J. A. Sans

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.

X-ray absorption of Zn1−xCoxO thin films: A local structure study

In this study, the application of synchrotron radiation microprobe to the analysis of Co incorporation in Zn1−xCoxO is reported. From the Co and Zn fluorescence line intensity ratio, the Co concentrations were deduced. A combination of fluorescence mapping with x-ray absorption spectroscopic techniques made possible to examine not only the uniform elemental distribution but also the short range structural order with the Co content. In Zn1−xCoxO films grown by pulsed laser depositions the Co atoms are in tetrahedral site configuration, corresponding to a substitutional site in wurtzite structure. Within the experimental accuracy, no significant bond length deviation from the pure ZnO structure was observed in the considered compositional range.

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.

Charge-transfer absorption band in Zn1−xMxO (M: Co, Mn) investigated by means of photoconductivity, Ga doping, and optical measurements under pressure

The nature of the charge-transfer absorption band in undoped and Ga-doped Zn1−xMxO (M: Co, Mn) thin films is investigated by means of photoconductivity and optical absorption measurements under pressure. Internal transitions in the crystal field split Co 3d shell do not contribute to the photoconductivity spectrum and have very low pressure coefficient. Broad absorption bands at photon energies just below the band gap in both ZnMnO and ZnCoO clearly contribute to the photoconductivity spectra, indicating that they create free carriers and are consequently charge-transfer transitions. Under pressure, charge transfer bands have a pressure coefficient close to or larger than the band gap, in contrast to the expected low or negative pressure coefficient in a valence-band-to-localized level transition. Finally, the expected Burstein–Moss shift in the fundamental edge of heavily Ga-doped samples of ZnMO is associated to a larger shift and intensity decrease in the pre-edge band, confirming that charge-transfer ...

X-ray linear dichroism of defects in GaN:Mg using hard x-ray nanoprobe

In this study, we report the application of synchrotron radiation nanoprobe techniques to the structural analysis of pyramidal defects in Mg doped GaN. A combination of fluorescence mapping with spectroscopic techniques enabled us to examine not only the elemental nature but also their crystallographic orientation on the submicron scale. Our observations provide strong evidence for the diffusion and subsequent segregation of background impurities of Fe and Cr on the sample surface, with the absence of pronounced sequential stacking compared with Ga atoms. The strong polarization-dependent x-ray absorption near-edge structure features showed the preservation of the hexagonal crystalline structure in both defect-free and hexagonal pyramids. From the spatially resolved x-ray linear dichroism, no preferential disorder was observed in the direction parallel or perpendicular to the crystal growth.

Dependence Of Electrically Active Centers Content With The Growth Temperature In Heavily Ga‐doped ZnO Thin Films: Correlation Between Optical, Structural And Transport Properties

This work proposes an explanation for the decrease of the electrically active Ga content with the growth temperature observed in ZnO:Ga thin films grown by pulsed laser deposition (PLD). Studies of optical, structural and transport properties have led us to propose the possible segregation of most of Ga atoms in an intermediate phase of spinel ZnGa2O4.

Chemical effects of band filling and band‐gap renormalization on heavily doped ZnO:MIII (Al, Ga and In): A band anti‐crossing approach

Previous results show that the discrepancy between the experimental measurements and standard model theoretical calculations can be related with our assumption of unperturbed dispersion in the conduction band of the material. For overcoming this limitation, it has been proposed the band anti‐crossing (BAC) model, based on the interaction of an antibonding state of Ga‐O with the conduction band. Extending this model to other doping elements from III‐group (Al, Ga and In), coherent results for the optical band‐gap energy were obtained. These results have been supported by theoretical calculus of the electronic band structure, carried out by numerical atomic orbitals density functional theory (DFT) approach. This method is designed for efficient calculations in large systems and implemented in the SIESTA code.