alternative polycrystalline solar cells as compared with their single-crystal counterparts are not fully understood, but are thought to originate from structural and chemical heterogeneities at various length scales. Here, it is demonstrated that multimodal focused nanobeam X-ray microscopy can be used to reveal multiple aspects of the problem in a single measurement by mapping chemical makeup, lattice structure and charge collection efficiency simultaneously in a working solar cell. This approach was applied to micrometre-sized individual grains in a Cu(In,Ga)Se2 polycrystalline film packaged in a working device. It was found that, near grain boundaries, collection efficiency is increased, and that in these regions the lattice parameter of the material is expanded. These observations are discussed in terms of possible physical models and future experiments.

Correlating dynamic strain and photoluminescence of solid-state defects with stroboscopic x-ray diffraction microscopy.

Control of local lattice perturbations near optically-active defects in semiconductors is a key step to harnessing the potential of solid-state qubits for quantum information science and nanoscale sensing. We report the development of a stroboscopic scanning X-ray diffraction microscopy approach for real-space imaging of dynamic strain used in correlation with microscopic photoluminescence measurements. We demonstrate this technique in 4H-SiC, which hosts long-lifetime room temperature vacancy spin defects. Using nano-focused X-ray photon pulses synchronized to a surface acoustic wave launcher, we achieve an effective time resolution of ~100 ps at a 25 nm spatial resolution to map micro-radian dynamic lattice curvatures. The acoustically induced lattice distortions near an engineered scattering structure are correlated with enhanced photoluminescence responses of optically-active SiC quantum defects driven by local piezoelectric effects. These results demonstrate a unique route for directly imaging local strain in nanomechanical structures and quantifying dynamic structure-function relationships in materials under realistic operating conditions.

High-resolution three-dimensional structural microscopy by single-angle Bragg ptychography.

Coherent X-ray microscopy by phase retrieval of Bragg diffraction intensities enables lattice distortions within a crystal to be imaged at nanometre-scale spatial resolutions in three dimensions. While this capability can be used to resolve structure-property relationships at the nanoscale under working conditions, strict data measurement requirements can limit the application of current approaches. Here, we introduce an efficient method of imaging three-dimensional (3D) nanoscale lattice behaviour and strain fields in crystalline materials with a methodology that we call 3D Bragg projection ptychography (3DBPP). This method enables 3D image reconstruction of a crystal volume from a series of two-dimensional X-ray Bragg coherent intensity diffraction patterns measured at a single incident beam angle. Structural information about the sample is encoded along two reciprocal-space directions normal to the Bragg diffracted exit beam, and along the third dimension in real space by the scanning beam. We present our approach with an analytical derivation, a numerical demonstration, and an experimental reconstruction of lattice distortions in a component of a nanoelectronic prototype device.

Efficient modeling of Bragg coherent x-ray nanobeam diffraction.

X-ray Bragg diffraction experiments that utilize tightly focused coherent beams produce complicated Bragg diffraction patterns that depend on scattering geometry, characteristics of the sample, and properties of the x-ray focusing optic. Here, we use a Fourier-transform-based method of modeling the 2D intensity distribution of a Bragg peak and apply it to the case of thin films illuminated with a Fresnel zone plate in three different Bragg scattering geometries. The calculations agree well with experimental coherent diffraction patterns, demonstrating that nanodiffraction patterns can be modeled at nonsymmetric Bragg conditions with this approach--a capability critical for advancing nanofocused x-ray diffraction microscopy.

Nanomembrane-based materials for Group IV semiconductor quantum electronics.

Strained-silicon/relaxed-silicon-germanium alloy (strained-Si/SiGe) heterostructures are the foundation of Group IV-element quantum electronics and quantum computation, but current materials quality limits the reliability and thus the achievable performance of devices. In comparison to conventional approaches, single-crystal SiGe nanomembranes are a promising alternative as substrates for the epitaxial growth of these heterostructures. Because the nanomembrane is truly a single crystal, in contrast to the conventional SiGe substrate made by compositionally grading SiGe grown on bulk Si, significant improvements in quantum electronic-device reliability may be expected with nanomembrane substrates. We compare lateral strain inhomogeneities and the local mosaic structure (crystalline tilt) in strained-Si/SiGe heterostructures that we grow on SiGe nanomembranes and on compositionally graded SiGe substrates, with micro-Raman mapping and nanodiffraction, respectively. Significant structural improvements are found using SiGe nanomembranes.

Coherent Bragg nanodiffraction at the hard X-ray Nanoprobe beamline.

Bragg coherent diffraction with nanofocused hard X-ray beams provides unique opportunities for quantitative in situ studies of crystalline structure in nanoscale regions of complex materials and devices by a variety of diffraction-based techniques. In the case of coherent diffraction imaging, a major experimental challenge in using nanoscale coherent beams is maintaining a constant scattering volume such that coherent fringe visibility is maximized and maintained over the course of an exposure lasting several seconds. Here, we present coherent Bragg diffraction patterns measured from different nanostructured thin films at the Sector 26 Nanoprobe beamline at the Advanced Photon Source and demonstrate that with nanoscale positional control, coherent diffraction patterns can be measured with source-limited fringe visibilities more than 50% suitable for imaging by coherent Bragg ptychography techniques.

Imaging local polarization in ferroelectric thin films by coherent x-ray Bragg projection ptychography.

We used x-ray Bragg projection ptychography (BPP) to map spatial variations of ferroelectric polarization in thin film PbTiO3, which exhibited a striped nanoscale domain pattern on a high-miscut (001) SrTiO3 substrate. By converting the reconstructed BPP phase image to picometer-scale ionic displacements in the polar unit cell, a quantitative polarization map was made that was consistent with other characterization. The spatial resolution of 5.7 nm demonstrated here establishes BPP as an important tool for nanoscale ferroelectric domain imaging, especially in complex environments accessible with hard x rays.

Quantitative nanoscale imaging of lattice distortions in epitaxial semiconductor heterostructures using nanofocused X-ray Bragg projection ptychography.

We imaged nanoscale lattice strain in a multilayer semiconductor device prototype with a new X-ray technique, nanofocused Bragg projection ptychography. Applying this technique to the epitaxial stressor layer of a SiGe-on-SOI structure, we measured the internal lattice behavior in a targeted region of a single device and demonstrated that its internal strain profile consisted of two competing lattice distortions. These results provide the strongest nondestructive test to date of continuum modeling predictions of nanoscale strain distributions.