J. Segura-Ruiz

Status of the hard X-ray microprobe beamline ID22 of the European Synchrotron Radiation Facility

The ESRF synchrotron beamline ID22, dedicated to hard X-ray microanalysis and consisting of the combination of X-ray fluorescence, X-ray absorption spectroscopy, diffraction and 2D/3D X-ray imaging techniques, is one of the most versatile instruments in hard X-ray microscopy science. This paper describes the present beamline characteristics, recent technical developments, as well as a few scientific examples from recent years of the beamline operation. The upgrade plans to adapt the beamline to the growing needs of the user community are briefly discussed.

Nano-X-ray absorption spectroscopy of single Co-implanted ZnO nanowires.

We report on the local structure of single Co-implanted ZnO nanowires studied using a hard X-ray nanoprobe. X-ray fluorescence maps show uniform Zn and Co distributions along the wire within the length scale of the beam size. The X-ray fluorescence data allow the estimation of the Co content within the nanowire. Polarization dependent X-ray absorption near edge structure shows no structural disorder induced neither in the radial nor axial directions of the implanted nanowires after subsequent annealing. Co2+ ions occupy Zn sites into the wurtzite ZnO lattice. Extended X-ray absorption fine structure data reveal high structural order in the host lattice without distortion in their interatomic distances, confirming the recovery of the radiation damaged ZnO structure through thermal annealing.

Phase Separation in Single InxGa1–xN Nanowires Revealed through a Hard X-ray Synchrotron Nanoprobe

In this work, we report on the composition, short- and long-range structural order of single molecular beam epitaxy grown In(x)Ga(1-x)N nanowires using a hard X-ray synchrotron nanoprobe. Nano-X-ray fluorescence mapping reveals an axial and radial heterogeneous elemental distribution in the single wires with Ga accumulation at their bottom and outer regions. Polarization-dependent nano-X-ray absorption near edge structure demonstrates that despite the elemental modulation, the tetrahedral order around the Ga atoms remains along the nanowires. Nano-X-ray diffraction mapping on single nanowires shows the existence of at least three different phases at their bottom: an In-poor shell and two In-rich phases. The alloy homogenizes toward the top of the wires, where a single In-rich phase is observed. No signatures of In-metallic precipitates are observed in the diffraction spectra. The In-content along the single nanowires estimated from X-ray fluorescence and diffraction data are in good agreement. A rough picture of these phenomena is briefly presented. We anticipate that this methodology will contribute to a greater understanding of the underlying growth concepts not only of nanowires but also of many nanostructures in materials science.

Crossed Ga2O3/SnO2 multiwire architecture: a local structure study with nanometer resolution.

Crossed nanowire structures are the basis for high-density integration of a variety of nanodevices. Owing to the critical role of nanowires intersections in creating hybrid architectures, it has become a challenge to investigate the local structure in crossing points in metal oxide nanowires. Thus, if intentionally grown crossed nanowires are well-patterned, an ideal model to study the junction is formed. By combining electron and synchrotron beam nanoprobes, we show here experimental evidence of the role of impurities in the coupling formation, structural modifications, and atomic site configuration based on crossed Ga2O3/SnO2 nanowires. Our experiment opens new avenues for further local structure studies with both nanometer resolution and elemental sensitivity.

Probing quantum confinement within single core-multishell nanowires.

Theoretically core-multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices.

Spontaneous core–shell elemental distribution in In-rich InxGa1−xN nanowires grown by molecular beam epitaxy

The elemental distribution of self-organized In-rich In(x)Ga1-xN nanowires grown by plasma-assisted molecular beam epitaxy has been investigated using three different techniques with spatial resolution on the nanoscale. Two-dimensional images and elemental profiles of single nanowires obtained by x-ray fluorescence and energy-dispersive x-ray spectroscopy, respectively, have revealed a radial gradient in the alloy composition of each individual nanowire. The spectral selectivity of resonant Raman scattering has been used to enhance the signal from very small volumes with different elemental composition within single nanowires. The combination of the three techniques has provided sufficient sensitivity and spatial resolution to prove the spontaneous formation of a core–shell nanowire and to quantify the thicknesses and alloy compositions of the core and shell regions. A theoretical model based on continuum elastic theory has been used to estimate the strain fields present in such inhomogeneous nanowires. These results suggest new strategies for achieving high quality nonpolar heterostructures.

Exploring single semiconductor nanowires with a multimodal hard X-ray nanoprobe.

Semiconductor nanowires offer new opportunities for optoelectronic and spintronic nanodevices. However, their full potential is ultimately dictated by our ability to control multiple property-function relationships taking place at the nanoscale in the spatial and time domains. Only a combination of high-resolution analytical techniques can provide a comprehensive understanding of their complex functionalities. Here we describe how a multimodal hard X-ray nanoprobe addresses fundamental questions in nanowire research. Selected topics ranging from cluster formation, dopant segregation, and phase separations to quantum confinement effects are investigated with sub-100 nm spatial resolution and sub-50 ps temporal resolution. This approach opens new avenues for structural, composition and optical studies with broad applicability in materials science.

Local lattice distortions in single Co-implanted ZnO nanowires

This work reports on the local structure of as-implanted and thermally-treated single Co:ZnO nanowires studied using a hard X-ray nanoprobe. Although the Co ions are incorporated into the wurtzite ZnO lattice, X-ray absorption near edge structure data show high structural disorder in the as-implanted nanowires compared with the annealed ones. In particular, extended X-ray absorption fine structure from single wires reveals a lattice distortion around Zn sites of the as-implanted nanowires, which involves an expansion of the stable wurtzite lattice. The observed local lattice response confirms good recovery of the implantation-induced damage within the ZnO lattice through a thermal treatment.

Synchrotron microanalysis techniques applied to potential photovoltaic materials

X-ray synchrotron radiation techniques are used to characterize photovoltaic-related semiconductors. Micro-X-ray-fluorescence and X-ray beam induced current mapping of multicrystalline silicon photovoltaic cells show metallic impurities accumulating at the interface of crystallographic defects, and current variations over the cell that are attributed to bulk defects and structural variation of the silicon. Similarly, studies on a single-crystal GaAs using X-ray fluorescence and X-ray excited optical luminescence show an inhomogeneous As distribution correlated with the photoluminescence signal, with higher As concentration regions having stronger photoluminescence signal. Both examples show how the combination of synchrotron microanalysis techniques can contribute to a better understanding of the optical properties of photovoltaic materials.